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To achieve multiple inheritance, we must use interfaces, but why don't interface methods have bodies and why do they have to be overridden in the derived class?
I really want a lucid answer , not involving too much computer jargon , i cant seem to understand this , i have referred various references
Because Java, in contrast to languages like C++ or Eiffel, only has multiple inheritance of types (i.e. interfaces as well as one class), not multiple inheritance of state and behaviour. The latter of which add enormous complexity (especially state).
The Java designers (and C#, for that matter) opted to not include it as it presented C++ programmers often with very hard to debug issues. You can solve pretty much most problems that require true multiple inheritance with implementing multiple interfaces, so the tradeoff was deemed worth it.
Note that multiple inheritance of behaviour (not state) might come to Java 8 (unless they postpone it again like one of the many other things) in form of virtual extension methods where an interface can declare a method that delegates to one in another class, which then exists on all types that implement that interface.
Interfaces declare WHAT services the implementing class provides, not HOW (that's the job of the implementing class). Multiple inheritance is regarded bad, as it leads to complicated code and class hierarchies.
Interfaces only have constant variables(public + static + final) and abstract methods(public & abstract). These are meant to be used by the classes which implement the interfaces.
Interfaces simply say 'Am a contract', which if you wish to use, should stick to some rules(give implementation to all abstract methods).
Multiple inheritance is omitted in Java by making sure that a class can extend only 1 class, in order to avoid the diamond problem. You can anyways have multiple inheritance of types in Java by using interfaces.
A Java interface contains a list of methods that must be implemented by the class that implements the interface. Thus, the methods have no body: the body of each method is in the implementing class(es).
Simple Answer:
An interface provides a standard for implementation.
Explanation:
In Java an interface is similar to an abstract class in that its members are not implemented. For example,
public interface Comparable
{ boolean less(Object m);
boolean greater(Object m);
boolean lessEqual(Object m);
boolean greaterEqual(Object m);
}
An interface provides a standard for implementation.
Benefit of using interfaces is that they simulate multiple inheritance. All classes in Java must have exactly one base class, the only exception being java.lang.Object (the root class of the Java type system); multiple inheritance of classes is not allowed in java.
All instance methods are implicitly public and abstract. You can mark them as such, but are discouraged from doing so as the marking is considered obsolete practice. The interfaces themselves need not be public and several interfaces in the standard libraries are not public and thus used only internally.
An interface creates a protocol that classes may implement. Note that one can extend an interface (to get a new interface) just as you can extend a class. One can actually extend several interfaces. Interfaces thus enjoy the benefits of multiple inheritance. (Classes do not.) There are almost no disadvantages to multiple inheritance of interface (small name conflict problems are one exception). There are large disadvantages to multiple inheritance of implementation as in C++. These include efficiency considerations as well as the semantic difficulty of determining just what code will be executed in some circumstances.
The Polynomial class that implements Comparable will need to implement all of the functions declared in the interface.
public class Polynomial implements Comparable
{ . . .
boolean less(Object m){ . . . }
boolean greater(Object m){ . . . }
boolean lessEqual(Object m){ . . . }
boolean greaterEqual(Object m){ . . . }
Polynomial multiply(Polynomial P){ . . . }
. . .
}
A class may choose to implement any number of interfaces. A class that implements an interface must provide bodies for all methods of that interface. Also, We expect that an abstract class can choose to implement part of an interface leaving the rest for non-abstract subclasses.
The usefulness of interfaces goes far beyond simply publishing protocols for other programmers. Any function can have parameters that are of interface type. Any object from a class that implements the interface may be passed as an argument.
References:
Interface
Interfaces
Interface Wiki
Interface methods has no body
like
public interface Flyable
{
public void fly();
}
because interface itself not going to do anything
interface is defines contract.
an interface, on other hand, defines what a class can do,
not what it is. So interface is about verbs usually.
So the Flyable interface is doing nothing but defines the contract that the implanted
FlyableObjects are going to fly.
like:
class Rocket implements Flyable
{
public void fly()
{
// do the stuffs.
}
}
interface
and ofcourse we can achieve multiple inheritance also only and only through interface.
if interfaces had bodies then it would have brought back the Deadly Daimond of Death problem.
Consider this example having interfaces with bodies
interface A {
void print(){ System.out.print("A") }
}
interface B {
void print(){ System.out.print("B") }
}
interface C extends A, B {
// now since A and B have bodies interfaces would have had choice to not to override the default behavior
}
public class C_Implementer implements C{
public static void main(String args[]){
C c = new C_Implementer();
c.print(); // gotcha!!!!! what should it print, A or B????
}
}
You are asking "Why does Java not support multiple inheritance of implementation?"
This is discussed in the Java Tutorials, Multiple Inheritance of State, Implementation, and Type, but I wanted to give a specific example of the problems of multiple inheritance of implementation (as well as a new language feature solution at the end).
