Assigning to a Parameter

A parameter is a "one-way message" that the caller uses to send values to a method.

Within the body of a method, a parameter is used just like any variable. It can be used in arithmetic expressions, in assignment statements, and so on.

However, changes made to the parameter do not have any effect outside the method body. A parameter is a local copy of whatever value the caller passed into the method. Any changes made to it affect only this local copy. For example:

class CheckingAccount
{
  . . . .
  private int balance;

  public void processCheck( int  amount  )
  {                          
    int charge;
    if ( balance < 100000 )
      charge = 15; 
    else
      charge = 0;
    balance =  balance -  amount  - charge  ;

    // change the local copy of the value in "amount"
    amount = 0 ; 
  }
}

class CheckingTester
{

  public static void main ( String[] args )
  {
    CheckingAccount act;
    int check = 5000;
    act = new CheckingAccount( "123-345-99", 
        "Wanda Fish",  100000 );
           
    System.out.println( "check:" + check );     // prints "5000"

    // call processCheck with a copy of the value 5000
    act.processCheck( check );             

    System.out.println( "check:" + check );     // prints "5000" --- "check" was not changed
  }
}

The formal parameter amount is the name used for the value 5000 that processCheck() has been sent. That method changes the value held in amount, but this has no effect on any other variable.