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new operator in Java - GeeksforGeeks

Last Updated : 30 May, 2018

When you are declaring a class in java, you are just creating a new data type. A class provides the blueprint for objects. You can create an object from a class. However obtaining objects of a class is a two-step process :

1. Declaration : First, you must declare a variable of the class type. This variable does not define an object. Instead, it is simply a variable that can refer to an object. Below is general syntax of declaration with an example :

   Syntax :
   class-name var-name;
   
   Example :
   // declare reference to an object of class Box
   Box mybox;
   

   A variable in this state, which currently references no object, can be illustrated as follows (the variable name, mybox, plus a reference pointing to nothing):
2. Instantiation and Initialization : Second, you must acquire an actual, physical copy of the object and assign it to that variable. You can do this using the new operator. The new operator instantiates a class by dynamically allocating(i.e, allocation at run time) memory for a new object and returning a reference to that memory. This reference is then stored in the variable. Thus, in Java, all class objects must be dynamically allocated. The new operator is also followed by a call to a class constructor, which initializes the new object. A constructor defines what occurs when an object of a class is created. Constructors are an important part of all classes and have many significant attributes. In below example we will use the default constructor. Below is general syntax of instantiation and initialization with an example :

   Syntax :
   var-name = new class-name();
   
   Example :
   // instantiation via new operator and 
   // initialization via default constructor of class Box
   mybox = new Box();
   

   Before understanding, how new dynamically allocates memory, let us see class Box prototype.

   class Box
   {
       double width;
       double height;
       double depth;
   }
   

   A variable after second step, currently refer to a class object, can be illustrated as follows (the variable name, mybox, plus a reference pointing to Box object):

Hence declaration of a class variable, instantiation of a class and initialization of an object of class can be together illustrated as follows :

1. The above two statements can be rewritten as one statement.

   Box mybox = new Box();
   

2. The reference returned by the new operator does not have to be assigned to a class variable. It can also be used directly in an expression. For example:

   double height = new Box().height;
   

3. Since arrays are object in java, hence while instantiating arrays, we use new operator. For example:

   int arr[] = new int[5];
   

4. At this point, you might be wondering why you do not need to use new operator for primitives data types. The answer is that Java’s primitive types are not implemented as objects. Rather, they are implemented as "normal" variables. This is done in the interest of efficiency. For object versions of the primitive data types, refer Wrapper Classes.
5. The phrase "instantiating a class" means the same thing as "creating an object." When you create an object, you are creating an "instance" of a class, therefore "instantiating" a class.

Assigning object reference variables

When you assign one object reference variable to another object reference variable, you are not creating a copy of the object, you are only making a copy of the reference. Let us understand this with an example.

// Java program to demonstrate assigning 
// of object reference variables

// Box class
class Box
{
    double width;
    double height;
    double depth;
}

// Driver class
public class Test
{
    // Driver method
    public static void main(String[] args) 
    {
        // creating box object
        Box b1 = new Box();
        
        // assigning b2 to b1
        Box b2 = b1;
        
        // height via b1 and b2
        System.out.println(b1.height);
        System.out.println(b2.height);
        
        // changing height via b2
        b2.height = 20;
        
        // height via b1 and b2
        // after modification through b2
        System.out.println(b1.height);
        System.out.println(b2.height);
    }
    
}

Output :

0.0
0.0
20.0
20.0

First let us understand what the following fragment does in above program.

Box b1 = new Box();
Box b2 = b1;

You might think that

is being assigned a reference to a copy of the object referred to by

. That is, you might think that

and

refer to separate and distinct objects. However, this would be wrong. Instead, after this fragment executes,

and

will both refer to the same object. The assignment of

to

did not allocate any memory or copy any part of the original object. It simply makes

refer to the same object as does

. Thus, any changes made to the object through

will affect the object to which

is referring, since they are the same object. Same can be verified by output when we change

of box via

. This situation can be illustrated as follows :

Although

and

both refer to the same object, they are not linked in any other way. For example, a subsequent assignment to

will simply

from the original object without affecting the object or affecting

.For example :

Box b1 = new Box();
Box b2 = b1;
// ...
b1 = null;

Here,

has been set to

, but

still points to the original object.

Passing object references variables to methods

When we pass object reference to a method, the parameter that receives it will refer to the same object as that referred to by the argument. To know more with examples, refer

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