Imagine two interfaces (in our proposed version of Java that allows interface method bodies) that define a method with the same name.
public interface FaceOne {
public void method() {
System.out.println("FaceOne Version");
}
}
public interface FaceTwo {
public void method() {
System.out.println("FaceTwo Version");
}
}
And a class implements both interfaces, but doesn't override the method.
public class Inheriter implements FaceOne, FaceTwo {
}
When I call Inheriter.method(), which works since the class inherits the method from its ancestors, the problem arises: does the output print "FaceOne Version" or "FaceTwo Version"?
In addition, if the class were to override the method, but wanted to also call its ancestor's version using super, the compiler would again have trouble choosing between a version of the method.
This is why Java does not support multiple inheritance of implementation.
As an aside, I think an elegant way to implement this into the language would be as follows:
Continue to force implementing classes to override their ancestor interface's methods. This solves the first problem of a non-overridden method.
Then, use a similar notation as that of accessing an enclosing instance for an inner class to access a specific ancestor interface with super. The Inheriter class would then have multiple options:
Do not call super.method(), but rather only use newly-defined implementation.
Use FaceOne.super.method() to make the default inherited implementation output "FaceOne Version".
Use FaceTwo.super.method() to make the default inherited implementation output "FaceTwo Version".
Use a combination of the above:
One implementation could be:
#Override
public void method() {
FaceOne.super.method();
FaceTwo.super.method();
System.out.println("Inheriter Version");
}
Outputting:
FaceOne Version
FaceTwo Version
Inheriter Version
Edit: According to this question this is apparently exactly how default implementations are structured in Java 8.
I'm porting some Python code to Java, and I am having trouble dealing with the following problem:
I have some classes which need to have abilities A, B, or C. Class 1 needs ability A, class 2 needs A, B and C, and class 3 needs B and C. Most importantly, I want to easily be able to change what class can have what ability in the future.
I solved this problem pretty easily with multiple inheritance in Python. I'm trying to figure out the best way to do it in Java, but I can't come up with as good of a solution. I know multiple inheritance is frowned-upon, so I'd appreciate being taught a better way.
Thanks!
It depends on your actual use case, but have you already considered decorators?
http://en.wikipedia.org/wiki/Decorator_pattern
Multiple-inheritance ain't frowned upon. What is frowned upon is "implementation inheritance" (also known as "code reuse"), because it leads to the unsolvable "diamond problem". And because, well, code-reuse really hasn't much to do with OO.
What you want to do can be solved using multiple inheritance (and, say, delegation if you need to do "code reuse").
interface A {
void move();
}
interface B {
void eat();
}
interface C {
void think();
}
class One implements A { ... }
class Two implements B { ... }
class Three implements B, C { ... }
Any OOA/OOD using multiple inheritance can be trivially translated to Java. The part where you say that you need to change the "ability" all the time is a bit scary: if, say, a Car can move(), why would it suddenly need to be able to think()?
You can use AspectJ's mixin syntax fairly easily to emulate multiple inheritance (and at compile time too). First, declare an interface for the functionality you want to mixin:
public interface A{
String getSomethingForA();
}
then define an annotation which you can use to signify that you want the mixin applied to a given class:
public #interface WithA {}
then add the annotation to the class you want to use:
#WithA
public class MyClass {}
then, to actually add some functionality:
#Aspect
public class MixinA {
public static class AImpl implements A{
public String getSomethingForA() {
return "it worked!";
}
}
#DeclareMixin("#WithA *")
public static A get() {
return new AImpl();
}
}
You'll need to use the aspectj jars and run the aspects as part of your compile process, but this lets you create truly modularized functionality and then forcibly merge it into your classes later. To access your class with the new functionality, do the following:
MyClass obj = new MyClass();
((A)obj).getSomethingForA();
You can apply the same annotation to another class and cast it as well:
#WithA
#WithB //let's pretend we created this with some other functionality
public class AnotherClass {}
AnotherClass anotherObj = new AnotherClass();
((A)anotherObj).getSomethingForA();
((B)anotherObj).andSetSomethingElseForB("something else");
Multiple inheritance is almost always a bad idea, as its effects can usually be achieved through other mechanisms. Based upon your description of the problem, it sounds like you want to
Use interfaces to define behavior (public interface A) in this scenario, each behavior should probably have its own interface.
If 2 behaviors are tightly coupled (say A & B), define an interface that implements those two atomic interfaces (public interface CombinedAandB extends A, B)
Define an abstract base class that implements the interface to provide default implementations for behaviors
public abstract class BaseAB implements A, B
{
#Override
public void A() { add(0,1); }
#Override
public void B() {add(1,0); }
private void add(int a, int b) //it doesn't return. no soup for you.
