In any nontrivial software project, bugs are simply a fact of life. Careful planning, programming, and testing can help reduce their pervasiveness, but somehow, somewhere, they’ll always find a way to creep into your code. This becomes especially apparent as new features are introduced and your code base grows in size and complexity.
\nFortunately, some bugs are easier to detect than others. Compile-time bugs, for example, tell you immediately that something is wrong; you can use the compiler’s error messages to figure out what the problem is and fix it, right then and there. Runtime bugs, however, can be much more problematic; they don’t always surface immediately, and when they do, it may be at a point in time that’s far removed from the actual cause of the problem. <\/P>
\nGenerics add stability to your code by making more of your bugs detectable at compile time. Some programmers choose to learn generics by studying the Java Collections Framework; after all, generics are<\/I> heavily used by those classes. However, since we haven’t yet covered collections, this chapter will focus primarily on simple “collections-like” examples that we’ll design from scratch. This hands-on approach will teach you the necessary syntax and terminology while demonstrating the various kinds of problems that generics were designed to solve. <\/P>
\nA Simple Box Class<\/H3>Let’s begin by designing a nongeneric
Box<\/CODE> class that operates on objects of any type. It need only provide two methods:add<\/CODE>, which adds an object to the box, andget<\/CODE>, which retrieves it:public class Box {<\/p>\nprivate Object object;<\/p>\n
public void add(Object object) {
\n this.object = object;
\n }<\/p>\npublic Object get() {
\n return object;
\n }
\n }
\n<\/PRE>Since its methods accept or returnObject<\/CODE>, you’re free to pass in whatever you want, provided that it’s not one of the primitive types. However, should you need to restrict the contained type to something specific (likeInteger<\/CODE>), your only option would be to specify the requirement in documentation (or in this case, a comment), which of course the compiler knows nothing about:
\npublic class BoxDemo1 {<\/p>\npublic static void main(String[] args) {<\/p>\n
\/\/ ONLY place Integer objects into this box!
\n Box integerBox = new Box();<\/p>\nintegerBox.add(new Integer(10));
\n Integer someInteger = (Integer)integerBox.get();
\n System.out.println(someInteger);
\n }
\n}
\n<\/PRE><\/BLOCKQUOTE>TheBoxDemo1<\/CODE><\/A> program creates anInteger<\/CODE> object, passes it toadd<\/CODE>, then assigns that same object tosomeInteger<\/CODE> by the return value ofget<\/CODE>. It then prints the object’s value (10) to standard output. We know that the cast fromObject<\/CODE> toInteger<\/CODE> is correct because we’ve honored the “contract” specified in the comment. But remember, the compiler knows nothing about this \u2014 it just trusts that our cast is correct. Furthermore, it will do nothing to prevent a careless programmer from passing in an object of the wrong type, such asString<\/CODE>:
\npublic class BoxDemo2 {<\/p>\npublic static void main(String[] args) {<\/p>\n
\/\/ ONLY place Integer objects into this box!
\n Box integerBox = new Box();<\/p>\n\/\/ Imagine this is one part of a large application
\n \/\/ modified by one programmer.
\n integerBox.add(“10”); \/\/ note how the type is now String<\/p>\n\/\/ … and this is another, perhaps written
\n \/\/ by a different programmer
\n Integer someInteger = (Integer)integerBox.get();
\n System.out.println(someInteger);
\n }
\n}
\n<\/PRE><\/BLOCKQUOTE>InBoxDemo2<\/CODE><\/A> we’ve stored the number 10 as aString<\/CODE>, which could be the case when, say, a GUI collects input from the user. However, the existing cast fromObject<\/CODE> toInteger<\/CODE> has mistakenly been overlooked. This is clearly a bug, but because the code still compiles, you wouldn’t know anything is wrong until runtime, when the application crashes with aClassCastException<\/CODE>:Exception in thread “main”
\n java.lang.ClassCastException:
\n java.lang.String cannot be cast to java.lang.Integer
\n at BoxDemo2.main(BoxDemo2.java:6)
\n<\/PRE>
\nIf the
Box<\/CODE> class had been designed with generics in mind, this mistake would have been caught by the compiler, instead of crashing the application at runtime. <\/P><\/BLOCKQUOTE>
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