Java Abstract Classes

感觉分析的很好

http://tutorials.jenkov.com/java/abstract-classes.html

Abstract classes in Java are classes which cannot be instantiated, meaning you cannot create new instances of an abstract class. The purpose of an abstract class is to function as a base for subclasses. This text gets into the purpose of abstract classes in more detail towards the end of this text.

Declaring an Abstract Class

You declare that a class is abstract by adding the abstract keyword to the class declaration. Here is an example:

public abstract class MyAbstractClass {

}

That is all there is to it. Now you cannot create instances of MyAbstractClass. Thus, the following Java code is no longer valid:

MyAbstractClass myClassInstance = new MyAbstractClass();  //not valid

If you try to compile the code above, the Java compiler will generate an error.

Abstract Methods

An abstract class can have abstract methods. You declare a method abstract by adding the abstract keyword in front of the method declaration. Here is how that looks:

public abstract class MyAbstractClass {

    public abstract void abstractMethod();
}

An abstract method has no implementation. It just has a method signature.

If a class has an abstract method, the whole class must be declared abstract. Not all methods have to be abstract, even if the class is abstract. An abstract class can have a mixture of abstract and non-abstract classes.

Subclasses of an abstract class must implement (override) all abstract methods of its abstract superclass. The non-abstract methods of the superclass are just inherited as they are. They can also be overridden, if needed.

Here is an example subclass of MyAbstractClass:

public class MySubClass extends MyAbstractClass {

    public void abstractMethod() {
        System.out.println("My method implementation");
    }
}

Notice how MySubClass has to implement the abstract method abstractMethod() from its superclassMyAbstractClass.

The only time a subclass of an abstract class is not forced to implement all abstract methods of its superclass, is if the subclass is also an abstract class.

The Purpose of Abstract Classes

The purpose of abstract classes is to function as base classes which can be extended by subclasses to create a full implementation. For instance, imagine that a certain process requires 3 steps:

1. The step before the action.
2. The action.
3. The step after the action.

If the steps before and after the action are always the same, the 3-step process could be implemented in an abstract superclass like this:

public abstract MyAbstractProcess {

    public void process() {
        stepBefore();
        action();
        stepAfter();
    }

    public void stepBefore() {
        //implementation directly in abstract superclass
    }

    public abstract void action(); // implemented by subclasses

    public void stepAfter() {
        //implementation directly in abstract superclass
    }
}

Notice how the action() method is abstract. Subclasses of MyAbstractProcess can now extendMyAbstractProcess and just override the action() method.

When the process() method of the subclass is called, the full process is executed, including the action() method of the subclass.

Of course, the MyAbstractProcess did not have to be an abstract class to function as a base class. Nor did theaction() method have to be abstract either. You could have just used an ordinary class. However, by making the method to implement abstract, and thus the class too, you signal clearly to the programmer, that this class should not be used as it is, but be used as a base class for a subclass, and that the abstract method should be implemented in the subclass.

The above example did not have a default implementation for the action() method. In some cases your superclass might actually have a default implementation for the method that subclasses are supposed to override. In that case, you cannot make the method abstract. You can still make the superclass abstract though, even if it contains no abstract methods.

Here is a more concrete example that opens a URL, processes it and closes the connection to the URL afterwards.

public abstract class URLProcessorBase {

    public void process(URL url) throws IOException {
        URLConnection urlConnection = url.openConnection();
        InputStream input = urlConnection.getInputStream();

        try{
            processURLData(input);
        } finally {
            input.close();
        }
    }

    protected abstract void processURLData(InputStream input)
        throws IOException;

}

Notice how the processURLData() method is abstract. Subclasses of URLProcessorBase has to implement this method.

Subclasses of URLProcessorBase can process data downloaded from URL’s without worrying about opening and closing the network connection to the URL. This is done by the URLProcessorBase. Subclasses only need to worry about processing the data from the InputStream passed to the processURLData() metod. This makes it easier to implement classes that processes data from URL’s.

Here is an example subclass:

public class URLProcessorImpl extends URLProcessorBase {

    @Override
    protected void processURLData(InputStream input) throws IOException {
        int data = input.read();
        while(data != -1){
            System.out.println((char) data);
            data = input.read();
        }
    }
}

Notice how the subclass only implements the processURLData() method, and nothing more. The rest of the code is inherited from the URLProcessorBase superclass.

Here is an example of how to use the URLProcessorImpl class:

URLProcessorImpl urlProcessor = new URLProcessorImpl();

urlProcessor.process(new URL("http://jenkov.com"));

The process() method is called, which is implemented in the URLProcessorBase superclass. This method in turn calls the processURLData() in the URLProcessorImpl class.

Abstract Classes and the Template Method Design Pattern

The example I showed you above with the URLProcessorBase class is actually an example of the Template Method design pattern. The Template Method design pattern provides a partial implementation of some process, which subclasses can complete when extending the Template Method base class.

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