{ a + b; //If you know why this is wrong, high five yourself. }
}
Define a concrete class that extends the abstract base class, implements another interface, and provides its own behavior.
public class IDoABAndC extends BaseAB implements C
{
//stuff, etc
}
You can define the abilities in interfaces and implement them in your classes.
In java you don't have multiple inheritance, instead you can implement multiple interfaces.
So your class 1 will implement interface A and B. Class 2 will implement interface A, B and C. Class 3 will implement B and C.
If what you need is interface inheritance, then as mentioned before, you can always implement multiple interfaces.
If you're looking for implementation inheritance, you're somewhat out of luck. The best solution is probably to use delegation — replace the extra superclasses with fields, and implement methods that just delegate to those fields. It does require writing a lot of repetitive delegation methods, but it's rather unavoidable in Java (without resorting to AspectJ or other bytecode-munging tricks; careful, this way madness lies …).
This is a bit tangential, but you can have python code running in Java via Jython (http://www.jython.org/). This addresses the porting to Java part, not the solving multiple inheritance part (I think you need to determine which is relevant)
I'm working with a certain API library in Java. It has a base class A, as well as B and C which both extend A. B & C provide similar but distinct functionality, all three classes are in the library.
public abstract class A
{
virtual foo();
}
public class B extends A {}
public class C extends A {}
How do I get elements of A, B, and C in my class? If I use interfaces to implement the classes, there is a lot of duplicate code, and inner classes will not allow me to override existing methods so that the calling interfaces of A, B, and C are preserved.
How do I implement multiple inheritence in Java?
EDIT:
Thanks for edit George, its more clear now, forgot to mention one critical requirement: my classes must have A as a base so that they can be managed by platform API.
To recap, you have:
class A
{
public void foo() {}
}
class B extends A
{
public specificAMethod() {}
}
class C extends A
{
public specificCMethod() {}
}
The above classes are in a library that you can't access or modify.
You want to get the behaviour of both B and C in a third class D, as if it were possible to write:
class D extends B, C
{
}
Right?
What about using B and C instead of inheriting? Do you really need inheritance? You want to call private B and C methods?
class D
{
private B b;
private C c;
}
If you take the general approach of "Prefer Composition over Inheritance", you may find out that one or both of the classes shouldn't be actually "Inherited" anyway. In both cases, do the classes have a strict "is-a" relationship? If you can say "Has-a" out loud and it doesn't sound stupid, you probably want composition.
For instance, if you are implementing a deck of cards, the 3 of spades is-a 3 and it is-a spade, but you can also think of it as it has-a 3 and it has-a suit (spade). Since you can think of it that way, you should probably prefer the has-a interpretation and use composition.
Try to stay away from inheritance trees more than a few objects deep--and if you ever feel the desire to use multiple inheritance, try to work around it. This is one of the cases where Java kind of forces you to design your code to be a little more readable by not supplying a feature (but admittedly when you really want it, it's not there and that can hurt! Some level of mixin would be nice).
Sounds like you want to extend A - call it D - override its foo(), and then have new subclasses E & F that extend D and add their own functionality.
You might think about extracting common interfaces and reusing those. A good IDE with refactoring capability will make it easy to do.
Multiple class inheritance is not possible in Java, however you can use multiple inheritance for interfaces. Using Delegation pattern you can combine behavior of several other classes into one.
Completely different approach is applied in COM: all objects inherit from IUnknown, which has a method that could be translated to Java as:
Object queryInterface(Class<?> clazz)
The simplest implementation of this method can be:
if(clazz.isAssignableFrom(this.getClass()))
return this;
else
return null;
Where single inheritance won't work, it's just enough to add:
else if(class == SomeClass.class) {
return something;
} else ...
This way even most complex multiple inheritance cases can be tackled and you get full control over what is returned and when, so you avoid many problems with 'classical' multiple inheritance from languages like C++, like fork-join problem.
Since Java 1.8 you can use interfaces with default methods. It's very handy and I tend to use it a lot. Covariant return types are supported, too. There are a fiew restricktions (see below), but you dont have to implement the inheritence by your self and let the compiler work for you.
public interface A {
default A foo() {
return this;
}
}
public interface B extends A {
#Override
default B foo() {
return this;
}
}
public interface C extends A {
#Override
default C foo() {
return this;
}
}
public interface D extends B, C {
#Override
// if the return type does not implement B and C
// the comiler will throw an error here
default D foo() {
return this;
}
}
Note that this but note that you cant call super.foo or define fields or private members (until Java 9) since its still interfaces. If you get along with this restrictions, this opens up you a new level of object oriented programming.
I am reading "The Java Tutorial" (for the 2nd time). I just got through the section on Interfaces (again), but still do not understand how Java Interfaces simulate multiple inheritance. Is there a clearer explanation than what is in the book?
Suppose you have 2 kinds of things in your domain : Trucks and Kitchens
Trucks have a driveTo() method and Kitchens a cook() method.
Now suppose Pauli decides to sell pizzas from the back of a delivery truck. He wants a thing where he can driveTo() and cook() with.
In C++ he would use multiple inheritance to do this.
In Java that was considered to be too dangerous so you can inherit from a main class, but you can "inherit" behaviors from interfaces, which are for all intents and purposes abstract classes with no fields or method implementations.
So in Java we tend to implement multiple inheritance using delegations :
Pauli subclasses a truck and adds a kitchen to the truck in a member variable called kitchen. He implements the Kitchen interface by calling kitchen.cook().
class PizzaTruck extends Truck implements Kitchen {
Kitchen kitchen;
public void cook(Food foodItem) {
kitchen.cook(foodItem);
}
}
He is a happy man because he can now do things like ;
pizzaTruck.driveTo(beach);
pizzaTruck.cook(pizzaWithExtraAnchovies);
Ok, this silly story was to make the point that it is no simulation of multiple inheritance, it is real multiple inheritance with the proviso that you can only inherit the contract, only inherit from empty abstract base classes which are called interfaces.
(update: with the coming of default methods interfaces now can also provide some behavior to be inherited)
You're probably confused because you view multiple inheritance locally, in terms of one class inheriting implementation details from multiple parents. This is not possible in Java (and often leads to abuse in languages where it's possible).
Interfaces allow multiple inheritance of types, e.g. a class Waterfowl extends Bird implements Swimmer can be used by other classes as if it were a Bird and as if it were a Swimmer. This is the the deeper meaning of multiple inheritance: allowing one object to act like it belongs to several unrelated different classes at once.
Here is a way to achieve multiple inheritance through interfaces in java.
What to achieve?
class A extends B, C // this is not possible in java directly but can be achieved indirectly.
class B{
public void getValueB(){}
}
class C{
public void getValueC(){}
}
interface cInterface{
public getValueC();
}
class cChild extends C implemets cInterface{
public getValueC(){
// implementation goes here, call the super class's getValueC();
}
}
// Below code is **like** class A extends B, C
class A extends B implements cInterface{
cInterface child = new cChild();
child.getValueC();
}
given the two interfaces below...
interface I1 {
abstract void test(int i);
}
interface I2 {
abstract void test(String s);
}
We can implement both of these using the code below...
public class MultInterfaces implements I1, I2 {
public void test(int i) {
System.out.println("In MultInterfaces.I1.test");
}
public void test(String s) {
System.out.println("In MultInterfaces.I2.test");
}
public static void main(String[] a) {
MultInterfaces t = new MultInterfaces();
t.test(42);
t.test("Hello");
}
}
We CANNOT extend two objects, but we can implement two interfaces.
Interfaces don't simulate multiple inheritance. Java creators considered multiple inheritance wrong, so there is no such thing in Java.
If you want to combine the functionality of two classes into one - use object composition. I.e.
public class Main {
private Component1 component1 = new Component1();
private Component2 component2 = new Component2();
}
And if you want to expose certain methods, define them and let them delegate the call to the corresponding controller.
Here interfaces may come handy - if Component1 implements interface Interface1 and Component2 implements Interface2, you can define
class Main implements Interface1, Interface2
So that you can use objects interchangeably where the context allows it.
It's pretty simple. You can implement more than one interface in a type. So for example, you could have an implementation of List that is also an instance of Deque (and Java does...LinkedList).
You just can't inherit implementations from multiple parents (i.e. extend multiple classes). Declarations (method signatures) are no problem.
You know what, coming from the perspective of a JavaScript dev trying to understand what the heck is going on with this stuff, I'd like to point out a couple things and somebody please tell me what I'm missing here if I'm way off the mark.
Interfaces are really simple. Stupidly, insanely simple. They're as stupidly, insanely simple as people initially think, which is why there are so many duplicate questions on this exact subject because the one reason to use them has been made unclear by people trying to make more of them than they are and there is widespread misuse in every Java server-side code-base I've ever been exposed to.
So, why would you want to use them? Most of the time you wouldn't. You certainly wouldn't want to use them ALL the time as many seem to think. But before I get to when you would, let's talk about what they're NOT.
Interfaces are NOT:
in any way a workaround for any sort of inheritance mechanism that Java lacks. They have nothing to do with inheritance, they never did, and in no way simulate anything inheritance-like.
necessarily something that helps you with stuff you wrote, so much as it helps the other guy write something meant to be interfaced by your stuff.
They really are as simple as you think they are on first glance. People misuse stupidly all the time so it's hard to understand what the point is. It's just validation/testing. Once you've written something conforms to an interface and works, removing that "implements" code won't break anything.
But if you're using interfaces correctly, you wouldn't want to remove it because having it there gives the next developer a tool for writing an access layer for another set of databases or web services that they want the rest of your app to continue using because they know their class will fail until they get the 100% complete-as-expected-interface in place. All interfaces do is validate your class and establish that you have in fact implemented an interface as you promised you would. Nothing more.
They're also portable. By exposing your interface definitions you can give people wanting to use your unexposed code a set of methods to conform to in order for their objects to use it correctly. They don't have to implement the interfaces. They could just jot them down on a piece of notepad paper and double-check that. But with the interface you have more of a guarantee nothing is going to try to work until it has a proper version of the interface in question.
So, any interface not likely to ever be implemented more than once? Completely useless. Multiple-inheritance? Stop reaching for that rainbow. Java avoids them for a reason in the first place and composited/aggregate objects are more flexible in a lot of ways anyway. That's not to say interfaces can't help you model in ways that multiple-inheritance allows but it's really not inheritance in any way shape or form and shouldn't be seen as such. It's just guaranteeing that your code won't work until you've implemented all of the methods you established that you would.
It's not a simulation of multiple inheritance. In java you can't inherit from two classes, but if you implements two interfaces "it seems like you inherited from two different classes" because you can use your class as any of your two intefaces.
For example
interface MyFirstInteface{
void method1();
}
interface MySecondInteface{
void method2();
}
class MyClass implements MyFirstInteface, MySecondInteface{
public void method1(){
//Method 1
}
public void method2(){
//Method 2
}
public static void main(String... args){
MyFirstInterface mfi = new MyClass();
MySecondInterface msi = new MyClass();
}
}
This will work and you can use mfi and msi, it seems like a multi inheritance, but it's not because you don't inherit anything, you just rewrite public methods provided by the interfaces.
You need to be precise:
Java allows multiple inheritance of interface, but only single inheritance of implementation.
You do multiple inheritance of interface in Java like this:
public interface Foo
{
String getX();
}
public interface Bar
{
String getY();
}
public class MultipleInterfaces implements Foo, Bar
{
private Foo foo;
private Bar bar;
public MultipleInterfaces(Foo foo, Bar bar)
{
this.foo = foo;
this.bar = bar;
}
public String getX() { return this.foo.getX(); }
public String getY() { return this.bar.getY(); }
}
Just by the way, the reason why Java does not implement full multiple inheritance is because it creates ambiguities. Suppose you could say "A extends B, C", and then both B and C have a function "void f(int)". Which implementation does A inherit? With Java's approach, you can implement any number of interfaces, but interfaces only declare a signature. So if two interfaces include functions with the same signature, fine, your class must implement a function with that signature. If interfaces you inherit have functions with different signatures, then the functions have nothing to do with each other, so there is no question of a conflict.
I'm not saying this is the only way. C++ implements true multiple inheritance by establishing precedence rules of which implementation wins. But the authors of Java decided to eliminate the ambiguity. Whether because of a philosophical belief that this made for cleaner code, or because they didn't want to do all the extra work, I don't know.
It's not fair to say that interfaces 'simulate' multiple inheritance.
Sure, your type can implement multiple interfaces and act as many different types polymorphically. However, you obviously won't inherit behaviour or implementations under this arrangement.
Generally look at composition where you think you may need multiple inheritance.
OR A potential solution to achieving something multiple inheritance like is the Mixin interface - http://csis.pace.edu/~bergin/patterns/multipleinheritance.html. Use with care!
They don't.
I think that the confusion comes from people believing that implementing an interface constitutes some form of inheritance. It doesn't; the implementation can simply be blank, no behavior is forced by the act or guaranteed through any contract. A typical example is the Clonable-interface, which while alluding to lots of great functionality, which defines so little that's it's essentially useless and potentially dangerous.
What do you inherit by implementing an interface? Bubkes! So in my opinion, stop using the words interface and inheritance in the same sentence. As Michael Borgwardt said, an interface is not a definition but an aspect.
You can actually "inherit" from multiple concrete classes if they implement interfaces themselves. innerclasses help you achieve that:
interface IBird {
public void layEgg();
}
interface IMammal {
public void giveMilk();
}
class Bird implements IBird{
public void layEgg() {
System.out.println("Laying eggs...");
}
}
class Mammal implements IMammal {
public void giveMilk() {
System.out.println("Giving milk...");
}
}
class Platypus implements IMammal, IBird {
private class LayingEggAnimal extends Bird {}
private class GivingMilkAnimal extends Mammal {}
private LayingEggAnimal layingEggAnimal = new LayingEggAnimal();
private GivingMilkAnimal givingMilkAnimal = new GivingMilkAnimal();
#Override
public void layEgg() {
layingEggAnimal.layEgg();
}
#Override
public void giveMilk() {
givingMilkAnimal.giveMilk();
}
}
I'd like to point out something that bit me in the behind, coming from C++ where you can easily inherit many implementations too.
