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Chapter 14. Blocks and Statements

Chapter 14. Blocks and Statements

The sequence of execution of a program is controlled by statements, which are executed for their effect and do not have values.

Some statements contain other statements as part of their structure; such other statements are substatements of the statement. We say that statement S immediately contains statement U if there is no statement T different from S and U such that S contains T and T contains U. In the same manner, some statements contain expressions (§15 (Expressions)) as part of their structure.

The first section of this chapter discusses the distinction between normal and abrupt completion of statements (§14.1). Most of the remaining sections explain the various kinds of statements, describing in detail both their normal behavior and any special treatment of abrupt completion.

Blocks are explained first (§14.2), followed by local class declarations (§14.3) and local variable declaration statements (§14.4).

Next a grammatical maneuver that sidesteps the familiar "dangling else" problem (§14.5) is explained.

The last section (§14.21) of this chapter addresses the requirement that every statement be reachable in a certain technical sense.

14.1. Normal and Abrupt Completion of Statements

Every statement has a normal mode of execution in which certain computational steps are carried out. The following sections describe the normal mode of execution for each kind of statement.

If all the steps are carried out as described, with no indication of abrupt completion, the statement is said to complete normally. However, certain events may prevent a statement from completing normally:

If such an event occurs, then execution of one or more statements may be terminated before all steps of their normal mode of execution have completed; such statements are said to complete abruptly.

An abrupt completion always has an associated reason, which is one of the following:

The terms "complete normally" and "complete abruptly" also apply to the evaluation of expressions (§15.6). The only reason an expression can complete abruptly is that an exception is thrown, because of either a throw with a given value (§14.18) or a run-time exception or error (§11 (Exceptions), §15.6).

If a statement evaluates an expression, abrupt completion of the expression always causes the immediate abrupt completion of the statement, with the same reason. All succeeding steps in the normal mode of execution are not performed.

Unless otherwise specified in this chapter, abrupt completion of a substatement causes the immediate abrupt completion of the statement itself, with the same reason, and all succeeding steps in the normal mode of execution of the statement are not performed.

Unless otherwise specified, a statement completes normally if all expressions it evaluates and all substatements it executes complete normally.

A block is a sequence of statements, local class declarations, and local variable declaration statements within braces.

A block is executed by executing each of the local variable declaration statements and other statements in order from first to last (left to right). If all of these block statements complete normally, then the block completes normally. If any of these block statements complete abruptly for any reason, then the block completes abruptly for the same reason.

14.3. Local Class Declarations

A local class is a nested class (§8 (Classes)) that is not a member of any class and that has a name (§6.2, §6.7).

All local classes are inner classes (§8.1.3).

Every local class declaration statement is immediately contained by a block (§14.2). Local class declaration statements may be intermixed freely with other kinds of statements in the block.

It is a compile-time error if a local class declaration contains any of the access modifiers public, protected, or private (§6.6), or the modifier static (§8.1.1).

The scope and shadowing of a local class declaration is specified in §6.3 and §6.4.

Example 14.3-1. Local Class Declarations

Here is an example that illustrates several aspects of the rules given above:

class Global {
    class Cyclic {}

    void foo() {
        new Cyclic(); // create a Global.Cyclic
        class Cyclic extends Cyclic {} // circular definition

        {
            class Local {}
            {
                class Local {} // compile-time error
            }
            class Local {} // compile-time error
            class AnotherLocal {
                void bar() {
                    class Local {} // ok
                }
            }
        }
        class Local {} // ok, not in scope of prior Local
    }
}

The first statement of method foo creates an instance of the member class Global.Cyclic rather than an instance of the local class Cyclic, because the statement appears prior to the scope of the local class declaration.

The fact that the scope of a local class declaration encompasses its whole declaration (not only its body) means that the definition of the local class Cyclic is indeed cyclic because it extends itself rather than Global.Cyclic. Consequently, the declaration of the local class Cyclic is rejected at compile time.

Since local class names cannot be redeclared within the same method (or constructor or initializer, as the case may be), the second and third declarations of Local result in compile-time errors. However, Local can be redeclared in the context of another, more deeply nested, class such as AnotherLocal.

The final declaration of Local is legal, since it occurs outside the scope of any prior declaration of Local.


14.4. Local Variable Declaration Statements

A local variable declaration statement declares one or more local variable names.

LocalVariableDeclarationStatement:

LocalVariableDeclaration:

See §8.3 for UnannType. The following productions from §4.3, §8.3, and §8.4.1 are shown here for convenience:

Every local variable declaration statement is immediately contained by a block. Local variable declaration statements may be intermixed freely with other kinds of statements in the block.

Apart from local variable declaration statements, a local variable can be declared by the header of a basic for statement (§14.14.1), an enhanced for statement (§14.14.2), or a try-with-resources statement (§14.20.3).

The rules for annotation modifiers on a local variable declaration are specified in §9.7.4 and §9.7.5.

It is a compile-time error if final appears more than once as a modifier for a local variable declaration.

It is a compile-time error if the LocalVariableType is var and any of the following are true:

Example 14.4-1. Local Variables Declared With var

The following code illustrates these rules restricting the use of var:

var a = 1;            // Legal
var b = 2, c = 3.0;   // Illegal: multiple declarators
var d[] = new int[4]; // Illegal: extra bracket pairs
var e;                // Illegal: no initializer
var f = { 6 };        // Illegal: array initializer
var g = (g = 7);      // Illegal: self reference in initializer

These restrictions help to avoid confusion about the type being represented by var.


14.4.1. Local Variable Declarators and Types

Each declarator in a local variable declaration declares one local variable, whose name is the Identifier that appears in the declarator.

If the optional keyword final appears at the start of the declaration, the variable being declared is a final variable (§4.12.4).

The declared type of a local variable is determined as follows:

Example 14.4.1-1. Type of Local Variables Declared With var

The following code illustrates the typing of variables declared with var:

var a = 1;                // a has type 'int'
var b = java.util.List.of(1, 2);  // b has type 'List<Integer>'
var c = "x".getClass();   // c has type 'Class<? extends String>' 
                          // (see JLS 15.12.2.6)
var d = new Object() {};  // d has the type of the anonymous class
var e = (CharSequence & Comparable<String>) "x";
                          // e has type CharSequence & Comparable<String>
var f = () -> "hello";    // Illegal: lambda not in an assignment context
var g = null;             // Illegal: null type

Note that some variables declared with var cannot be declared with an explicit type, because the type of the variable is not denotable.

