Study and learn Interview MCQ Questions and Answers on object in java . Attend job interviews easily with these Multiple Choice Questions.

object in java

Objects are always passed by Reference. Reference is nothing but the starting address of a memory location where the actual object lies. You can create any number of references to just One Object. Let us go by an example.

We create a Class “Cloud” with a single Property called “color”. It is a String object. We create 3 objects of the Cloud type.

public Class Cloud
{
  String color;
  public static void main(String[] args)
  {
    Cloud obj1 = new Cloud();
    obj1.color = "RED";

    Cloud obj2, obj3;
    obj2 = obj1; //Assign obj1 to obj2
    obj3 = obj2; //Assign obj2 to obj3
    System.out.println("OBJ2= " + obj2.color + ", OBJ3= " + obj3.color);
    obj2=null; obj3=null;
    System.out.print("Color= " + obj1.color);
  }

}
//OUTPUT
//OBJ2= RED, OBJ3= RED
//Color= RED

Notice that all three references obj1, obj2 and obj3 are pointing to the same Cloud object. Thus the above example outputs the same color-property value “RED“. Later, we made the references obj2 and obj3 to point to the null location. Still the original object pointed or referenced by obj1 holds the value. So the final PRINT statement outputs “RED” as usual.

Note: Reference is also called a Variable whether Primitive or Object.

This concludes that a Java object assignment happens by Reference. But the actual value of memory location pointed by References are copied by Value only. So we are actually copying memory locations from one Reference variable to another Reference variable by Value. Interviewers ask this famous Java question just to test your knowledge. If someone asks in C language point of view, say that it is by Reference. If they ask in Java point of view, say that it is by Value. 

object in java Passing by Value or Reference to a Method Explained

You already learnt that Object assignment in Java happens by Reference. Let us try to pass Objects to another method as a parameter or argument.

class Tractor
{
  int height;

  void change(Tractor obj)
  {
    obj.height = 200;
  }
  public static void main(String[] args)
  {
    Tractor obj1 = new Tractor();
    obj1.height = 100;
    System.out.println("Before Height= " + obj1.height);
    obj1.change(obj1);
    System.out.println("After Height= " + obj1.height);
  }
}
//OUTPUT
//Before Height= 100
//After Height= 200;

In the above example, we have created an object “obj1” of type Tractor. We assigned value of 100 to the property “height”. We printed the value before passing the object to another method. Now, we passed the object to another method called “change” from the calling-method “main“. We modified the height property to 200 in the called-method. Now, we tried to print the value of the property in the Calling method.

Similarly, we can pass Java objects to the Constructor using Pass by Reference or simply Reference. 

Here also, we pass the memory location pointed by one Reference variable to another Reference variable of a Called method as a Value only. So, you are actually passing Values (memory locations represented by integer or long data type). Interviewers take advantage of this tricky logic to confuse you. If someone asks in C language point of view, say that it is by Reference. If they ask in Java point of view, say that it is by Value.

object in java

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