Having a "wide" interface with many methods means that you'll have to implement a lot of methods in your concrete classes and you can't share these easily across implementations.
For instance:
interface Herbivore {
void munch(Vegetable v);
};
interface Carnivore {
void devour(Prey p);
}
interface AllEater : public Herbivore, Carnivore { };
class Fox implements AllEater {
...
};
class Bear implements AllEater {
...
};
In this example, Fox and Bear cannot share a common base implementation for both it's interface methods munch and devour.
If the base implementations look like this, we'd maybe want to use them for Fox and Bear:
class ForestHerbivore implements Herbivore
void munch(Vegetable v) { ... }
};
class ForestCarnivore implements Carnivore
void devour(Prey p) { ... }
};
But we can't inherit both of these. The base implementations need to be member variables in the class and methods defined can forward to that. I.e:
class Fox implements AllEater {
private ForestHerbivore m_herbivore;
private ForestCarnivore m_carnivore;
void munch(Vegetable v) { m_herbivore.munch(v); }
void devour(Prey p) { m_carnivore.devour(p); }
}
This gets unwieldy if interfaces grow (i.e. more than 5-10 methods...)
A better approach is to define an interface as an aggregation of interfaces:
interface AllEater {
Herbivore asHerbivore();
Carnivore asCarnivore();
}
This means that Fox and Bear only has to implement these two methods, and the interfaces and base classes can grow independetly of the aggregate AllEater interface that concerns the implementing classes.
Less coupling this way, if it works for your app.
I don't think they do.
Inheritance is specifically an implementation-oriented relationship between implementations. Interfaces do not provide any implementation information at all, but instead define a type. To have inheritance, you need to specifically inherit some behaviors or attributes from a parent class.
I believe there is a question here somewhere specifically about the role of interfaces and multiple inheritance, but I can't find it now...
There's really no simulation of multiple inheritance in Java.
People will sometimes say that you can simulate multiple inheritance using Interfaces because you can implement more than one interface per class, and then use composition (rather than inheritance) in your class to achieve the behaviors of the multiple classes that you were trying to inherit from to begin with.
If it makes sense in your object model, you can of course inherit from one class and implement 1 or more interfaces as well.
There are cases where multiple-inheritance turns to be very handy and difficult to replace with interfaces without writing more code. For example, there are Android apps that use classes derived from Activity and others from FragmentActivity in the same app. If you have a particular feature you want to share in a common class, in Java you will have to duplicate code instead of let child classes of Activity and FragmentsActivity derive from the same SharedFeature class. And the poor implementation of generics in Java doesn't help either because writing the following is illegal:
public class SharedFeature<T> extends <T extends Activity>
...
...
There is no support for multiple inheritance in java.
This story of supporting multiple inheritance using interface is what we developers cooked up. Interface gives flexibility than concrete classes and we have option to implement multiple interface using single class. This is by agreement we are adhering to two blueprints to create a class.
This is trying to get closer to multiple inheritance. What we do is implement multiple interface, here we are not extending (inheriting) anything. The implementing class is the one that is going to add the properties and behavior. It is not getting the implementation free from the parent classes. I would simply say, there is no support for multiple inheritance in java.
No, Java does not support multiple inheritance.
Neither using class nor using interface. Refer to this link for more info
https://devsuyed.wordpress.com/2016/07/21/does-java-support-multiple-inheritance
I also have to say that Java doesn't support multiple inheritance.
You have to differentiate the meaning between extends and implements keywords in Java. If we use extends, we are actually inheriting the class after that keyword. But, in order to make everything simple, we can't use extends more than once. But you can implement as many Interfaces as you wish.
If you implement an interface, there's a zero chance that you will miss the implementation of all the methods in each interface (Exception: default implementations of interface methods introduced in Java 8) So, you are now fully aware of what is happening with the things that you have embedded to your fresh class.
Why Java doesn't allow multiple inheritance is actually, multiple inheritance makes the code somewhat complex. Sometimes, two methods of parent classes might conflict due to having the same signatures. But if you are forced to implement all the methods manually, you will get the full understanding about what's going on, as I mentioned above. It makes your code more understandable to you.
If you need more info on Java interfaces, check out this article, http://www.geek-programmer.com/introduction-to-java-interfaces/
Between two Java class multiple Inheritance directly is not possible. In this case java recommend Use to interface and declare method inside interface and implement method with Child class.
interface ParentOne{
public String parentOneFunction();
}
interface ParentTwo{
public String parentTwoFunction();
}
class Child implements ParentOne,ParentTwo{
#Override
public String parentOneFunction() {
return "Parent One Finction";
}
#Override
public String parentTwoFunction() {
return "Parent Two Function";
}
public String childFunction(){
return "Child Function";
}
}
public class MultipleInheritanceClass {
public static void main(String[] args) {
Child ch = new Child();
System.out.println(ch.parentOneFunction());
System.out.println(ch.parentTwoFunction());
System.out.println(ch.childFunction());
}
}
Java doesn't allow multiple inheritance, but it allows implementing multiple interfaces. Why?