Upward projection is applied to the type of the initializer when determining the type of the variable. If the type of the initializer contains capture variables, this projection maps the type of the initializer to a supertype that does not contain capture variables.

While it would be possible to allow the type of the variable to mention capture variables, by projecting them away we enforce an attractive invariant that the scope of a capture variable is never larger than the statement containing the expression whose type is captured. Informally, capture variables cannot "leak" into subsequent statements.

A local variable of type float always contains a value that is an element of the float value set (§4.2.3); similarly, a local variable of type double always contains a value that is an element of the double value set. It is not permitted for a local variable of type float to contain an element of the float-extended-exponent value set that is not also an element of the float value set, nor for a local variable of type double to contain an element of the double-extended-exponent value set that is not also an element of the double value set.

The scope and shadowing of a local variable declaration is specified in §6.3 and §6.4.

14.4.2. Execution of Local Variable Declarations

A local variable declaration statement is an executable statement. Every time it is executed, the declarators are processed in order from left to right. If a declarator has an initializer, the initializer is evaluated and its value is assigned to the variable.

If a declarator does not have an initializer, then every reference to the variable must be preceded by execution of an assignment to the variable, or a compile-time error occurs by the rules of §16 (Definite Assignment).

Each initializer (except the first) is evaluated only if evaluation of the preceding initializer completes normally.

Execution of the local variable declaration completes normally only if evaluation of the last initializer completes normally.

If the local variable declaration contains no initializers, then executing it always completes normally.

There are many kinds of statements in the Java programming language. Most correspond to statements in the C and C++ languages, but some are unique.

As in C and C++, the if statement of the Java programming language suffers from the so-called "dangling else problem," illustrated by this misleadingly formatted example:

if (door.isOpen())
    if (resident.isVisible())
        resident.greet("Hello!");
else door.bell.ring();  // A "dangling else"

The problem is that both the outer if statement and the inner if statement might conceivably own the else clause. In this example, one might surmise that the programmer intended the else clause to belong to the outer if statement.

The Java programming language, like C and C++ and many programming languages before them, arbitrarily decrees that an else clause belongs to the innermost if to which it might possibly belong. This rule is captured by the following grammar:

StatementWithoutTrailingSubstatement:

The following productions from §14.9 are shown here for convenience:

IfThenElseStatementNoShortIf:

Statements are thus grammatically divided into two categories: those that might end in an if statement that has no else clause (a "short if statement") and those that definitely do not.

Only statements that definitely do not end in a short if statement may appear as an immediate substatement before the keyword else in an if statement that does have an else clause.

This simple rule prevents the "dangling else" problem. The execution behavior of a statement with the "no short if" restriction is identical to the execution behavior of the same kind of statement without the "no short if" restriction; the distinction is drawn purely to resolve the syntactic difficulty.

14.6. The Empty Statement

An empty statement does nothing.

Execution of an empty statement always completes normally.

Statements may have label prefixes.

LabeledStatementNoShortIf:

The Identifier is declared to be the label of the immediately contained Statement.

Unlike C and C++, the Java programming language has no goto statement; identifier statement labels are used with break or continue statements (§14.15, §14.16) appearing anywhere within the labeled statement.

The scope of a label of a labeled statement is the immediately contained Statement.

It is a compile-time error if the name of a label of a labeled statement is used within the scope of the label as a label of another labeled statement.

There is no restriction against using the same identifier as a label and as the name of a package, class, interface, method, field, parameter, or local variable. Use of an identifier to label a statement does not obscure (§6.4.2) a package, class, interface, method, field, parameter, or local variable with the same name. Use of an identifier as a class, interface, method, field, local variable or as the parameter of an exception handler (§14.20) does not obscure a statement label with the same name.

A labeled statement is executed by executing the immediately contained Statement.

If the statement is labeled by an Identifier and the contained Statement completes abruptly because of a break with the same Identifier, then the labeled statement completes normally. In all other cases of abrupt completion of the Statement, the labeled statement completes abruptly for the same reason.

Example 14.7-1. Labels and Identifiers

The following code was taken from a version of the class String and its method indexOf, where the label was originally called test. Changing the label to have the same name as the local variable i does not obscure the label in the scope of the declaration of i. Thus, the code is valid.

class Test {
    char[] value;
    int offset, count;
    int indexOf(TestString str, int fromIndex) {
        char[] v1 = value, v2 = str.value;
        int max = offset + (count - str.count);
        int start = offset + ((fromIndex < 0) ? 0 : fromIndex);
    i:
        for (int i = start; i <= max; i++) {
            int n = str.count, j = i, k = str.offset;
            while (n-- != 0) {
                if (v1[j++] != v2[k++])
                    continue i;
            } 
            return i - offset;
        }
        return -1;
    }
}

The identifier max could also have been used as the statement label; the label would not obscure the local variable max within the labeled statement.


14.8. Expression Statements

Certain kinds of expressions may be used as statements by following them with semicolons.

An expression statement is executed by evaluating the expression; if the expression has a value, the value is discarded.

Execution of the expression statement completes normally if and only if evaluation of the expression completes normally.

Unlike C and C++, the Java programming language allows only certain forms of expressions to be used as expression statements. For example, it is legal to use a method invocation expression (§15.12):

System.out.println("Hello world");  // OK

but it is not legal to use a parenthesized expression (§15.8.5):

(System.out.println("Hello world"));  // illegal

Note that the Java programming language does not allow a "cast to void" - void is not a type - so the traditional C trick of writing an expression statement such as:

(void)... ;  // incorrect!

does not work. On the other hand, the Java programming language allows all the most useful kinds of expressions in expression statements, and it does not require a method invocation used as an expression statement to invoke a void method, so such a trick is almost never needed. If a trick is needed, either an assignment statement (§15.26) or a local variable declaration statement (§14.4) can be used instead.

The if statement allows conditional execution of a statement or a conditional choice of two statements, executing one or the other but not both.

IfThenElseStatementNoShortIf:

The Expression must have type boolean or Boolean, or a compile-time error occurs.

14.9.1. The if-then Statement

An if-then statement is executed by first evaluating the Expression. If the result is of type Boolean, it is subject to unboxing conversion (§5.1.8).

If evaluation of the Expression or the subsequent unboxing conversion (if any) completes abruptly for some reason, the if-then statement completes abruptly for the same reason.

Otherwise, execution continues by making a choice based on the resulting value:

14.9.2. The if-then-else Statement

An if-then-else statement is executed by first evaluating the Expression. If the result is of type Boolean, it is subject to unboxing conversion (§5.1.8).

If evaluation of the Expression or the subsequent unboxing conversion (if any) completes abruptly for some reason, then the if-then-else statement completes abruptly for the same reason.

