Passing objects as parameters to methods

Here is an example of a method with an object reference variable as parameter: 

    public static void main(String[] args)
    {
        Circle circle1 = new Circle(1);
        Circle circle2 = new Circle(4);
        
        printCircle(circle1);  // Pass object as argument
        printCircle(circle2);  // Pass object as argument
    }

    // Method with an object as parameter
    public static void printCircle( Circle c )
    {
        System.out.println("The area of the circle of radius "
                           + c.radius + " is " + c.getArea());
    }

In Java, the object reference in the actual parameter is copied to the object reference argument

Reminder:   assigning (= copying) reference typed variables

  • Assigning to variables of reference type will make a alias:

  • Update using circle1.radius will also change circle2.radius: 

        public static void main(String[] args)
    {
        Circle circle1 = new Circle(5);  
        Circle circle2 = circle1;
        
        circle2.radius = 99;
        System.out.println( circle1.radius ); // Prints: 99
    }

What happens when a method updates its object passed as a parameter

Since the formal parameter is an alias for the actual parameter, the original object will be updated: 

    public static void main(String[] args)
    {
        Circle circle1 = new Circle(1);

	System.out.println( circle1.getArea() );  // 3.14159

	incrementRadius( circle1 );  // increment radius by 1

	System.out.println( circle1.getArea() );  // 4 × 3.14159
    }

    // Method updates object passed as parameter
    public static void incrementRadius( Circle c )
    {
        c.radius++;   // Increment radius by 1
    }

In Java, the formal parameter c is an alias of the actual parameter circle1

So c.radius++ will also updates circle1.radius

Review:    passing primitive variables to methods

  • In Java (this may be different in other programming languages):

      • When you invoke a method with an argument, the value of the argument is copied (= assigned) to the corresponding parameter.

Review:    passing primitive variables to methods

  • The value of x is copied into the parameter variable a and program control is transfered to the method increment( ):

    Note: x and a are different variables

Review:    passing primitive variables to methods

  • The execution of increment( ) method in increment the parameter variable a inside the increment( ) method:

    Note: the x in main( ) is not affected !!!

Review:    passing primitive variables to methods

Demo program that shows that arguments as passed to method by copying the value: 

    public static void main(String[] args) 
    {
        int x;
        
        x = 4;
	System.out.println(x);

	increment( x );  // Pass x by copying: a = x

	System.out.println(x);
    } 
    
    public static void increment(int a )
    {
	a = a + 1;
    }

Demo with BlueJ: show the activation record of main( ) and increment( ) when stopped in increment( )