Notes on Declaring and Using Methods

There are three ways to call a method:

1. Using a method name by itself to call another method of the same classsuch as maximum( number1, number2, number3 ) in line 21 of Fig. 6.3.
2. Using a variable that contains a reference to an object, followed by a dot (.) and the method name to call a method of the referenced objectsuch as the method call in line 10 of Fig. 6.4, maximumFinder.determineMaximum(), which calls a method of class MaximumFinder from the main method of MaximumFinderTest.
3. Using the class name and a dot (.) to call a static method of a classsuch as Math.sqrt( 900.0 ) in Section 6.3.

Note that a static method can call only other static methods of the same class directly (i.e., using the method name by itself) and can manipulate only static fields in the same class directly. To access the class's non-static members, a static method must use a reference to an object of the class. Recall that static methods relate to a class as a whole, whereas non-static methods are associated with a specific instance (object) of the class and may manipulate the instance variables of that object. Many objects of a class, each with its own copies of the instance variables, may exist at the same time. Suppose a static method were to invoke a non-static method directly. How would the method know which object's instance variables to manipulate? What would happen if no objects of the class existed at the time the non-static method was invoked? Clearly, such a situation would be problematic. Thus, Java does not allow a static method to access non-static members of the same class directly.

There are three ways to return control to the statement that calls a method. If the method does not return a result, control returns when the program flow reaches the method-ending right brace or when the statement

 return;

is executed. If the method returns a result, the statement

      return expression;

evaluates the expression, then returns the result to the caller.

Common Programming Error 6.4

Declaring a method outside the body of a class declaration or inside the body of another method is a syntax error.

Common Programming Error 6.5

Omitting the return-value-type in a method declaration is a syntax error.

Common Programming Error 6.6

Placing a semicolon after the right parenthesis enclosing the parameter list of a method declaration is a syntax error.

Common Programming Error 6.7

Redeclaring a method parameter as a local variable in the method's body is a compilation error.

Common Programming Error 6.8

Forgetting to return a value from a method that should return a value is a compilation error. If a return value type other than void is specified, the method must contain a return statement that returns a value consistent with the method's return-value-type. Returning a value from a method whose return type has been declared void is a compilation error.