Otherwise, execution continues by making a choice based on the resulting value:

14.10. The assert Statement

An assertion is an assert statement containing a boolean expression. An assertion is either enabled or disabled. If an assertion is enabled, execution of the assertion causes evaluation of the boolean expression and an error is reported if the expression evaluates to false. If the assertion is disabled, execution of the assertion has no effect whatsoever.

To ease the presentation, the first Expression in both forms of the assert statement is referred to as Expression1. In the second form of the assert statement, the second Expression is referred to as Expression2.

It is a compile-time error if Expression1 does not have type boolean or Boolean.

It is a compile-time error if, in the second form of the assert statement, Expression2 is void (§15.1).

An assert statement that is executed after its class or interface has completed initialization is enabled if and only if the host system has determined that the top level class or interface that lexically contains the assert statement enables assertions.

Whether a top level class or interface enables assertions is determined no later than the earliest of (i) the initialization of the top level class or interface, and (ii) the initialization of any class or interface nested in the top level class or interface. Whether a top level class or interface enables assertions cannot be changed after it has been determined.

An assert statement that is executed before its class or interface has completed initialization is enabled.

This rule is motivated by a case that demands special treatment. Recall that the assertion status of a class is set no later than the time it is initialized. It is possible, though generally not desirable, to execute methods or constructors prior to initialization. This can happen when a class hierarchy contains a circularity in its static initialization, as in the following example:

public class Foo {
    public static void main(String[] args) {
        Baz.testAsserts(); 
        // Will execute after Baz is initialized.
    }
}
class Bar {
    static {
        Baz.testAsserts(); 
        // Will execute before Baz is initialized!
    }
}
class Baz extends Bar {
    static void testAsserts() {
        boolean enabled = false;
        assert  enabled = true;
        System.out.println("Asserts " + 
			   (enabled ? "enabled" : "disabled"));
    }
}

Invoking Baz.testAsserts() causes Baz to be initialized. Before this can happen, Bar must be initialized. Bar's static initializer again invokes Baz.testAsserts(). Because initialization of Baz is already in progress by the current thread, the second invocation executes immediately, though Baz is not initialized (§12.4.2).

Because of the rule above, if the program above is executed without enabling assertions, it must print:

Asserts enabled
Asserts disabled

A disabled assert statement does nothing. In particular, neither Expression1 nor Expression2 (if it is present) are evaluated. Execution of a disabled assert statement always completes normally.

An enabled assert statement is executed by first evaluating Expression1. If the result is of type Boolean, it is subject to unboxing conversion (§5.1.8).

If evaluation of Expression1 or the subsequent unboxing conversion (if any) completes abruptly for some reason, the assert statement completes abruptly for the same reason.

Otherwise, execution continues by making a choice based on the value of Expression1:

Typically, assertion checking is enabled during program development and testing, and disabled for deployment, to improve performance.

Because assertions may be disabled, programs must not assume that the expressions contained in assertions will be evaluated. Thus, these boolean expressions should generally be free of side effects. Evaluating such a boolean expression should not affect any state that is visible after the evaluation is complete. It is not illegal for a boolean expression contained in an assertion to have a side effect, but it is generally inappropriate, as it could cause program behavior to vary depending on whether assertions were enabled or disabled.

In light of this, assertions should not be used for argument checking in public methods. Argument checking is typically part of the contract of a method, and this contract must be upheld whether assertions are enabled or disabled.

A secondary problem with using assertions for argument checking is that erroneous arguments should result in an appropriate run-time exception (such as IllegalArgumentException, ArrayIndexOutOfBoundsException, or NullPointerException). An assertion failure will not throw an appropriate exception. Again, it is not illegal to use assertions for argument checking on public methods, but it is generally inappropriate. It is intended that AssertionError never be caught, but it is possible to do so, thus the rules for try statements should treat assertions appearing in a try block similarly to the current treatment of throw statements.

14.11. The switch Statement

The switch statement transfers control to one of several statements depending on the value of an expression.

SwitchBlockStatementGroup:

The type of the Expression must be char, byte, short, int, Character, Byte, Short, Integer, String, or an enum type (§8.9), or a compile-time error occurs.

The body of a switch statement is known as a switch block. Any statement immediately contained by the switch block may be labeled with one or more switch labels, which are case or default labels. Every case label has a case constant, which is either a constant expression or the name of an enum constant. Switch labels and their case constants are said to be associated with the switch statement.

Given a switch statement, all of the following must be true or a compile-time error occurs:

The prohibition against using null as a case constant prevents code being written that can never be executed. If the switch statement's Expression is of a reference type, that is, String or a boxed primitive type or an enum type, then an exception will be thrown will occur if the Expression evaluates to null at run time. In the judgment of the designers of the Java programming language, this is a better outcome than silently skipping the entire switch statement or choosing to execute the statements (if any) after the default label (if any).

A Java compiler is encouraged (but not required) to provide a warning if a switch on an enum-valued expression lacks a default label and lacks case labels for one or more of the enum's constants. Such a switch will silently do nothing if the expression evaluates to one of the missing constants.

In C and C++ the body of a switch statement can be a statement and statements with case labels do not have to be immediately contained by that statement. Consider the simple loop:

for (i = 0; i < n; ++i) foo();

where n is known to be positive. A trick known as Duff's device can be used in C or C++ to unroll the loop, but this is not valid code in the Java programming language:

int q = (n+7)/8;
switch (n%8) {
    case 0: do { foo();    // Great C hack, Tom,
    case 7:      foo();    // but it's not valid here.
    case 6:      foo();
    case 5:      foo();
    case 4:      foo();
    case 3:      foo();
    case 2:      foo();
    case 1:      foo();
            } while (--q > 0);
}

Fortunately, this trick does not seem to be widely known or used. Moreover, it is less needed nowadays; this sort of code transformation is properly in the province of state-of-the-art optimizing compilers.

A switch statement is executed by first evaluating the Expression. If the Expression evaluates to null, a NullPointerException is thrown and the entire switch statement completes abruptly for that reason. Otherwise, if the result is of type Character, Byte, Short, or Integer, it is subject to unboxing conversion (§5.1.8).

If evaluation of the Expression or the subsequent unboxing conversion (if any) completes abruptly for some reason, the switch statement completes abruptly for the same reason.

Otherwise, execution continues by comparing the value of the Expression with each case constant, and there is a choice:

If any statement immediately contained by the Block body of the switch statement completes abruptly, it is handled as follows:

Example 14.11-1. Fall-Through in the switch Statement

As in C and C++, execution of statements in a switch block "falls through labels."

