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Overriding Methods
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Inheriting, or extending, a class is a very good approach for factoring out common functionality between classes. Specific classes extending more general classes allow code to be reused in a project. As stated before, this helps keep the project more maintainable and less prone to bugs as the development cycle progresses. The problem with this approach is that the subclass that inherits the methods of the superclass is sometimes slightly different. For example, if a Fish class extends the Animal class, the move() method would not work since it is implemented by code that walks and a fish would need to swim. A class that extends another class may override any inherited method. This is done by defining another method called move() with the same arguments. When the move() method is invoked, the one that is implemented in the Fish class will be used. A class may override all, none, or just some of the methods it inherits from a parent class. The following is an example of the Fish class extending the Animal class and then overriding the move() method:
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public class Fish extends Animal { public void move(int direction) { /* Swim in the direction given as a parameter */ } }
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Note in the preceding example of the Fish class that the move() method signature is the same as in the Animal class. The move() method in the Fish class is overriding the move() method in the Animal class. When a Fish object is created and the move() method is called, it will execute the code that is located in the Fish class. To override a method, the method signatures must be identical.
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Inheritance and Class Type
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When a subclass overrides a method, it has the option of calling the method that is being overridden. This can be achieved by using the super keyword. The super keyword works just like the this keyword, but instead of referring to the current class, super refers to the superclass. When super is used, it must pass the correct arguments to the parent method. The following is an example of super being used in the Horse3 class. Since horses normally rest standing, the Horse2 class from earlier can be further modified to put the horse in a standing position before it performs the rest() method.
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public class Horse3 extends Animal{ public void rest(){ /* Stand before rest */ super.rest(); } public void neigh() { /* Neigh */ } }
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When a Horse3 object has its rest() method called, it will execute the code inside the rest() method of the Horse3 class. This is because the rest() method overrides the rest() method in the Animal class. The Horse3 s rest() method makes the horse stand and then uses super to call the rest() method in the Animal class.
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Abstract Classes
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So far, all the examples presented use concrete classes. A concrete class is a regular class that can be instantiated. Java has another class type called an abstract class. An abstract class is different from a concrete class because it cannot be instantiated and must be extended. An abstract class may contain abstract methods. Abstract methods are methods that are not implemented. They have a valid method signature but must be overridden and implemented in the class that extends the abstract class. The following is an example of an abstract class:
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public abstract class MusicPlayer { public abstract void play(); public abstract void stop(); public void changeVolume(int volumeLevel) { /* Set volume to volumeLevel */} }
7:
Understanding Class Inheritance
The preceding example is an abstract class for a music player. This is intended to be the base class for different music-playing devices such as MP3 players or CD players. Notice how the class is defined; the keyword abstract is used to indicate that this is an abstract class. This class provides some functionality with the changeVolume() method. It also contains two abstract methods. An abstract method can only exist in an abstract class. The abstract keyword is used to mark a method as such. Every abstract method must be implemented in the subclass that extends it. The purpose of an abstract method is to define the required functionality that any subclass must have. In this case, any music player must be able to play and stop. The functionality cannot be implemented in the MusicPlayer class because it is different from player to player. The following example is of two classes extending the MusicPlayer class:
public class MP3Player extends MusicPlayer{ public void play() { /* Start decoding and playing MP3 */ } public void stop() { /* Stop decoding and playing MP3 */ } } public class CDPlayer extends MusicPlayer { public void play() { /* Start reading and playing disc */ } public void stop() { /* Stop reading disc */ } }
The MP3Player and CDPlayer classes are both types of music players. By extending the MusicPlayer class, they are required to implement the play() and stop() methods by overriding the abstract classes in the base class.
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