I want to have a Class object, but I want to force whatever class it represents to extend class A and also class B.
I can do
<T extends ClassA & ClassB>
but it is not possible to extend from both classes, Is there a way to do this?
In java you cannot have a class which extends from two classes, since it doesn't support multiple inheritance. What you can have, is something like so:
public class A
...
public class B extends A
...
public class C extends B
...
In your generic signature, you can then specify that T must extend C: <T extends C>.
You could give a look at Default Methods (if you are working with Java 8), which essentially are methods declared within interfaces, and in Java, a class can implement multiple interfaces.
A simple way for this problem is inheritance.
public class A { // some codes }
public class B extends A { }
<T extends A>
Java does not have multiple inheritance as a design decision, but you may implement multiple interfaces.
As of Java 8 these interfaces may have implementations.
To use multiple classes there are other patterns:
If the parent classes are purely intended for that child class, but handle entirely different aspects, and were therefore separated, place them in a single artificial hierarchy. I admit to doing this once.
Use delegation; duplicate the API and delegate.
Use a lookup/discovery mechanism for very dynamic behaviour.
public T lookup(Class klazz);
This would need an API change, but uncouples classes, and is dynamic.
Related
Can an interface extend multiple interfaces in Java? This code appears valid in my IDE and it does compile:
interface Foo extends Runnable, Set, Comparator<String> { }
but I had heard that multiple inheritance was not allowed in Java. Why does there appear to be an exception for interfaces?
Yes, you can do it. An interface can extend multiple interfaces, as shown here:
interface Maininterface extends inter1, inter2, inter3 {
// methods
}
A single class can also implement multiple interfaces. What if two interfaces have a method defining the same name and signature?
There is a tricky point:
interface A {
void test();
}
interface B {
void test();
}
class C implements A, B {
#Override
public void test() {
}
}
Then single implementation works for both :).
Read my complete post here:
http://codeinventions.blogspot.com/2014/07/can-interface-extend-multiple.html
An interface can extend multiple interfaces.
A class can implement multiple interfaces.
However, a class can only extend a single class.
Careful how you use the words extends and implements when talking about interface and class.
From the Oracle documentation page about multiple inheritance type,we can find the accurate answer here.
Here we should first know the type of multiple inheritance in java:-
Multiple inheritance of state.
Multiple inheritance of implementation.
Multiple inheritance of type.
Java "doesn't support the multiple inheritance of state, but it support multiple inheritance of implementation with default methods since java 8 release and multiple inheritance of type with interfaces.
Then here the question arises for "diamond problem" and how Java deal with that:-
In case of multiple inheritance of implementation java compiler gives compilation error and asks the user to fix it by specifying the interface name.
Example here:-
interface A {
void method();
}
interface B extends A {
#Override
default void method() {
System.out.println("B");
}
}
interface C extends A {
#Override
default void method() {
System.out.println("C");
}
}
interface D extends B, C {
}
So here we will get error as:-
interface D inherits unrelated defaults for method() from types B and C interface D extends B, C
You can fix it like:-
interface D extends B, C {
#Override
default void method() {
B.super.method();
}
}
In multiple inheritance of type java allows it because interface doesn't contain mutable fields and only one implementation will belong to the class so java doesn't give any issue and it allows you to do so.
In Conclusion we can say that java doesn't support multiple inheritance of state but it does support multiple inheritance of implementation and multiple inheritance of type.
Can an interface extend multiple interfaces in java?
Answer is: Yes.
According to JLS
An interface may be declared to be a direct extension of one or more
other interfaces, meaning that it implicitly specifies all the member
types, abstract methods, and constants of the interfaces it extends,
except for any member types and constants that it may hide.
You can extend multiple Interfaces but you cannot extend multiple classes.
The reason that it is not possible in Java to extending multiple classes, is the bad experience from C++ where this is possible.
The alternative for multipe inheritance is that a class can implement multiple interfaces (or an Interface can extend multiple Interfaces)
I think your confusion lies with multiple inheritance, in which it is bad practise to do so and in Java this is also not possible. However, implementing multiple interfaces is allowed in Java and it is also safe.