For example, the program:

class TooMany {
    static void howMany(int k) {
        switch (k) {
            case 1: System.out.print("one ");
            case 2: System.out.print("too ");
            case 3: System.out.println("many");
        }
    }
    public static void main(String[] args) {
        howMany(3);
        howMany(2);
        howMany(1);
    }
}

contains a switch block in which the code for each case falls through into the code for the next case. As a result, the program prints:

many
too many
one too many

If code is not to fall through case to case in this manner, then break statements should be used, as in this example:

class TwoMany {
    static void howMany(int k) {
        switch (k) {
            case 1: System.out.println("one");
                    break;  // exit the switch
            case 2: System.out.println("two");
                    break;  // exit the switch
            case 3: System.out.println("many");
                    break;  // not needed, but good style
        }
    }
    public static void main(String[] args) {
        howMany(1);
        howMany(2);
        howMany(3);
    }
}

This program prints:

one
two
many

14.12. The while Statement

The while statement executes an Expression and a Statement repeatedly until the value of the Expression is false.

The Expression must have type boolean or Boolean, or a compile-time error occurs.

A while statement is executed by first evaluating the Expression. If the result is of type Boolean, it is subject to unboxing conversion (§5.1.8).

If evaluation of the Expression or the subsequent unboxing conversion (if any) completes abruptly for some reason, the while statement completes abruptly for the same reason.

Otherwise, execution continues by making a choice based on the resulting value:

14.12.1. Abrupt Completion of while Statement

Abrupt completion of the contained Statement is handled in the following manner:

The do statement executes a Statement and an Expression repeatedly until the value of the Expression is false.

The Expression must have type boolean or Boolean, or a compile-time error occurs.

A do statement is executed by first executing the Statement. Then there is a choice:

Executing a do statement always executes the contained Statement at least once.

14.13.1. Abrupt Completion of do Statement

Abrupt completion of the contained Statement is handled in the following manner:

Example 14.13-1. The do Statement

The following code is one possible implementation of the toHexString method of class Integer:

public static String toHexString(int i) {
    StringBuffer buf = new StringBuffer(8);
    do {
        buf.append(Character.forDigit(i & 0xF, 16));
        i >>>= 4;
    } while (i != 0);
    return buf.reverse().toString();
}

Because at least one digit must be generated, the do statement is an appropriate control structure.


The for statement has two forms:

14.14.1. The basic for Statement

The basic for statement executes some initialization code, then executes an Expression, a Statement, and some update code repeatedly until the value of the Expression is false.

BasicForStatementNoShortIf:

The Expression must have type boolean or Boolean, or a compile-time error occurs.

The scope and shadowing of a local variable declared in the ForInit part of a basic for statement is specified in §6.3 and §6.4.

14.14.1.1. Initialization of for Statement

A for statement is executed by first executing the ForInit code:

14.14.1.2. Iteration of for Statement

Next, a for iteration step is performed, as follows:

If the Expression is not present, then the only way a for statement can complete normally is by use of a break statement.

14.14.1.3. Abrupt Completion of for Statement

Abrupt completion of the contained Statement is handled in the following manner:

14.14.2. The enhanced for statement

The enhanced for statement has the form:

EnhancedForStatementNoShortIf:

The following productions from §4.3, §8.3, §8.4.1, and §14.4 are shown here for convenience:

The header of the enhanced for statement declares a local variable, whose name is the identifier given by VariableDeclaratorId.

If the keyword final appears at the start of the declaration, the variable being declared is a final variable (§4.12.4).

It is a compile-time error if the LocalVariableType is var and the VariableDeclaratorId has one or more bracket pairs.

The type of the Expression must be a subtype of the raw type Iterable or an array type (§10.1), or a compile-time error occurs.

The type of the local variable is determined as follows:

The scope and shadowing of the local variable is specified in §6.3 and §6.4.

When an enhanced for statement is executed, the local variable is initialized, on each iteration of the loop, to successive elements of the array or Iterable produced by the expression. The precise meaning of the enhanced for statement is given by translation into a basic for statement, as follows:

Example 14.14-1. Enhanced for And Arrays

The following program, which calculates the sum of an integer array, shows how enhanced for works for arrays:

int sum(int[] a) {
    int sum = 0;
    for (int i : a) sum += i;
    return sum;
}

Example 14.14-2. Enhanced for And Unboxing Conversion

The following program combines the enhanced for statement with auto-unboxing to translate a histogram into a frequency table:

Map<String, Integer> histogram = ...;
double total = 0;
for (int i : histogram.values())
    total += i;
for (Map.Entry<String, Integer> e : histogram.entrySet())
    System.out.println(e.getKey() + " " + e.getValue() / total);
}

14.15. The break Statement

A break statement transfers control out of an enclosing statement.

A break statement with no label attempts to transfer control to the innermost enclosing switch, while, do, or for statement of the immediately enclosing method or initializer; this statement, which is called the break target, then immediately completes normally.

To be precise, a break statement with no label always completes abruptly, the reason being a break with no label.

If no switch, while, do, or for statement in the immediately enclosing method, constructor, or initializer contains the break statement, a compile-time error occurs.

A break statement with label Identifier attempts to transfer control to the enclosing labeled statement (§14.7) that has the same Identifier as its label; this statement, which is called the break target, then immediately completes normally. In this case, the break target need not be a switch, while, do, or for statement.

To be precise, a break statement with label Identifier always completes abruptly, the reason being a break with label Identifier.

A break statement must refer to a label within the immediately enclosing method, constructor, initializer, or lambda body. There are no non-local jumps. If no labeled statement with Identifier as its label in the immediately enclosing method, constructor, initializer, or lambda body contains the break statement, a compile-time error occurs.

It can be seen, then, that a break statement always completes abruptly.

The preceding descriptions say "attempts to transfer control" rather than just "transfers control" because if there are any try statements (§14.20) within the break target whose try blocks or catch clauses contain the break statement, then any finally clauses of those try statements are executed, in order, innermost to outermost, before control is transferred to the break target. Abrupt completion of a finally clause can disrupt the transfer of control initiated by a break statement.

Example 14.15-1. The break Statement

In the following example, a mathematical graph is represented by an array of arrays. A graph consists of a set of nodes and a set of edges; each edge is an arrow that points from some node to some other node, or from a node to itself. In this example it is assumed that there are no redundant edges; that is, for any two nodes P and Q, where Q may be the same as P, there is at most one edge from P to Q.

