visual basic barcode printing 2: Declarations and Access Control in Java

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2: Declarations and Access Control
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Coding with abstract class types (including interfaces, discussed later in this chapter) let s you take advantage of polymorphism, and gives you the greatest degree of flexibility and extensibility. You ll learn more about polymorphism in 5. You can t mark a class as both abstract and final. They have nearly opposite meanings. An abstract class must be subclassed, whereas a final class must not be subclassed. If you see this combination of abstract and final modifiers, used for a class or method declaration, the code will not compile.
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EXERCISE 2-1 Creating an Abstract Superclass and Concrete Subclass
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The following exercise will test your knowledge of public, default, final, and abstract classes. Create an abstract superclass named Fruit and a concrete subclass named Apple. The superclass should belong to a package called food and the subclass can belong to the default package (meaning it isn t put into a package explicitly). Make the superclass public and give the subclass default access.
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1. Create the superclass as follows:
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package food; public abstract class Fruit{ /* any code you want */}
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2. Create the subclass in a separate file as follows:
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import food.Fruit; class Apple extends Fruit{ /* any code you want */}
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3. Create a directory called food off the directory in your class path setting. 4. Attempt to compile the two files. If you want to use the Apple class, make sure you place the Fruit.class file in the food subdirectory.
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Method and Variable Declarations and Modifiers
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We ve looked at what it means to use a modifier in a class declaration, and now we ll look at what it means to modify a method or variable declaration.
Declarations and Modifiers (Exam Objective 1.2)
Methods and instance (nonlocal) variables are collectively known as members. You can modify a member with both access and nonaccess modifiers, and you have more modifiers to choose from (and combine) than when you re declaring a class.
Member Access
Because method and variable members are usually given access control in exactly the same way, we ll cover both in this section. Whereas a class can use just two of the four access control levels (default or public), members can use all four:
public protected default private
Default protection is what you get when you don t type an access modifier in the member declaration. The default and protected access control types have almost identical behavior, except for one difference that will be mentioned later.
It s crucial that you know access control inside and out for the exam. There will be quite a few questions with access control playing a role. Some questions test several concepts of access control at the same time, so not knowing one small part of access control could blow an entire question.
What does it mean for code in one class to have access to a member of another class For now, ignore any differences between methods and variables. If class A has access to a member of class B, it means that class B s member is visible to class A. When a class does not have access to another member, the compiler will slap you for trying to access something that you re not even supposed to know exists! You need to understand two different access issues:
Whether method code in one class can access a member of another class Whether a subclass can inherit a member of its superclass
The first type of access is when a method in one class tries to access a method or a variable of another class, using the dot operator (.) to invoke a method or retrieve a variable. For example,
2: Declarations and Access Control
class Zoo { public String coolMethod() { return "Wow baby"; } } class Moo { public void useAZoo() { Zoo z = new Zoo(); // If the preceding line compiles Moo has access // to the Zoo class // But does it have access to the coolMethod() System.out.println("A Zoo says, " + z.coolMethod()); // The preceding line works because Moo can access the // public method } }
The second type of access revolves around which, if any, members of a superclass a subclass can access through inheritance. We re not looking at whether the subclass can, say, invoke a method on an instance of the superclass (which would just be an example of the first type of access). Instead, we re looking at whether the subclass inherits a member of its superclass. Remember, if a subclass inherits a member, it s exactly as if the subclass actually declared the member itself. In other words, if a subclass inherits a member, the subclass has the member.
class Zoo { public String coolMethod() { return "Wow baby"; } }
class Moo extends Zoo { public void useMyCoolMethod() { // Does an instance of Moo inherit the coolMethod() System.out.println("Moo says, " + this.coolMethod()); // The preceding line works because Moo can inherit the public method // Can an instance of Moo invoke coolMethod() on an instance of Zoo Zoo z = new Zoo(); System.out.println("Zoo says, " + z.coolMethod()); // coolMethod() is public, so Moo can invoke it on a Foo reference } }
Declarations and Modifiers (Exam Objective 1.2)
Figure 2-1 compares the effect of access modifiers on whether a class can inherit a member of another class, or access a member of another class using a reference of an instance of that class. Much of access control (both types) centers on whether the two classes involved are in the same or different packages. Don t forget, though, if class A itself can t be accessed by class B, then no members within class A can be accessed by class B.
FIGURE 2-1
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