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Class Declarations and Modifiers
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We ll start this objective by looking at how to declare and modify a class. Although nested (often called inner) classes are on the exam, we ll save nested class declarations for 8. You re going to love that chapter. No, really. Seriously. No kidding around. Before we dig into class declarations, let s do a quick review of the rules:
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There can be only one public class per source code file. The name of the file must match the name of the public class. If the class is part of a package, the package statement must be the first line
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in the source code file.
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If there are import statements, they must go between the package statement
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and the class declaration. If there isn t a package statement, then the import statement(s) must be the first line(s) in the source code file. If there are no package or import statements, the class declaration must be the first line in the source code file. (Comments don t count; they can appear anywhere in the source code file.)
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Import and package statements apply to all classes within a source code file.
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The following code is a bare-bones class declaration:
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class MyClass { }
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This code compiles just fine, but you can also add modifiers before the class declaration. Modifiers fall into two categories:
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Access modifiers: public, protected, private Nonaccess modifiers (including strictfp, final, and abstract)
We ll look at access modifiers first, so you ll learn how to restrict or allow access to a class you create. Access control in Java is a little tricky because there are four access controls (levels of access) but only three access modifiers. The fourth access control level (called default or package access) is what you get when you don t use any of the
2: Declarations and Access Control
three access modifiers. In other words, every class, method, and instance variable you declare has an access control, whether you explicitly type one or not. Although all four access controls (which means all three modifiers) work for most method and variable declarations, a class can be declared with only public or default access; the other two access control levels don t make sense for a class, as you ll see.
Java is a package-centric language; the developers assumed that for good organization and name scoping, you would put all your classes into packages. They were right, and you should. Imagine this nightmare: three different programmers, in the same company but working on different parts of a project, write a class named Utilities. If those three Utilities classes have not been declared in any explicit package, and are in the classpath, you won t have any way to tell the compiler or JVM which of the three you re trying to reference. Sun recommends that developers use reverse domain names, appended with division and/or project names. For example, if your domain name is geeksanonymous.com, and you re working on the client code for the TwelvePointOSteps program, you would name your package something like com.geeksanonymous.steps.client. That would essentially change the name of your class to com.geeksanonymous.steps.client.Utilities. You might still have name collisions within your company, if you don t come up with your own naming schemes, but you re guaranteed not to collide with classes developed outside your company (assuming they follow Sun s naming convention, and if they don t, well, Really Bad Things could happen).
Class Access
What does it mean to access a class When we say code from one class (class A) has access to another class (class B), it means class A can do one of three things:
Create an instance of class B Extend class B (in other words, become a subclass of class B) Access certain methods and variables within class B, depending on the access
control of those methods and variables. In effect, access means visibility. If class A can t see class B, the access level of the methods and variables within class B won t matter; class A won t have any way to access those methods and variables.
Declarations and Modifiers (Exam Objective 1.2)
Default Access A class with default access has no modifier preceding it in the declaration. In other words, it s the access control you get when you don t type a modifier in the class declaration. Think of default access as package-level access, because a class with default access can be seen only by classes within the same package. For example, if class A and class B are in different packages, and class A has default access, class B won t be able to create an instance of class A, or even declare a variable or return type of class A. In fact, class B has to pretend that class A doesn t even exist, or the compiler will complain. Look at the following source file:
package cert; class Beverage { }
Now look at the second source file:
package exam.stuff; import cert.Beverage; class Tea extends Beverage { }
As you can see, the superclass (Beverage) is in a different package from the subclass (Tea). The import statement at the top of the Tea file is trying (fingers crossed) to import the Beverage class. The Beverage file compiles fine, but watch what happens when we try to compile the Tea file:
>javac Tea.java Tea.java:1: Can't access class cert.Beverage. Class or interface must be public, in same package, or an accessible member class. import cert.Beverage; ..
Tea won t compile because its superclass, Beverage, has default access and is in a different package. You can do one of two things to make this work. You could put both classes in the same package, or declare Beverage as public, as the next section describes.
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