Review:    objects in Java

  • In Java, an object consists of 2 components:

    1. An object reference variable   (which is not an object, but references to an object)

    2. The actual object which consists of a number of instance variables

  • Schematically:

  • This storage structure of objects makes it a bit complicated to make a copy of an object

What does it mean to copy an object

  • To copy an object means:

      • Make a duplicate of an object where the duplicate object contains the same data as the original object

      • Furthermore:   updating the instance variables in the duplicate object must not affect the values in the original object

  • Schematically: before making a copy of the object circle1

What does it mean to copy an object

  • To copy an object means:

      • Make a duplicate of an object where the duplicate object contains the same data as the original object

      • Furthermore:   updating the instance variables in the duplicate object must not affect the values in the original object

  • Schematically: after making a copy of the object circle1

How to copy an object in Java    step-by-step instructions

  • Initial state: 

        public static void main(String[] args) 
    {
        Circle circle1 = new Circle(4);  // The original object
     

        
 
 
    }

How to copy an object in Java    step-by-step instructions

  • Step 1: create an object to store the copy 

        public static void main(String[] args) 
    {
        Circle circle1 = new Circle(4);  // The original object
      
        Circle circle2 = new Circle();

  
 
    }

How to copy an object in Java    step-by-step instructions

  • Step 2: copy the instance variables from the original to the copy object 

        public static void main(String[] args) 
    {
        Circle circle1 = new Circle(4);  // The original object
      
        Circle circle2 = new Circle();

        circle2.radius = circle1.radius; 

    }

 

How to copy an object in Java    step-by-step instructions

  • Result: updates on the object copy will not affect the original object: 

        public static void main(String[] args) 
    {
        Circle circle1 = new Circle(4);  // The original object
      
        Circle circle2 = new Circle();

        circle2.radius = circle1.radius; 
	circle2.radius++; // Will not affect circle1.radius
    }

Demo: demo/10-classes/10-copy/Demo.java + Circle.java

A common error in copying objects

  • Variables of primitive data types (such as int, double, etc) can be copied with an assignment statement: 

        int x = 4;   // int is a primitive data type
    int xCopy;

    xCopy = x;   // Copies x to xCopy

  • However, in Java, the assignment statement will not copy objects of non-primitive data types: 

        Circle circle1 = new Circle(4);  // Non-primitive data type

    Circle circle2;

    circle2 = circle1;   // Does not copy an object 
                           // ** This copies the reference
			   // ** in circle1 to circle2

Review:   the effect of assignment with a reference variable

  • Initially we have the object circle1:
      

        Circle circle1 = new Circle(4);

    Schematically:

     

Review:   the effect of assignment with a reference variable

  • The definition Circle circle2 defines another reference variable:                                         

        Circle circle1 = new Circle(4);

    Circle circle2;

    Schematically:

     

Review:   the effect of assignment with a reference variable

  • The assignment circle2 = circle1 will make circle2 reference to the object that circle1 is referencing: 

        Circle circle1 = new Circle(4);

    Circle circle2;

    circle2 = circle1;   // Does not copy an object object

    Schematically:

    I.e.: circle1 and circle2 are aliases

Review:   the effect of assignment with a reference variable

  • Because circle2 and circle1 reference to the same (Circle) object, updates made using circle2.radius will also affect circle1.radius and vice verse: 

        public static void main(String[] args) 
    {
        Circle circle1 = new Circle(4);

        Circle circle2;

        circle2 = circle1;   // Does not copy an object object

	circle2.radius = 999; // Update will also affect myList[1]

	System.out.println( circle1.radius );  // Changed !
	System.out.println( circle2.radius );
    }

    The fact that circle1.radius is change proves that circle1 and circle2 are aliases

Demo: demo/10-classes/10-copy/Demo2.java + Circle.java