In Java programs, classes are the basic building blocks. When defining a class, you describe all the parts and characteristics of one of those building blocks. To use most classes, you have to create objects. An object is a runtime instance of a class in memory. All the various objects of all the different classes represent the state of your program.
In the following sections, we'll look at fields, methods, and comments. We'll also explore the relationship between classes and files.
Fields and Methods
Java classes have two primary elements: methods, often called functions or procedures in other languages, and fields, more generally known as variables. Together these are called the members of the class. Variables hold the state of the program, and methods operate on that state. If the change is important to remember, a variable stores that change. That's all classes really do. It's the programmer who creates and arranges these elements in such a way that the resulting code is useful and, ideally, easy for other programmers to understand.
Other building blocks include interfaces, which you'll learn about in Chapter 5, “Class Design,” and enums, which you'll learn about when you start studying for the OCP exam.
The simplest Java class you can write looks like this:
1: public class Animal { 2: }
Java calls a word with special meaning a keyword. The public keyword on line 1 means the class can be used by other classes. The class keyword indicates you're defining a class. Animal gives the name of the class. Granted, this isn't a very interesting class, so add your first field:
1: public class Animal { 2: String name; 3: }
The line numbers aren't part of the program; they're just there to make the code easier to talk about.
On line 2, we define a variable named name. We also define the type of that variable to be a String. A String is a value that we can put text into, such as "this is a string ". String is also a class supplied with Java. Next you can add methods:
1: public class Animal { 2: String name; 3: public String getName() { 4: return name; 5: } 6: public void setName(String newName) { 7: name = newName; 8: } 9: }
On lines 3–5, you've defined your first method. A method is an operation that can be called. Again, public is used to signify that this method may be called from other classes. Next comes the return type—in this case, t...
