What happens to the argument if we update the parameter variable ?

Consider a method increment( ) that updates the parameter variable: 

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

	increment( x );  // Pass x

	System.out.println(x);  // What value will be printed ?
    } 
    
    public static void increment(int a ) 
    {
	a = a + 1;  // Changes the parameter variable
    }

DEMO: demo/06-methods/03-pass-by-value/Demo.java     --     Run in BlueJ

Background info: making a copy of a variable

Program that shows the effect of copying a variable: 

public class Copy
{
    public static void main(String[] args) 
    {
        int a, x;

        x = 4;      // CHANGES the value in x to 4

        a = x;      // CHANGES the value in a in x (i.e.: copy)

        a = a + 1;  // CHANGES the value in a to a+1
    }   
}

DEMO: demo/06-methods/03-pass-by-value/Copy.java     --     Step in BlueJ

Background info: making a copy of a variable

The statement a = x makes a copy of the variable x in the variable a 

public class Copy
{
    public static void main(String[] args) 
    {
        int a, x;

        x = 4;      // CHANGES the value in x to 4

        a = x;      // Makes a copy of x in a

        a = a + 1;  // CHANGES the value in a to a+1
    }   
}

Important:   a copy and its original are independent
                      when the copy in variable a is changed by a = a + 1, the value in variable x is not changed

DEMO: demo/06-methods/03-pass-by-value/Copy.java     --     Step in BlueJ

How Java passes arguments to methods

Java passes the arguments to method invocations by copying each argument to the parameter variable: 

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

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

	System.out.println(x);  // x UNCHANGED !  Explained in more details next
    } 
    
    public static void increment(int a )  //  a  and  x  are different variables !!
    {
	a = a + 1;
    }

DEMO: demo/06-methods/03-pass-by-value/PassByCopy.java     --     Step in BlueJ

Passing arguments by value

  • In Java (note: 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.

    Schematically:

    The value of the argument x (in main's activation record) is copied to the parameter variable a (in increment's activation record)

Arguments passed by value cannot be updated by the invoked method

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

    Note:   x and a are different variables

Arguments passed by value cannot be updated by the invoked method

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

    It did not change the argument x in main( ) and therefore is not affected !!!

Important note:   variable names in different methods are completely separate and unrelated

Each method can name their own variables independently from other methods: 

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

	increment( x );  // Pass main's x by copying

	System.out.println(x);
    } 
    
    public static void increment(int x )
    {
	x = x + 1;  // Uses increment's x in operation
    }

Variables defined in different methods are independent (different) variables

DEMO: demo/06-methods/03-pass-by-value/Copy.java

Demo with BlueJ: show the different value in variable x in the activation records when stopped after x++