A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.
for example, take a look here: http://www.tutorialspoint.com/java/java_interfaces.htm
I am confused regarding a concept in multiple inheritance.
I have three classes A, B and C.
Class A {
// ...
}
Class B extends A {
// ...
}
Class C extends B {
// ...
}
I know this is a bad practice of multiple inheritance and I also read java allows multiple inheritance through interfaces. But I am not getting any error in the above code. Please can anyone explain me with a clear example without using interface.
Thanks!!
This is not multi-inheritance. Each class has exactly one direct super class. If your example was considered multi-inheritance, you wouldn't be able to use the extends keyword at all, since each class already extends by default the Object class.
Multi-inheritence would be
class C extends A,B {}
And that's illegal in Java.
Your code does not contain multiple inheritance, and is, indeed, legal Java syntax. Multiple inheritance refers to a case where a class directly extends two superclasses. For example:
public class MyClass extends MyFather, MyMother {
}
Note that this is, of course, illegal Java syntax.
"Multiple inheritance" in Java basically means inheriting multiple interfaces, not inheriting multiple implementations.
Now, there is a new feature of Java 8 that allows you to do something like multiple inheritance of actual implementations, via interfaces and something called default methods. I would strongly encourage you to really master the basics of Java first before trying them. Once you are ready, here is a good tutorial on default methods.
The code you have written above is an example of Multilevel inheritance not Multiple Inheritance.
Multiple Inheritance is like :
class A extends B,C {
//this code is not valid in JAVA
}
And, if you want to use interfaces for implementing a structure like multiple inheritance, then you could use:
interface test_interface1{
/*all the methods declared here in this interface should be the part
** of the class which is implementing this current interface
*/
}
Similarly :
interface test_interface2{
}
So, create a class TestClass like :
class TestClass implements test_interface1,test_interface2 {
//now you have to use each and every method(s) declared in both the interfaces
// i.e. test_interface1 & test_interface2
}
You could also use a syntax like:
class TestClass extends AnyClass implements test_interface1,test_interface2 {
/* but do keep in mind - use extends keyword before implements
** and now you know you cannot use more than 1 class names with extends keyword
** in java.
*/
Can an interface extend multiple interfaces in Java? This code appears valid in my IDE and it does compile:
interface Foo extends Runnable, Set, Comparator<String> { }
but I had heard that multiple inheritance was not allowed in Java. Why does there appear to be an exception for interfaces?
Yes, you can do it. An interface can extend multiple interfaces, as shown here:
interface Maininterface extends inter1, inter2, inter3 {
// methods
}
A single class can also implement multiple interfaces. What if two interfaces have a method defining the same name and signature?
There is a tricky point:
interface A {
void test();
}
interface B {
void test();
}
class C implements A, B {
#Override
public void test() {
}
}
Then single implementation works for both :).
Read my complete post here:
http://codeinventions.blogspot.com/2014/07/can-interface-extend-multiple.html
An interface can extend multiple interfaces.
A class can implement multiple interfaces.
However, a class can only extend a single class.
Careful how you use the words extends and implements when talking about interface and class.
From the Oracle documentation page about multiple inheritance type,we can find the accurate answer here.
Here we should first know the type of multiple inheritance in java:-
Multiple inheritance of state.
Multiple inheritance of implementation.
Multiple inheritance of type.
Java "doesn't support the multiple inheritance of state, but it support multiple inheritance of implementation with default methods since java 8 release and multiple inheritance of type with interfaces.
Then here the question arises for "diamond problem" and how Java deal with that:-
In case of multiple inheritance of implementation java compiler gives compilation error and asks the user to fix it by specifying the interface name.
