Class extending more than one class Java?

Question

I know that a class can implement more than one interface, but is it possible to extend more than one class? For example I want my class to extend both TransformGroup and a class I created. Is this possible in Java? Both statements class X extends TransformGroup extends Y and class X extends TransformGroup, Y receive an error. And if it is not possible, why? TransformGroup extends Group but I guess it also extends Node since it inherits fields from Node and it can be passed where a Node object is required. Also, like all classes in Java, they extend Object class. So why wouldn't it be possible to extend with more than one class?

TransformGroup inheritance

So, if that is possible, what is the proper way to do it? And if not, why and how should I solve the problem?

Answer 1

Assume B and C are overriding inherited method and their own implementation. Now D inherits both B & C using multiple inheritance. D should inherit the overridden method.The Question is which overridden method will be used? Will it be from B or C? Here we have an ambiguity. To exclude such situation multiple inheritance was not used in Java.

Answer 2

Most of the answers given seem to assume that all the classes we are looking to inherit from are defined by us.

But what if one of the classes is not defined by us, i.e. we cannot change what one of those classes inherits from and therefore cannot make use of the accepted answer, what happens then?

Well the answer depends on if we have at least one of the classes having been defined by us. i.e. there exists a class A among the list of classes we would like to inherit from, where A is created by us.

In addition to the already accepted answer, I propose 3 more instances of this multiple inheritance problem and possible solutions to each.

Inheritance type 1

Ok say you want a class C to extend classes, A and B, where B is a class defined somewhere else, but A is defined by us. What we can do with this is to turn A into an interface then, class C can implement A while extending B.

class A {}
class B {} // Some external class
class C {}

Turns into

interface A {}
class AImpl implements A {}
class B {} // Some external class
class C extends B implements A

Inheritance type 2

Now say you have more than two classes to inherit from, well the same idea still holds - all but one of the classes has to be defined by us. So say we want class A to inherit from the following classes, B, C, ... X where X is a class which is external to us, i.e. defined somewhere else. We apply the same idea of turning all the other classes but the last into an interface then we can have:

interface B {}
class BImpl implements B {}
interface C {}
class CImpl implements C {}
...
class X {}
class A extends X implements B, C, ...

Inheritance type 3

Finally, there is also the case where you have just a bunch of classes to inherit from, but none of them are defined by you. This is a bit trickier, but it is doable by making use of delegation. Delegation allows a class A to pretend to be some other class B but any calls on A to some public method defined in B, actually delegates that call to an object of type B and the result is returned. This makes class A what I would call a Fat class

How does this help?

Well it's simple. You create an interface which specifies the public methods within the external classes which you would like to make use of, as well as methods within the new class you are creating, then you have your new class implement that interface. That may have sounded confusing, so let me explain better.

Initially we have the following external classes B, C, D, ..., X, and we want our new class A to inherit from all those classes.

class B {
    public void foo() {}
}

class C {
    public void bar() {}
}

class D {
    public void fooFoo() {}
}

...

class X {
    public String fooBar() {}
}

Next we create an interface A which exposes the public methods that were previously in class A as well as the public methods from the above classes

interface A {
    void doSomething(); // previously defined in A
    String fooBar(); // from class X
    void fooFoo(); // from class D
    void bar(); // from class C
    void foo(); // from class B
}

Finally, we create a class AImpl which implements the interface A.

class AImpl implements A {
    // It needs instances of the other classes, so those should be
    // part of the constructor
    public AImpl(B b, C c, D d, X x) {}
    ... // define the methods within the interface
}

And there you have it! This is sort of pseudo-inheritance because an object of type A is not a strict descendant of any of the external classes we started with but rather exposes an interface which defines the same methods as in those classes.

You might ask, why we didn't just create a class that defines the methods we would like to make use of, rather than defining an interface. i.e. why didn't we just have a class A which contains the public methods from the classes we would like to inherit from? This is done in order to reduce coupling. We don't want to have classes that use A to have to depend too much on class A (because classes tend to change a lot), but rather to rely on the promise given within the interface A.

Answer 3

Hello please note like real work.

Children can not have two mother

So in java, subclass can not have two parent class.

Answer 4

Java does not allow extending multiple classes.

Let's assume C class is extending A and B classes. Then if suppose A and B classes have method with same name(Ex: method1()). Consider the code:

C obj1 = new C(); obj1.method1(); - here JVM will not understand to which method it need to access. Because both A and B classes have this method. So we are putting JVM in dilemma, so that is the reason why multiple inheritance is removed from Java. And as said implementing multiple classes will resolve this issue.

Hope this has helped.

Answer 5

There is no concept of multiple inheritance in Java. Only multiple interfaces can be implemented.

Answer 6

In Java multiple inheritance is not permitted for implementations (classes) only for interfaces:

interface A extends B, C

E.g. MouseInputListener extends MouseListener and MouseMotionListener

And, of course, a class can implement several interfaces:

class X implements A, F

Answer 7

In Java multiple inheritance is not permitted. It was excluded from the language as a design decision, primarily to avoid circular dependencies.

Scenario1: As you have learned the following is not possible in Java:

public class Dog extends Animal, Canine{

}

Scenario 2: However the following is possible:

public class Canine extends Animal{

}

public class Dog extends Canine{

}

The difference in these two approaches is that in the second approach there is a clearly defined parent or super class, while in the first approach the super class is ambiguous.

Consider if both Animal and Canine had a method drink(). Under the first scenario which parent method would be called if we called Dog.drink()? Under the second scenario, we know calling Dog.drink() would call the Canine classes drink method as long as Dog had not overridden it.

Answer 8

Multiple inheritance is not possible with class, you can achieve it with the help of interface but not with class. It is by design of java language. Look a comment by James gosling.

by James Gosling in February 1995 gives an idea on why multiple inheritance is not supported in Java.

JAVA omits many rarely used, poorly understood, confusing features of C++ that in our experience bring more grief than benefit. This primarily consists of operator overloading (although it does have method overloading), multiple inheritance, and extensive automatic coercions.

Answer 9

java can not support multiple inheritence.but u can do this in this way

class X
{
}
class Y extends X
{
}
class Z extends Y{
}

Answer 10

Java didn't provide multiple inheritance.
When you say A extends B then it means that A extends B and B extends Object.
It doesn't mean A extends B, Object.

class A extends Object
class B extends A

Answer 11

No it is not possible in java (Maybe in java 8 it will be avilable). Except the case when you extend in a tree. For example:

class A
class B extends A
class C extends B