Nodes are represented by integers, and there is an edge from node i to node edges[i][j] for every i and j for which the array reference edges[i][j] does not throw an ArrayIndexOutOfBoundsException.

The task of the method loseEdges, given integers i and j, is to construct a new graph by copying a given graph but omitting the edge from node i to node j, if any, and the edge from node j to node i, if any:

class Graph {
    int edges[][];
    public Graph(int[][] edges) { this.edges = edges; }

    public Graph loseEdges(int i, int j) {
        int n = edges.length;
        int[][] newedges = new int[n][];
        for (int k = 0; k < n; ++k) {
edgelist:
{
            int z;
search:
{
            if (k == i) {
                for (z = 0; z < edges[k].length; ++z) {
                    if (edges[k][z] == j) break search;
                }
            } else if (k == j) {
                for (z = 0; z < edges[k].length; ++z) {
                    if (edges[k][z] == i) break search;
                }
            }

            // No edge to be deleted; share this list.
            newedges[k] = edges[k];
            break edgelist;
} //search

            // Copy the list, omitting the edge at position z.
            int m = edges[k].length - 1;
            int ne[] = new int[m];
            System.arraycopy(edges[k], 0, ne, 0, z);
            System.arraycopy(edges[k], z+1, ne, z, m-z);
            newedges[k] = ne;
} //edgelist
        }
        return new Graph(newedges);
    }
}

Note the use of two statement labels, edgelist and search, and the use of break statements. This allows the code that copies a list, omitting one edge, to be shared between two separate tests, the test for an edge from node i to node j, and the test for an edge from node j to node i.


14.16. The continue Statement

A continue statement may occur only in a while, do, or for statement; statements of these three kinds are called iteration statements. Control passes to the loop-continuation point of an iteration statement.

A continue statement with no label attempts to transfer control to the innermost enclosing while, do, or for statement of the immediately enclosing method, constructor, or initializer; this statement, which is called the continue target, then immediately ends the current iteration and begins a new one.

To be precise, such a continue statement always completes abruptly, the reason being a continue with no label.

If no while, do, or for statement of the immediately enclosing method, constructor, or initializer contains the continue statement, a compile-time error occurs.

A continue statement with label Identifier attempts to transfer control to the enclosing labeled statement (§14.7) that has the same Identifier as its label; that statement, which is called the continue target, then immediately ends the current iteration and begins a new one.

To be precise, a continue statement with label Identifier always completes abruptly, the reason being a continue with label Identifier.

The continue target must be a while, do, or for statement, or a compile-time error occurs.

A continue statement must refer to a label within the immediately enclosing method, constructor, initializer, or lambda body. There are no non-local jumps. If no labeled statement with Identifier as its label in the immediately enclosing method, constructor, initializer, or lambda body contains the continue statement, a compile-time error occurs.

It can be seen, then, that a continue statement always completes abruptly.

See the descriptions of the while statement (§14.12), do statement (§14.13), and for statement (§14.14) for a discussion of the handling of abrupt termination because of continue.

The preceding descriptions say "attempts to transfer control" rather than just "transfers control" because if there are any try statements (§14.20) within the continue target whose try blocks or catch clauses contain the continue statement, then any finally clauses of those try statements are executed, in order, innermost to outermost, before control is transferred to the continue target. Abrupt completion of a finally clause can disrupt the transfer of control initiated by a continue statement.

Example 14.16-1. The continue Statement

In the Graph class in §14.15, one of the break statements is used to finish execution of the entire body of the outermost for loop. This break can be replaced by a continue if the for loop itself is labeled:

class Graph {
    int edges[][];
    public Graph(int[][] edges) { this.edges = edges; }

    public Graph loseEdges(int i, int j) {
        int n = edges.length;
        int[][] newedges = new int[n][];
edgelists:
        for (int k = 0; k < n; ++k) {
            int z;
search:
{
            if (k == i) {
                for (z = 0; z < edges[k].length; ++z) {
                    if (edges[k][z] == j) break search;
                }
            } else if (k == j) {
                for (z = 0; z < edges[k].length; ++z) {
                    if (edges[k][z] == i) break search;
                }
            }

            // No edge to be deleted; share this list.
            newedges[k] = edges[k];
            continue edgelists;
} //search

            // Copy the list, omitting the edge at position z.
            int m = edges[k].length - 1;
            int ne[] = new int[m];
            System.arraycopy(edges[k], 0, ne, 0, z);
            System.arraycopy(edges[k], z+1, ne, z, m-z);
            newedges[k] = ne;
        } //edgelists
        return new Graph(newedges);
    }
}

Which to use, if either, is largely a matter of programming style.


14.17. The return Statement

A return statement returns control to the invoker of a method (§8.4, §15.12) or constructor (§8.8, §15.9).

A return statement is contained in the innermost constructor, method, initializer, or lambda expression whose body encloses the return statement.

It is a compile-time error if a return statement is contained in an instance initializer or a static initializer (§8.6, §8.7).

A return statement with no Expression must be contained in one of the following, or a compile-time error occurs:

A return statement with no Expression attempts to transfer control to the invoker of the method, constructor, or lambda body that contains it. To be precise, a return statement with no Expression always completes abruptly, the reason being a return with no value.

A return statement with an Expression must be contained in one of the following, or a compile-time error occurs:

The Expression must denote a variable or a value, or a compile-time error occurs.

When a return statement with an Expression appears in a method declaration, the Expression must be assignable (§5.2) to the declared return type of the method, or a compile-time error occurs.

A return statement with an Expression attempts to transfer control to the invoker of the method or lambda body that contains it; the value of the Expression becomes the value of the method invocation. More precisely, execution of such a return statement first evaluates the Expression. If the evaluation of the Expression completes abruptly for some reason, then the return statement completes abruptly for that reason. If evaluation of the Expression completes normally, producing a value V, then the return statement completes abruptly, the reason being a return with value V.

If the expression is of type float and is not FP-strict (§15.4), then the value may be an element of either the float value set or the float-extended-exponent value set (§4.2.3). If the expression is of type double and is not FP-strict, then the value may be an element of either the double value set or the double-extended-exponent value set.

It can be seen, then, that a return statement always completes abruptly.

The preceding descriptions say "attempts to transfer control" rather than just "transfers control" because if there are any try statements (§14.20) within the method or constructor whose try blocks or catch clauses contain the return statement, then any finally clauses of those try statements will be executed, in order, innermost to outermost, before control is transferred to the invoker of the method or constructor. Abrupt completion of a finally clause can disrupt the transfer of control initiated by a return statement.

14.18. The throw Statement

A throw statement causes an exception (§11 (Exceptions)) to be thrown. The result is an immediate transfer of control (§11.3) that may exit multiple statements and multiple constructor, instance initializer, static initializer and field initializer evaluations, and method invocations until a try statement (§14.20) is found that catches the thrown value. If no such try statement is found, then execution of the thread (§17 (Threads and Locks)) that executed the throw is terminated (§11.3) after invocation of the uncaughtException method for the thread group to which the thread belongs.

The Expression in a throw statement must either denote a variable or value of a reference type which is assignable (§5.2) to the type Throwable, or denote the null reference, or a compile-time error occurs.

The reference type of the Expression will always be a class type (since no interface types are assignable to Throwable) which is not parameterized (since a subclass of Throwable cannot be generic (§8.1.2)).

At least one of the following three conditions must be true, or a compile-time error occurs:

The exception types that a throw statement can throw are specified in §11.2.2.

A throw statement first evaluates the Expression. Then:

It can be seen, then, that a throw statement always completes abruptly.

If there are any enclosing try statements (§14.20) whose try blocks contain the throw statement, then any finally clauses of those try statements are executed as control is transferred outward, until the thrown value is caught. Note that abrupt completion of a finally clause can disrupt the transfer of control initiated by a throw statement.

If a throw statement is contained in a method declaration or a lambda expression, but its value is not caught by some try statement that contains it, then the invocation of the method completes abruptly because of the throw.

If a throw statement is contained in a constructor declaration, but its value is not caught by some try statement that contains it, then the class instance creation expression that invoked the constructor will complete abruptly because of the throw (§15.9.4).

If a throw statement is contained in a static initializer (§8.7), then a compile-time check (§11.2.3) ensures that either its value is always an unchecked exception or its value is always caught by some try statement that contains it. If at run time, despite this check, the value is not caught by some try statement that contains the throw statement, then the value is rethrown if it is an instance of class Error or one of its subclasses; otherwise, it is wrapped in an ExceptionInInitializerError object, which is then thrown (§12.4.2).

If a throw statement is contained in an instance initializer (§8.6), then a compile-time check (§11.2.3) ensures that either its value is always an unchecked exception or its value is always caught by some try statement that contains it, or the type of the thrown exception (or one of its superclasses) occurs in the throws clause of every constructor of the class.

By convention, user-declared throwable types should usually be declared to be subclasses of class Exception, which is a subclass of class Throwable (§11.1.1).

14.19. The synchronized Statement

A synchronized statement acquires a mutual-exclusion lock (§17.1) on behalf of the executing thread, executes a block, then releases the lock. While the executing thread owns the lock, no other thread may acquire the lock.

The type of Expression must be a reference type, or a compile-time error occurs.

A synchronized statement is executed by first evaluating the Expression. Then:

The locks acquired by synchronized statements are the same as the locks that are acquired implicitly by synchronized methods (§8.4.3.6). A single thread may acquire a lock more than once.

Acquiring the lock associated with an object does not in itself prevent other threads from accessing fields of the object or invoking un-synchronized methods on the object. Other threads can also use synchronized methods or the synchronized statement in a conventional manner to achieve mutual exclusion.

Example 14.19-1. The synchronized Statement

class Test {
    public static void main(String[] args) {
        Test t = new Test();
        synchronized(t) {
            synchronized(t) {
                System.out.println("made it!");
            }
        }
    }
}

This program produces the output:

made it!

Note that this program would deadlock if a single thread were not permitted to lock a monitor more than once.


A try statement executes a block. If a value is thrown and the try statement has one or more catch clauses that can catch it, then control will be transferred to the first such catch clause. If the try statement has a finally clause, then another block of code is executed, no matter whether the try block completes normally or abruptly, and no matter whether a catch clause is first given control.

See §8.3 for UnannClassType. The following productions from §4.3, §8.3, and §8.4.1 are shown here for convenience:

The Block immediately after the keyword try is called the try block of the try statement.

The Block immediately after the keyword finally is called the finally block of the try statement.

A try statement may have catch clauses, also called exception handlers.

A catch clause declares exactly one parameter, which is called an exception parameter.

It is a compile-time error if final appears more than once as a modifier for an exception parameter declaration.

The scope and shadowing of an exception parameter is specified in §6.3 and §6.4.

An exception parameter may denote its type as either a single class type or a union of two or more class types (called alternatives). The alternatives of a union are syntactically separated by |.

A catch clause whose exception parameter is denoted as a single class type is called a uni-catch clause.

A catch clause whose exception parameter is denoted as a union of types is called a multi-catch clause.

Each class type used in the denotation of the type of an exception parameter must be the class Throwable or a subclass of Throwable, or a compile-time error occurs.

It is a compile-time error if a type variable is used in the denotation of the type of an exception parameter.

It is a compile-time error if a union of types contains two alternatives Di and Dj (i ≠ j) where Di is a subtype of Dj (§4.10.2).

The declared type of an exception parameter that denotes its type with a single class type is that class type.

The declared type of an exception parameter that denotes its type as a union with alternatives D1 | D2 | ... | Dn is lub(D1, D2, ..., Dn).

An exception parameter of a multi-catch clause is implicitly declared final if it is not explicitly declared final.

It is a compile-time error if an exception parameter that is implicitly or explicitly declared final is assigned to within the body of the catch clause.

An exception parameter of a uni-catch clause is never implicitly declared final, but it may be explicitly declared final or be effectively final (§4.12.4).

An implicitly final exception parameter is final by virtue of its declaration, while an effectively final exception parameter is (as it were) final by virtue of how it is used. An exception parameter of a multi-catch clause is implicitly declared final, so will never occur as the left-hand operand of an assignment operator, but it is not considered effectively final.

If an exception parameter is effectively final (in a uni-catch clause) or implicitly final (in a multi-catch clause), then adding an explicit final modifier to its declaration will not introduce any compile-time errors. On the other hand, if the exception parameter of a uni-catch clause is explicitly declared final, then removing the final modifier may introduce compile-time errors because the exception parameter, now considered to be effectively final, can no longer longer be referenced by anonymous and local class declarations in the body of the catch clause. If there are no compile-time errors, it is possible to further change the program so that the exception parameter is re-assigned in the body of the catch clause and thus will no longer be considered effectively final.

The exception types that a try statement can throw are specified in §11.2.2.

The relationship of the exceptions thrown by the try block of a try statement and caught by the catch clauses (if any) of the try statement is specified in §11.2.3.

Exception handlers are considered in left-to-right order: the earliest possible catch clause accepts the exception, receiving as its argument the thrown exception object, as specified in §11.3.

A multi-catch clause can be thought of as a sequence of uni-catch clauses. That is, a catch clause where the type of the exception parameter is denoted as a union D1|D2|...|Dn is equivalent to a sequence of n catch clauses where the types of the exception parameters are class types D1, D2, ..., Dn respectively. In the Block of each of the n catch clauses, the declared type of the exception parameter is lub(D1, D2, ..., Dn). For example, the following code:

try {
    ... throws ReflectiveOperationException ...
}
catch (ClassNotFoundException | IllegalAccessException ex) {
    ... body ...
}

is semantically equivalent to the following code:

try {
    ... throws ReflectiveOperationException ...
}
catch (final ClassNotFoundException ex1) {
    final ReflectiveOperationException ex = ex1;
    ... body ...
}
catch (final IllegalAccessException ex2) {
    final ReflectiveOperationException ex = ex2;
    ... body ...
}

where the multi-catch clause with two alternatives has been translated into two uni-catch clauses, one for each alternative. A Java compiler is neither required nor recommended to compile a multi-catch clause by duplicating code in this manner, since it is possible to represent the multi-catch clause in a class file without duplication.

A finally clause ensures that the finally block is executed after the try block and any catch block that might be executed, no matter how control leaves the try block or catch block. Handling of the finally block is rather complex, so the two cases of a try statement with and without a finally block are described separately (§14.20.1, §14.20.2).

A try statement is permitted to omit catch clauses and a finally clause if it is a try-with-resources statement (§14.20.3).

14.20.1. Execution of try-catch

A try statement without a finally block is executed by first executing the try block. Then there is a choice:

Example 14.20.1-1. Catching An Exception

class BlewIt extends Exception {
    BlewIt() { }
    BlewIt(String s) { super(s); }
}
class Test {
    static void blowUp() throws BlewIt { throw new BlewIt(); }

    public static void main(String[] args) {
        try {
            blowUp();
        } catch (RuntimeException r) {
            System.out.println("Caught RuntimeException");
        } catch (BlewIt b) {
            System.out.println("Caught BlewIt");
        }
    }
}

Here, the exception BlewIt is thrown by the method blowUp. The try-catch statement in the body of main has two catch clauses. The run-time type of the exception is BlewIt which is not assignable to a variable of type RuntimeException, but is assignable to a variable of type BlewIt, so the output of the example is:

Caught BlewIt

14.20.2. Execution of try-finally and try-catch-finally

A try statement with a finally block is executed by first executing the try block. Then there is a choice:

Example 14.20.2-1. Handling An Uncaught Exception With finally

class BlewIt extends Exception {
    BlewIt() { }
    BlewIt(String s) { super(s); }
}
class Test {
    static void blowUp() throws BlewIt {
        throw new NullPointerException();
    }
    public static void main(String[] args) {
        try {
            blowUp();
        } catch (BlewIt b) {
            System.out.println("Caught BlewIt");
        } finally {
            System.out.println("Uncaught Exception");
        }
    }
}

This program produces the output:

Uncaught Exception
Exception in thread "main" java.lang.NullPointerException
        at Test.blowUp(Test.java:7)
        at Test.main(Test.java:11)

The NullPointerException (which is a kind of RuntimeException) that is thrown by method blowUp is not caught by the try statement in main, because a NullPointerException is not assignable to a variable of type BlewIt. This causes the finally clause to execute, after which the thread executing main, which is the only thread of the test program, terminates because of an uncaught exception, which typically results in printing the exception name and a simple backtrace. However, a backtrace is not required by this specification.

The problem with mandating a backtrace is that an exception can be created at one point in the program and thrown at a later one. It is prohibitively expensive to store a stack trace in an exception unless it is actually thrown (in which case the trace may be generated while unwinding the stack). Hence we do not mandate a back trace in every exception.


14.20.3. try-with-resources

A try-with-resources statement is parameterized with local variables (known as resources) that are initialized before execution of the try block and closed automatically, in the reverse order from which they were initialized, after execution of the try block. catch clauses and a finally clause are often unnecessary when resources are closed automatically.

TryWithResourcesStatement:

The following productions from §8.4.1 and §14.4 are shown here for convenience:

A resource specification uses variables to denote resources for the try statement, either by declaring local variables with initializer expressions or by referring to suitable existing variables. An existing variable is referred to by either an expression name (§6.5.6) or a field access expression (§15.11).

It is a compile-time error for a resource specification to declare two variables with the same name.

It is a compile-time error if final appears more than once as a modifier for each variable declared in a resource specification.

A variable declared in a resource specification is implicitly declared final if it is not explicitly declared final (§4.12.4).

A resource denoted by an expression name or field access expression must be a final or effectively final variable that is definitely assigned before the try-with-resources statement (§16 (Definite Assignment)), or a compile-time error occurs.

It is a compile-time error if the LocalVariableType of a variable declared in a resource specification is var and the initializer expression contains a reference to the variable.

The type of a variable declared in a resource specification is determined as follows:

The type of a variable declared or referred to as a resource in a resource specification must be a subtype of AutoCloseable, or a compile-time error occurs.

The scope and shadowing of a variable declared in a resource specification is specified in §6.3 and §6.4.

Resources are initialized in left-to-right order. If a resource fails to initialize (that is, its initializer expression throws an exception), then all resources initialized so far by the try-with-resources statement are closed. If all resources initialize successfully, the try block executes as normal and then all non-null resources of the try-with-resources statement are closed.

Resources are closed in the reverse order from that in which they were initialized. A resource is closed only if it initialized to a non-null value. An exception from the closing of one resource does not prevent the closing of other resources. Such an exception is suppressed if an exception was thrown previously by an initializer, the try block, or the closing of a resource.

A try-with-resources statement whose resource specification indicates multiple resources is treated as if it were multiple try-with-resources statements, each of which has a resource specification that indicates a single resource. When a try-with-resources statement with n resources (n > 1) is translated, the result is a try-with-resources statement with n-1 resources. After n such translations, there are n nested try-catch-finally statements, and the overall translation is complete.

14.20.3.1. Basic try-with-resources

A try-with-resources statement with no catch clauses or finally clause is called a basic try-with-resources statement.

If a basic try-with-resource statement is of the form:

try (VariableAccess ...)
    Block

then the resource is first converted to a local variable declaration by the following translation:

try (T #r = VariableAccess ...) {
    Block
}

T is the type of the variable denoted by VariableAccess and #r is an automatically generated identifier that is distinct from any other identifiers (automatically generated or otherwise) that are in scope at the point where the try-with-resources statement occurs. The try-with-resources statement is then translated according to the rest of this section.

