Recurring Themes

In Chapter 9, we learned how while loops can be used to command a computer to execute millions of instructions without having to actually type in millions of instructions. Now that we know how to make a computer perform millions of operations, we would like to be able to write programs that process millions of pieces of information. We have seen how to manipulate small quantities of information in a program. Names have been essential to this process. In order to work with information, we must first associate a name with the value or object to be manipulated so that we can write instructions telling the computer to apply methods or operators to the data. It is, however, hard to imagine writing a program containing millions of instance variable declarations! There must be another way. We encounter programs that process large collections of information every day. The book you are reading contains roughly a million words. The program used to format this book has the ability to manage all of these words, decide how many can fit on each page, etc. When you go to Google and search for web pages about a particular topic, the software at Google somehow has to examine data describing millions of web pages to find the ones of interest to you. When you load a picture from your new 8 megapixel digital camera into your computer, the computer has to store the 8 million data values that describe the colors of the 8 million dots of color that make up the image and arrange to display the right colors on your screen. In this and the following chapter, we will explore two very di↵erent ways of manipulating large collections of information in a program. The technique presented in the following chapter involves a new feature of the Java language called arrays. The technique presented in this chapter, on the other hand, is interesting because it does not involve any new language features. It merely involves using features of Java you already know about in new ways. The technique we discuss in this chapter is called recursion. Recursion is a technique for defining new classes and methods. When we define new classes and methods, we usually use the names of other classes and methods to describe the behavior of the class being defined. For example, the definition of the very first class we presented, TouchyButton …