Java has 51 reserved words that have very specific meanings and cannot be used as identifiers in the application code. Also, 16 contextual keywords are treated as keywords when found in a specific context. Programmers should not use these keywords for anything other than what they are meant to be.
1. What is a Keyword in Java?The keywords are predefined, reserved words that have a very specific meaning for the compiler. These keywords cannot be used as variables, methods, classes, or any other identifiers.
In the following statement, int is a keyword that indicates that the variable age is of integer type (32-bit signed two’s complement integer). We can’t use int as a variable name etc. Using keywords as identifiers will result in compilation errors.
int age;const and goto are reserved, even though they are not currently used.true, false, and null might seem like keywords, but they are literal; we cannot use them as identifiers in our programs.strictfp was added in JDK 1.2.assert was added in JDK 1.4.enum was added in JDK 1.5.The following 51 keywords cannot be used as identifiers.
Keyword Description abstract Used with classes and methods. An abstract class cannot be instantiated. An abstract method is incomplete without the body and must be implemented in the child class to create an instance of the child class. assert enables us to test the assumptions about our program. boolean represents only one of two possible values i.e. eithertrue or false. break is used to terminate for, while, or do-while loop. It may also be used to terminate a switch statement as well. byte can store whole numbers from -128 and 127. case represents a block of code in switch statements. catch follows the try block and handles the checked exceptions thrown by try block and any possible unchecked exceptions. char used to store a single character. class defines a class. const is a reserved keyword for constant values. Use final instead. continue skips the current iteration of a for, while, or do-while loops and jumps to the next iteration. default used to specify the default block in a switch statement and default methods in functional interfaces. do used to contain the statements to execute repeatedly until the condition in the while statement is true. double used to declare a variable that can hold 64-bit floating-point number. else used to indicate the alternative branches in an if statement. enum is a type whose fields consist of a fixed set of constants. extends used for extending a class. final used with class variables, methods or classes. A final variable cannot be assigned another value after it has been initialized. A final method cannot be overridden in the child class. No class can subclass a final class. finally contains code to be executed everytime a try-catch block is completed – either with errors or without any error. float used to declare a variable that can hold a 32-bit floating-point number. for start a loop to execute a set of instructions repeatedly when a condition is true. If the number of iterations is known, it is recommended to use for loop. goto Currently, not in use. if used for writing conditional statements. implements used for implementing an interface. import import a package, class or interface to the current class. instanceof Checks whether an object is an instance of a specific class or an interface. int used to store a 32-bit integer value. interface declares an interface. long used to store a 64-bit integer value. native indicates native code (platform-specific). new creates a new object of the specified class. package declares a package for storing the related classes. private access modifier to indicate that a method or variable may be accessed only in the class in which it is declared. protected access modifier to indicate that a class, method or variable may be accessed only in the current package, or inherited outside the current package. public access modifier to indicate that a class, method or variable is accessible everywhere. return used to return from a method when its execution is complete. short used to store a 16-bit integer value. static indicates that a variable or method belongs to the class object, not to the individual instances of that class. strictfp used to restrict the floating-point calculations to ensure portability. super used to refer to parent class objects. switch help in providing multiple possible execution paths for a program. synchronized marks a block or method a critical section where one and only one thread is executing at a time. this used to refer to the current object. throw used to explicitly throw an exception from a method or constructor. throws used to declare the list of exceptions that may be thrown by that method or constructor. transient used on class attributes/variables to indicate that the serialization process of this class should ignore such variables. try contains the application code which is expected to work in normal conditions. void specifies that a method should not have a return value. volatile indicates that an attribute is not cached thread-locally, and is always read from the “main memory”. while continually executes a block of statements until a particular condition evaluates to true _ (Underscore) added in Java 9, to prevent writing underscores as an unused lambda, method, or catch formal parameter. 3. Contextual Keywords
The following 16 words can be interpreted as keywords or as other tokens, depending on the context in which they appear.
Keyword Description exports used for importing and exporting the modules. module used for declaring modules. non-sealed used to define sealed classes and interfaces. open used for declaring modules. opens used for importing and exporting the modules. permits used to define sealed classes and interfaces. provides used for importing and exporting the modules. record used to define new requires used for importing and exporting the modules. sealed used to define sealed classes and interfaces. to used for importing and exporting the modules. transitive recognized as a terminal in a RequiresModifier. uses used for importing and exporting the modules. var used to infer local variable types. with used for importing and exporting the modules. yield used to yield a value in a switch statement.module and open are used for declaring modules.exports, opens, provides, requires, to, uses, and with are used for importing and exporting the modules.transitive is recognized as a terminal in a RequiresModifier.var is used to infer local variable types.yield is used to yield a value in a switch statement.record is used to define new record types.non-sealed, permits, and sealed are used to define sealed classes and interfaces.Learning about all keywords on a single page is not possible. We will learn about each Java keyword in its dedicated tutorial.
Happy Learning !!
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