Example here:-
interface A {
void method();
}
interface B extends A {
#Override
default void method() {
System.out.println("B");
}
}
interface C extends A {
#Override
default void method() {
System.out.println("C");
}
}
interface D extends B, C {
}
So here we will get error as:-
interface D inherits unrelated defaults for method() from types B and C interface D extends B, C
You can fix it like:-
interface D extends B, C {
#Override
default void method() {
B.super.method();
}
}
In multiple inheritance of type java allows it because interface doesn't contain mutable fields and only one implementation will belong to the class so java doesn't give any issue and it allows you to do so.
In Conclusion we can say that java doesn't support multiple inheritance of state but it does support multiple inheritance of implementation and multiple inheritance of type.
Can an interface extend multiple interfaces in java?
Answer is: Yes.
According to JLS
An interface may be declared to be a direct extension of one or more
other interfaces, meaning that it implicitly specifies all the member
types, abstract methods, and constants of the interfaces it extends,
except for any member types and constants that it may hide.
You can extend multiple Interfaces but you cannot extend multiple classes.
The reason that it is not possible in Java to extending multiple classes, is the bad experience from C++ where this is possible.
The alternative for multipe inheritance is that a class can implement multiple interfaces (or an Interface can extend multiple Interfaces)
I think your confusion lies with multiple inheritance, in which it is bad practise to do so and in Java this is also not possible. However, implementing multiple interfaces is allowed in Java and it is also safe.
A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.
for example, take a look here: http://www.tutorialspoint.com/java/java_interfaces.htm
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?
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.
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
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
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 beneļ¬t. This
primarily consists of operator overloading (although it does have
method overloading), multiple inheritance, and extensive automatic
coercions.
There is no concept of multiple inheritance in Java. Only multiple interfaces can be implemented.
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.
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
java can not support multiple inheritence.but u can do this in this way
class X
{
}
class Y extends X
{
}
class Z extends Y{
}
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.
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.
Hello please note like real work.
Children can not have two mother
So in java, subclass can not have two parent class.
I have some base class Entity. I want to allow users of Entity to create instances of Entity that extend some given super class T (as if Entitys declaration was Class Entity extends T). I'm not aware of what T actually is, so I want Entity to be modular in that respect. If that's not possible I'm ok with Entity implementing a given interface T as well.
Things like public class Entity<T> extends T and public class Entity<T> implements T do not work. ("Cannot refer to the type parameter T as a supertype").
So my questions are:
1) is this at all possible to achieve in java ? some other language ?
2) if so, how ?
3) usually when java puts up hurdles like that it means something is wrong with my design, so how would you create this functionality (I guess what I'm looking for is basically multiple inheritance from Entity and T, can this be done ?).
EDIT: clarification - I want to achieve mixin type behavior while requiring as little as possible from the caller (creating a subclass of T which delegates calls to an Entity instance would demand way too much from the caller). is there no other way to do this ?
Java doesn't support multiple inheritance; a given class can't be a direct subclass of more than one class.
However, if Entity is an interface, you can do it using an intersection bound.
For example, to code a method that accepts an instance that is a subclass (not necessarily a direct subclass) of SomeClass and that implements Entity:
public <T extends Entity & SomeClass> void doSomething(T t) {
}
template <class A>
class B : public A
{
Possible in C++ at least.
I have used this once in my own work, to add behavior to two classes that are very closely related (one inheriting from the other). It struck me only much later that the Decorator Pattern might have worked as well. Perhaps you can also consider it.
The key in your question is when you said that you want to "create instances of Entity that extend some given super class T". An object (instance of a class) never extends a class. Only classes extend other classes.
What you probably want is to use interfaces. Let Entity be an interface and create classes that implement Entity and then instantiate those classes.
No thats not how it works in Java. You might want to consider composition instead of inheritance.
I'm kind of reading between the lines here, but are these Entities in a game engine? A component system would work well...
http://gameprogrammingpatterns.com/component.html
Here's a quick example...
class Entity {
List<Component> components;
void update() {
for(Component component: components) {
component.update(this);
}
}
}
interface Component {
void update(Entity parent);
}
class MovementComponent implements Component ...
class AnimationComponent ...
class BehaviourComponent ...