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Thinking in Java
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The argument list

The method argument list specifies what information you pass into the method. As you might guess, this information—like everything else in Java—takes the form of objects. So, what you must specify in the argument list are the types of the objects to pass in and the name to use for each one. As in any situation in Java where you seem to be handing objects around, you are actually passing references.[12] The type of the reference must be correct, however. If the argument is supposed to be a String, you must pass in a String or the compiler will give an error.

Consider a method that takes a String as its argument. Here is the definition, which must be placed within a class definition for it to be compiled:

int storage(String s) {
  return s.length() * 2;

This method tells you how many bytes are required to hold the information in a particular String. (Each char in a String is 16 bits, or two bytes, long, to support Unicode characters.) The argument is of type String and is called s. Once s is passed into the method, you can treat it just like any other object. (You can send messages to it.) Here, the length( ) method is called, which is one of the methods for Strings; it returns the number of characters in a string.

You can also see the use of the return keyword, which does two things. First, it means “leave the method, I’m done.” Second, if the method produces a value, that value is placed right after the return statement. In this case, the return value is produced by evaluating the expression s.length( ) * 2.

You can return any type you want, but if you don’t want to return anything at all, you do so by indicating that the method returns void. Here are some examples:

boolean flag() { return true; }
float naturalLogBase() { return 2.718f; }
void nothing() { return; }
void nothing2() {}

When the return type is void, then the return keyword is used only to exit the method, and is therefore unnecessary when you reach the end of the method. You can return from a method at any point, but if you’ve given a non-void return type, then the compiler will force you (with error messages) to return the appropriate type of value regardless of where you return.

At this point, it can look like a program is just a bunch of objects with methods that take other objects as arguments and send messages to those other objects. That is indeed much of what goes on, but in the following chapter you’ll learn how to do the detailed low-level work by making decisions within a method. For this chapter, sending messages will suffice.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire