Using Objects - Java Concepts - page 52

In the next section, you will see in greater detail how to supply method inputs and
obtain method outputs.
Let us look at another method of the String class. When you apply the
toUpperCase method to a String object, the method creates another String
object that contains the characters of the original string, with lowercase letters
converted to uppercase. For example, the sequence of statements
String river = ÐMississippiÑ;
String bigRiver = river.toUpperCase();
sets bigRiver to the String object ÐMISSISSIPPIÑ .
When you apply a method to an object, you must make sure that the method is defined
in the appropriate class. For example, it is an error to call
System.out.length(); // This method call is an error
The PrintStream class (to which System.out belongs) has no length method.
Let us summarize. In Java, every object belongs to a class. The class defines the
methods for the objects. For example, the String class defines the length and
toUpperCase methods (as well as other methodsȌyou will learn about most of
them in Chapter 4 ). The methods form the public interface of the class, telling you
what you can do with the objects of the class. A class also defines a private
implementation, describing the data inside its objects and the instructions for its
methods. Those details are hidden from the programmers who use objects and call
methods.
The public interface of a class specifies what you can do with its objects. The
hidden implementation describes how these actions are carried out.
Figure 4 shows two objects of the String class. Each object stores its own data
(drawn as boxes that contain characters). Both objects support the same set of
methodsȌthe interface that is specified by the String class.
S ELF C HECK
6. How can you compute the length of the string ÐMississippiÑ ?