Because interfaces specify only what the class is doing, not how it is doing it.
The problem with multiple inheritance is that two classes may define different ways of doing the same thing, and the subclass can't choose which one to pick.
One of my college instructors explained it to me this way:
Suppose I have one class, which is a Toaster, and another class, which is NuclearBomb. They both might have a "darkness" setting. They both have an on() method. (One has an off(), the other doesn't.) If I want to create a class that's a subclass of both of these...as you can see, this is a problem that could really blow up in my face here.
So one of the main issues is that if you have two parent classes, they might have different implementations of the same feature — or possibly two different features with the same name, as in my instructor's example. Then you have to deal with deciding which one your subclass is going to use. There are ways of handling this, certainly — C++ does so — but the designers of Java felt that this would make things too complicated.
With an interface, though, you're describing something the class is capable of doing, rather than borrowing another class's method of doing something. Multiple interfaces are much less likely to cause tricky conflicts that need to be resolved than are multiple parent classes.
Because inheritance is overused even when you can't say "hey, that method looks useful, I'll extend that class as well".
public class MyGodClass extends AppDomainObject, HttpServlet, MouseAdapter,
AbstractTableModel, AbstractListModel, AbstractList, AbstractMap, ...
The answer of this question is lies in the internal working of java compiler(constructor chaining).
If we see the internal working of java compiler:
public class Bank {
public void printBankBalance(){
System.out.println("10k");
}
}
class SBI extends Bank{
public void printBankBalance(){
System.out.println("20k");
}
}
After compiling this look like:
public class Bank {
public Bank(){
super();
}
public void printBankBalance(){
System.out.println("10k");
}
}
class SBI extends Bank {
SBI(){
super();
}
public void printBankBalance(){
System.out.println("20k");
}
}
when we extends class and create an object of it, one constructor chain will run till Object class.
Above code will run fine. but if we have another class called Car which extends Bank and one hybrid(multiple inheritance) class called SBICar:
class Car extends Bank {
Car() {
super();
}
public void run(){
System.out.println("99Km/h");
}
}
class SBICar extends Bank, Car {
SBICar() {
super(); //NOTE: compile time ambiguity.
}
public void run() {
System.out.println("99Km/h");
}
public void printBankBalance(){
System.out.println("20k");
}
}
In this case(SBICar) will fail to create constructor chain(compile time ambiguity).
For interfaces this is allowed because we cannot create an object of it.
For new concept of default and static method kindly refer default in interface.
Hope this will solve your query.
Thanks.
You can find accurate answer for this query in oracle documentation page about multiple inheritance
Multiple inheritance of state: Ability to inherit fields from multiple classes
One reason why the Java programming language does not permit you to extend more than one class is to avoid the issues of multiple inheritance of state, which is the ability to inherit fields from multiple classes
If multiple inheritance is allowed and When you create an object by instantiating that class, that object will inherit fields from all of the class's superclasses. It will cause two issues.
What if methods or constructors from different super classes instantiate the same field?
Which method or constructor will take precedence?
Multiple inheritance of implementation: Ability to inherit method definitions from multiple classes
Problems with this approach: name conflicts and ambiguity. If a subclass and superclass contain same method name (and signature), compiler can't determine which version to invoke.
But java supports this type of multiple inheritance with default methods, which have been introduced since Java 8 release. The Java compiler provides some rules to determine which default method a particular class uses.
Refer to below SE post for more details on resolving diamond problem:
What are the differences between abstract classes and interfaces in Java 8?
Multiple inheritance of type: Ability of a class to implement more than one interface.
Since interface does not contain mutable fields, you do not have to worry about problems that result from multiple inheritance of state here.
Java does not support multiple inheritance because of two reasons:
In java, every class is a child of Object class. When it inherits from more than one super class, sub class gets the ambiguity to
acquire the property of Object class..
In java every class has a constructor, if we write it explicitly or not at all. The first statement is calling super() to invoke the
supper class constructor. If the class has more than one super class, it
gets confused.
So when one class extends from more than one super class, we get compile time error.
Java supports multiple inheritance through interfaces only. A class can implement any number of interfaces but can extend only one class.
Multiple inheritance is not supported because it leads to deadly diamond problem. However, it can be solved but it leads to complex system so multiple inheritance has been dropped by Java founders.
In a white paper titled “Java: an Overview” by James Gosling in February 1995(link - page 2) gives an idea on why multiple inheritance is not supported in Java.
According to Gosling:
"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."