The meaning of a basic try-with-resources statement of the form:

try ({VariableModifier} R Identifier = Expression ...)
    Block

is given by the following translation to a local variable declaration and a try-catch-finally statement:

{
    final {VariableModifierNoFinal} R Identifier = Expression;
    Throwable #primaryExc = null;

    try ResourceSpecification_tail
        Block
    catch (Throwable #t) {
        #primaryExc = #t;
        throw #t;
    } finally {
        if (Identifier != null) {
            if (#primaryExc != null) {
                try {
                    Identifier.close();
                } catch (Throwable #suppressedExc) {
                    #primaryExc.addSuppressed(#suppressedExc);
                }
            } else {
                Identifier.close();
            }
        }
    }
}

{VariableModifierNoFinal} is defined as {VariableModifier} without final, if present.

#t, #primaryExc, and #suppressedExc are automatically generated identifiers that are distinct from any other identifiers (automatically generated or otherwise) that are in scope at the point where the try-with-resources statement occurs.

If the resource specification indicates one resource, then ResourceSpecification_tail is empty (and the try-catch-finally statement is not itself a try-with-resources statement).

If the resource specification indicates n > 1 resources, then ResourceSpecification_tail consists of the 2nd, 3rd, ..., n'th resources indicated in the resource specification, in the same order (and the try-catch-finally statement is itself a try-with-resources statement).

Reachability and definite assignment rules for the basic try-with-resources statement are implicitly specified by the translation above.

In a basic try-with-resources statement that manages a single resource:

In a basic try-with-resources statement that manages multiple resources:

14.20.3.2. Extended try-with-resources

A try-with-resources statement with at least one catch clause and/or a finally clause is called an extended try-with-resources statement.

The meaning of an extended try-with-resources statement:

try ResourceSpecification
    Block
[Catches]
[Finally]

is given by the following translation to a basic try-with-resources statement nested inside a try-catch or try-finally or try-catch-finally statement:

try {
    try ResourceSpecification
        Block
}
[Catches]
[Finally]

The effect of the translation is to put the resource specification "inside" the try statement. This allows a catch clause of an extended try-with-resources statement to catch an exception due to the automatic initialization or closing of any resource.

Furthermore, all resources will have been closed (or attempted to be closed) by the time the finally block is executed, in keeping with the intent of the finally keyword.

14.21. Unreachable Statements

It is a compile-time error if a statement cannot be executed because it is unreachable.

This section is devoted to a precise explanation of the word "reachable." The idea is that there must be some possible execution path from the beginning of the constructor, method, instance initializer, or static initializer that contains the statement to the statement itself. The analysis takes into account the structure of statements. Except for the special treatment of while, do, and for statements whose condition expression has the constant value true, the values of expressions are not taken into account in the flow analysis.

For example, a Java compiler will accept the code:

{
    int n = 5;
    while (n > 7) k = 2;
}

even though the value of n is known at compile time and in principle it can be known at compile time that the assignment to k can never be executed.

The rules in this section define two technical terms:

The definitions here allow a statement to complete normally only if it is reachable.

To shorten the description of the rules, the customary abbreviation "iff" is used to mean "if and only if."

A reachable break statement exits a statement if, within the break target, either there are no try statements whose try blocks contain the break statement, or there are try statements whose try blocks contain the break statement and all finally clauses of those try statements can complete normally.

This definition is based on the logic around "attempts to transfer control" in §14.15.

A continue statement continues a do statement if, within the do statement, either there are no try statements whose try blocks contain the continue statement, or there are try statements whose try blocks contain the continue statement and all finally clauses of those try statements can complete normally.

The rules are as follows:

One might expect the if statement to be handled in the following manner:

This approach would be consistent with the treatment of other control structures. However, in order to allow the if statement to be used conveniently for "conditional compilation" purposes, the actual rules differ.

As an example, the following statement results in a compile-time error:

while (false) { x=3; }

because the statement x=3; is not reachable; but the superficially similar case:

if (false) { x=3; }

does not result in a compile-time error. An optimizing compiler may realize that the statement x=3; will never be executed and may choose to omit the code for that statement from the generated class file, but the statement x=3; is not regarded as "unreachable" in the technical sense specified here.

The rationale for this differing treatment is to allow programmers to define "flag" variables such as:

static final boolean DEBUG = false;

and then write code such as:

if (DEBUG) { x=3; }

The idea is that it should be possible to change the value of DEBUG from false to true or from true to false and then compile the code correctly with no other changes to the program text.

Conditional compilation comes with a caveat. If a set of classes that use a "flag" variable - or more precisely, any static constant variable (§4.12.4) - are compiled and conditional code is omitted, it does not suffice later to distribute just a new version of the class or interface that contains the definition of the flag. The classes that use the flag will not see its new value, so their behavior may be surprising. In essence, a change to the value of a flag is binary compatible with pre-existing binaries (no LinkageError occurs) but not behaviorally compatible.

Another reason for "inlining" values of static constant variables is because of switch statements. They are the only kind of statement that relies on constant expressions, namely that each case label of a switch statement must be a constant expression whose value is different than every other case label. case labels are often references to static constant variables so it may not be immediately obvious that all the labels have different values. If it is proven that there are no duplicate labels at compile time, then inlining the values into the class file ensures there are no duplicate labels at run time either - a very desirable property.

Example 14.21-1. Conditional Compilation

If the example:

class Flags { static final boolean DEBUG = true; }
class Test {
    public static void main(String[] args) {
        if (Flags.DEBUG)
            System.out.println("DEBUG is true");
    }
}

is compiled and executed, it produces the output:

DEBUG is true

Suppose that a new version of class Flags is produced:

class Flags { static final boolean DEBUG = false; }

If Flags is recompiled but not Test, then running the new binary with the existing binary of Test produces the output:

DEBUG is true

because DEBUG is a static constant variable, so its value could have been used in compiling Test without making a reference to the class Flags.

This behavior would also occur if Flags was an interface, as in the modified example:

interface Flags { boolean DEBUG = true; }
class Test {
    public static void main(String[] args) {
        if (Flags.DEBUG)
            System.out.println("DEBUG is true");
    }
}

In fact, because the fields of interfaces are always static and final, we recommend that only constant expressions be assigned to fields of interfaces. We note, but do not recommend, that if a field of primitive type of an interface may change, its value may be expressed idiomatically as in:

interface Flags {
    boolean debug = Boolean.valueOf(true).booleanValue();
}

ensuring that this value is not a constant expression. Similar idioms exist for the other primitive types.


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