Java Methods

Java Methods

Method Invocation

When an object calls a method (either one of its own or another object's), the calling object is actually requesting that the receiving object perform some action; set a value, return a value, write to a file, or send an email, for example. This is also know as message passing; one object passes a message to a second object. The message contains the name of the method to execute on the receiving object and any required data that the receiving method requires. Data that are passed to a method are known as arguments; the required arguments for a method are defined by a method's parameter list.

Let's look at some examples of method invocations:

// Call method executeQuery() on myObject.
myObject.executeQuery();

// Call method set() on date, passing it the
// arguments Calendar.MONTH and 11.
date.set(Calendar.MONTH, 11);

// Call static method parseInt on Integer, passing
// it argument "32" and assigning the return
// value to variable age.
int age = Integer.parseInt("32");

Method Signature

The combined name and parameter list for each method in a class must be unique. The uniqueness of a parameter list takes the order of the parameters into account. So int myMethod (int x, String y) is unique from int myMethod (String y, int x ).

Let's take a look at the general syntax for a method declaration:

[modifiers] return_type method_name (parameter_list) [throws_clause] {

   [statement_list]
}

Everything within square brackets [] is optional. Of course, you don't include the square brackets in your code; they are included here only to indicate optional items. You can see that the minimal method declaration includes:

• Return Type: The return type is either a valid Java type (primitive or class) or void if no value is returned. If the method declares a return type, every exit path out of the method must have a return statement.
• Method Name: The method name must be a valid Java identifier. See Java Variables for the rules for Java identifiers.
• Parameter List: The parentheses following the method name contain zero or more type/identifier pairs that make up the parameter list. Each parameter is separated by a comma. Also, there can be zero parameters.
• Curly Braces: The method body is contained in a set of curly braces. Normally, the method body contains a sequence of semicolon delimited Java statements that are executed sequentially. Technically, though, the method body can be empty.

Note: If a method is declared with the abstract modifier or is defined in an interface instead of a class, there will be no method body; instead a semicolon will follow the parameter list's parentheses.

Other Modifiers

There are a number of optional modifiers that you can use when declaring a method. They are listed in the following table: