{"id":405,"date":"2007-10-21T00:00:00","date_gmt":"2007-10-21T00:00:00","guid":{"rendered":"http:\/\/www.strongd.net\/?p=405"},"modified":"2007-10-21T00:00:00","modified_gmt":"2007-10-21T00:00:00","slug":"Inheritance","status":"publish","type":"post","link":"https:\/\/www.strongd.net\/?p=405","title":{"rendered":"Inheritance"},"content":{"rendered":"

Inheritance<\/DIV>
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In the preceding lessons, you have seen inheritance<\/I> mentioned several times. In the Java language, classes can be derived<\/I> from other classes, thereby inheriting<\/I> fields and methods from those classes.
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\nDefinitions:<\/STRONG> 
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A class that is derived from another class is called a subclass<\/I> (also a derived class<\/I>, extended class<\/I>, or child class<\/I>). The class from which the subclass is derived is called a superclass<\/I> (also a base class<\/I> or a parent class<\/I>). <\/P>
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Excepting Object<\/CODE>, which has no superclass, every class has one and only one direct superclass (single inheritance). In the absence of any other explicit superclass, every class is implicitly a subclass of Object<\/CODE>. <\/P>
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Classes can be derived from classes that are derived from classes that are derived from classes, and so on, and ultimately derived from the topmost class, Object<\/CODE>. Such a class is said to be descended<\/I> from all the classes in the inheritance chain stretching back to Object<\/CODE>. <\/P>
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\n<\/BLOCKQUOTE>The idea of inheritance is simple but powerful: When you want to create a new class and there is already a class that includes some of the code that you want, you can derive your new class from the existing class. In doing this, you can reuse the fields and methods of the existing class without having to write (and debug!) them yourself.
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A subclass inherits all the members<\/I> (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass. <\/P>
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The Java Platform Class Hierarchy<\/H3>The Object<\/CODE><\/A> class, defined in the java.lang<\/CODE> package, defines and implements behavior common to all classes\u2014including the ones that you write. In the Java platform, many classes derive directly from Object<\/CODE>, other classes derive from some of those classes, and so on, forming a hierarchy of classes.
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\"All<\/P>
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All Classes in the Java Platform are Descendants of Object<\/P><\/CENTER>At the top of the hierarchy, Object<\/CODE> is the most general of all classes. Classes near the bottom of the hierarchy provide more specialized behavior.
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An Example of Inheritance<\/H3>Here is the sample code for a possible implementation of a Bicycle<\/CODE> class that was presented in the Classes and Objects lesson:
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public class Bicycle {<\/p>\n
    \/\/ the Bicycle class has three fields<\/I><\/B>
\n    public int cadence;
\n    public int gear;
\n    public int speed;<\/p>\n
    \/\/ the Bicycle class has one constructor<\/I><\/B>
\n    public Bicycle(int startCadence, int startSpeed, int startGear) {
\n        gear = startGear;
\n        cadence = startCadence;
\n        speed = startSpeed;
\n    }<\/p>\n
    \/\/ the Bicycle class has four methods<\/I><\/B>
\n    public void setCadence(int newValue) {
\n        cadence = newValue;
\n    }<\/p>\n
    public void setGear(int newValue) {
\n        gear = newValue;
\n    }<\/p>\n
    public void applyBrake(int decrement) {
\n        speed -= decrement;
\n    }<\/p>\n
    public void speedUp(int increment) {
\n        speed += increment;
\n    }<\/p>\n
}
\n<\/PRE><\/BLOCKQUOTE>
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A class declaration for a MountainBike<\/CODE> class that is a subclass of Bicycle<\/CODE> might look like this: <\/P>
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public class MountainBike extends Bicycle {<\/p>\n
    \/\/ the MountainBike subclass adds one field<\/I><\/B>
\n    public int seatHeight;<\/p>\n
    \/\/ the MountainBike subclass has one constructor<\/I><\/B>
\n    public MountainBike(int startHeight, int startCadence, int startSpeed, int startGear) {
\n        super(startCadence, startSpeed, startGear);
\n        seatHeight = startHeight;
\n    }\t<\/p>\n
    \/\/ the MountainBike subclass adds one method<\/I><\/B>
\n    public void setHeight(int newValue) {
\n        seatHeight = newValue;
\n    }\t<\/p>\n
}
\n<\/PRE><\/BLOCKQUOTE>MountainBike<\/CODE> inherits all the fields and methods of Bicycle<\/CODE> and adds the field seatHeight<\/CODE> and a method to set it. Except for the constructor, it is as if you had written a new MountainBike<\/CODE> class entirely from scratch, with four fields and five methods. However, you didn’t have to do all the work. This would be especially valuable if the methods in the Bicycle<\/CODE> class were complex and had taken substantial time to debug.
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What You Can Do in a Subclass<\/H3>A subclass inherits all of the public<\/I> and protected<\/I> members of its parent, no matter what package the subclass is in. If the subclass is in the same package as its parent, it also inherits the package-private<\/I> members of the parent. You can use the inherited members as is, replace them, hide them, or supplement them with new members:
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