In the preceding lessons, you have seen inheritance mentioned several times. In the Java language, classes can be derived from other classes, thereby inheriting fields and methods from those classes.
A class that is derived from another class is called a subclass (also a derived class, extended class, or child class). The class from which the subclass is derived is called a superclass (also a base class or a parent class).
Excepting Object
, 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
.
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
. Such a class is said to be descended from all the classes in the inheritance chain stretching back to Object
.
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.
A subclass inherits all the members (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.
The
Object
class, defined in the java.lang
package, defines and implements behavior common to all classesincluding the ones that you write. In the Java platform, many classes derive directly from Object
, other classes derive from some of those classes, and so on, forming a hierarchy of classes.
All Classes in the Java Platform are Descendants of Object
At the top of the hierarchy, Object
is the most general of all classes. Classes near the bottom of the hierarchy provide more specialized behavior.
Here is the sample code for a possible implementation of a Bicycle
class that was presented in
the Classes and Objects lesson:
public class Bicycle { // the Bicycle class has // three fields public int cadence; public int gear; public int speed; // the Bicycle class has // one constructor public Bicycle(int startCadence, int startSpeed, int startGear) { gear = startGear; cadence = startCadence; speed = startSpeed; } // the Bicycle class has // four methods public void setCadence(int newValue) { cadence = newValue; } public void setGear(int newValue) { gear = newValue; } public void applyBrake(int decrement) { speed -= decrement; } public void speedUp(int increment) { speed += increment; } }
A class declaration for a MountainBike
class that is a subclass of Bicycle
might look like this:
public class MountainBike extends Bicycle { // the MountainBike subclass adds // one field public int seatHeight; // the MountainBike subclass has one // constructor public MountainBike(int startHeight, int startCadence, int startSpeed, int startGear) { super(startCadence, startSpeed, startGear); seatHeight = startHeight; } // the MountainBike subclass adds // one method public void setHeight(int newValue) { seatHeight = newValue; } }
MountainBike
inherits all the fields and methods of Bicycle
and adds the field seatHeight
and a method to set it. Except for the constructor, it is as if you had written a new MountainBike
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
class were complex and had taken substantial time to debug.
A subclass inherits all of the public and protected 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 members of the parent. You can use the inherited members as is, replace them, hide them, or supplement them with new members:
super
.The following sections in this lesson will expand on these topics.
A subclass does not inherit the private
members of its parent class. However, if the superclass has public or protected methods for accessing its private fields, these can also be used by the subclass.
A nested class has access to all the private members of its enclosing classboth fields and methods. Therefore, a public or protected nested class inherited by a subclass has indirect access to all of the private members of the superclass.
We have seen that an object is of the data type of the class from which it was instantiated. For example, if we write
public MountainBike myBike = new MountainBike();
then myBike
is of type MountainBike
.
MountainBike
is descended from Bicycle
and Object
. Therefore, a MountainBike
is a Bicycle
and is also an Object
, and it can be used wherever Bicycle
or Object
objects are called for.
The reverse is not necessarily true: a Bicycle
may be a MountainBike
, but it isn't necessarily. Similarly, an Object
may be a Bicycle
or a MountainBike
, but it isn't necessarily.
Casting shows the use of an object of one type in place of another type, among the objects permitted by inheritance and implementations. For example, if we write
Object obj = new MountainBike();
then obj
is both an Object
and a Mountainbike
(until such time as obj
is assigned another object that is not a Mountainbike
). This is called implicit casting.
If, on the other hand, we write
MountainBike myBike = obj;
we would get a compile-time error because obj
is not known to the compiler to be a MountainBike
. However, we can tell the compiler that we promise to assign a MountainBike
to obj
by explicit casting:
MountainBike myBike = (MountainBike)obj;
This cast inserts a runtime check that obj
is assigned a MountainBike
so that the compiler can safely assume that obj
is a MountainBike
. If obj
is not a Mountainbike
at runtime, an exception will be thrown.
You can make a logical test as to the type of a particular object using the instanceof
operator. This can save you from a runtime error owing to an improper cast. For example:
if (obj instanceof MountainBike) { MountainBike myBike = (MountainBike)obj; }
Here the instanceof
operator verifies that obj
refers to a MountainBike
so that we can make the cast with knowledge that there will be no runtime exception thrown.