display: outer_x = 100
display: outer_x = 100
While nested classes are not applicable to all stiuations, they are particularly helpful when
handling events. We will return to the topic of nested classes in Chapter 22. There you will
see how inner classes can be used to simplify the code needed to handle certain types of
events. You will also learn about anonymous inner classes, which are inner classes that don't
have a name.
One final point: Nested classes were not allowed by the original 1.0 specification for Java.
They were added by Java 1.1.
Exploring the String Class
Although the String class will be examined in depth in Part II of this topic, a short exploration
of it is warranted now, because we will be using strings in some of the example programs
shown toward the end of Part I. String is probably the most commonly used class in Java's
class library. The obvious reason for this is that strings are a very important part of
The first thing to understand about strings is that every string you create is actually an
object of type String. Even string constants are actually String objects. For example, in the
System.out.println("This is a String, too");
the string "This is a String, too" is a String constant.
The second thing to understand about strings is that objects of type String are immutable;
once a String object is created, its contents cannot be altered. While this may seem like a
serious restriction, it is not, for two reasons:
If you need to change a string, you can always create a new one that contains
the modifications.
Java defines a peer class of String, called StringBuffer, which allows strings
to be altered, so all of the normal string manipulations are still available in Java.
(StringBuffer is described in Part II of this topic.)
Strings can be constructed in a variety of ways. The easiest is to use a statement like this:
String myString = "this is a test";
Once you have created a String object, you can use it anywhere that a string is allowed.
For example, this statement displays myString:
Java defines one operator for String objects: +. It is used to concatenate two strings.
For example, this statement
String myString = "I" + " like " + "Java.";
results in myString containing "I like Java."
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