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These learning object concern methods in Java. A method enables a sequence of statements to be collected together and then invoked by using the method's name.

Learning objects for methods

Concept Methods are the simplest construct for abstraction in Java. A method starts with a declaration that defines its signature : the name of the method, the number and types of the formal parameters and the return type . The body of the method consists of local variable declarations and of statements.A method is called or invoked by writing the name of the method followed by a list of values, called actual parameters , one for each formal parameter. A method can return a value or it can be declaredas void if no value is returned.

These source code of these learning objects can be found in method.zip .

LO Topic Java Files (.java) Prerequisites
"A void method" A void method Method01
"A method returning a value" A method returning a value Method02
"Calling one method from another" Calling one method from another Method03 1, 2
"Recursion" Recursion Method04 2
"Calling methods on an object" Calling methods on an object Method05 2, *
"Calling a method on the same object" Calling a method on the same object Method06 5, *
"Objects as parameters" Objects as parameters Method07 5, *
"Returning objects" Returning objects Method08 7, *
"Returning locally instantiated objects" Returning locally instantiated objects Method09 8, *

* This LO assumes knowledge of the declaration of classes and the instantiation of objects.

A void method

Concept When a method that is declared void is called, it allocates memory for its parameters and local variables, executesits statements and then returns. The call is a statement constructed from the name of the method followed by a list of actual parameters.

Program: Method01.java

// Learning Object Method01 //    void methodspublic class Method01 {     static void printMax(int a, int b) {        int max;         if (a > b)             max = a;        else             max = b;        System.out.println(max);     }      public static void main(/*String[] args*/) {         int x = 10, y = 20;        printMax(x, y);         Method01.printMax(10, y);    } }

The program computes the maximum of two integer values.

  • The variables x and y are allocated and initialized.
  • The method is called with the values of the actual parameters x and y .
  • Memory is allocated for the formal parameters of the method and the local variables. This is called an activation record and is displayed by Jeliot in the upper left hand part of the screen labeled Method Area . The new activation record hides the previous ones which are no longer accessible.
  • The actual parameters are used to initialize the formal parameters in the activation record.
  • The local variable max is allocated within the activation record.
  • The statements of the method are executed.
  • After the last statement has been executed, the method returns and the activation record is deallocated.
  • Execution continues with the statement after the method call. Here, the method is called again, this time with an integer literalas an actual parameter instead of a variable.

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Source:  OpenStax, Learning objects for java (with jeliot). OpenStax CNX. Dec 28, 2009 Download for free at http://cnx.org/content/col10915/1.2
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