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One of the most compelling features about Java is code reuse. But to be revolutionary, you’ve got to be able to do a lot more than copy code and change it.
That’s the approach used in procedural languages like C, and it hasn’t worked very well. Like everything in Java, the solution revolves around the class. You reuse code by creating new classes, but instead of creating them from scratch, you use existing classes that someone has already built and debugged.
The trick is to use the classes without soiling the existing code. In this chapter you’ll see two ways to accomplish this. The first is quite straightforward: You simply create objects of your existing class inside the new class. This is called composition because the new class is composed of objects of existing classes. You’re simply reusing the functionality of the code, not its form.
The second approach is more subtle. It creates a new class as a type of an existing class. You literally take the form of the existing class and add code to it without modifying the existing class. This magical act is called inheritance, and the compiler does most of the work. Inheritance is one of the cornerstones of object-oriented programming and has additional implications that will be explored in the next chapter.
It turns out that much of the syntax and behavior are similar for both composition and inheritance (which makes sense because they are both ways of making new types from existing types). In this chapter, you’ll learn about these code reuse mechanisms.