Implementing multiple interfaces is very useful and doesn't cause much problems to language implementers nor programmers. So it is allowed. Multiple inheritance while also useful, can cause serious problems to users (dreaded diamond of death). And most things you do with multiple inheritance can be also done by composition or using inner classes. So multiple inheritance is forbidden as bringing more problems than gains.
It is said that objects state is referred with respect to the fields in it and it would become ambiguous if too many classes were inherited. Here is the link
http://docs.oracle.com/javase/tutorial/java/IandI/multipleinheritance.html
Since this topic is not close I'll post this answer, I hope this helps someone to understand why java does not allow multiple inheritance.
Consider the following class:
public class Abc{
public void doSomething(){
}
}
In this case the class Abc does not extends nothing right? Not so fast, this class implicit extends the class Object, base class that allow everything work in java. Everything is an object.
If you try to use the class above you'll see that your IDE allow you to use methods like: equals(Object o), toString(), etc, but you didn't declare those methods, they came from the base class Object
You could try:
public class Abc extends String{
public void doSomething(){
}
}
This is fine, because your class will not implicit extends Object but will extends String because you said it. Consider the following change:
public class Abc{
public void doSomething(){
}
#Override
public String toString(){
return "hello";
}
}
Now your class will always return "hello" if you call toString().
Now imagine the following class:
public class Flyer{
public void makeFly(){
}
}
public class Bird extends Abc, Flyer{
public void doAnotherThing(){
}
}
Again class Flyer implicit extends Object which has the method toString(), any class will have this method since they all extends Object indirectly, so, if you call toString() from Bird, which toString() java would have to use? From Abc or Flyer? This will happen with any class that try to extends two or more classes, to avoid this kind of "method collision" they built the idea of interface, basically you could think them as an abstract class that does not extends Object indirectly. Since they are abstract they will have to be implemented by a class, which is an object (you cannot instanciate an interface alone, they must be implemented by a class), so everything will continue to work fine.
To differ classes from interfaces, the keyword implements was reserved just for interfaces.
You could implement any interface you like in the same class since they does not extends anything by default (but you could create a interface that extends another interface, but again, the "father" interface would not extends Object"), so an interface is just an interface and they will not suffer from "methods signature colissions", if they do the compiler will throw a warning to you and you will just have to change the method signature to fix it (signature = method name + params + return type).
public interface Flyer{
public void makeFly(); // <- method without implementation
}
public class Bird extends Abc implements Flyer{
public void doAnotherThing(){
}
#Override
public void makeFly(){ // <- implementation of Flyer interface
}
// Flyer does not have toString() method or any method from class Object,
// no method signature collision will happen here
}
For the same reason C# doesn't allow multiple inheritence but allows you to implement multiple interfaces.
The lesson learned from C++ w/ multiple inheritence was that it lead to more issues than it was worth.
An interface is a contract of things your class has to implement. You don't gain any functionality from the interface. Inheritence allows you to inherit the functionality of a parent class (and in multiple-inheritence, that can get extremely confusing).
Allowing multiple interfaces allows you to use Design Patterns (like Adapter) to solve the same types of issues you can solve using multiple inheritence, but in a much more reliable and predictable manner.
For example two class A,B having same method m1(). And class C extends both A, B.
class C extends A, B // for explaining purpose.
Now, class C will search the definition of m1. First, it will search in class if it didn't find then it will check to parents class. Both A, B having the definition So here ambiguity occur which definition should choose.
So JAVA DOESN'T SUPPORT MULTIPLE INHERITANCE.
in simple manner we all know, we can inherit(extends) one class but we can implements so many interfaces.. that is because in interfaces we don't give an implementation just say the functionality. suppose if java can extends so many classes and those have same methods.. in this point if we try to invoke super class method in the sub class what method suppose to run??, compiler get confused
example:- try to multiple extends
but in interfaces those methods don't have bodies we should implement those in sub class..
try to multiple implements
so no worries..
Multiple inheritance is not supported by class because of ambiguity.
(this point is explained clearly in above answers using super keyword)
Now for interfaces,
upto Java 7, interfaces could not define the implementation of methods. So if class implements from multiple interfaces having same method signature then implementation of that method is to be provided by that class.
from java 8 onwards, interfaces can also have implementation of methods. So if class implements two or more interfaces having same method signature with implementation, then it is mandated to implement the method in that class also.
From Java 9 onwards, interfaces can contain Static methods, Private methods, Private Static methods.
Modifications in features of Interfaces (over java version-7,8,9)
Because an interface is just a contract. And a class is actually a container for data.
Consider a scenario where Test1, Test2 and Test3 are three classes. The Test3 class inherits Test2 and Test1 classes. If Test1 and Test2 classes have same method and you call it from child class object, there will be ambiguity to call method of Test1 or Test2 class but there is no such ambiguity for interface as in interface no implementation is there.
Java does not support multiple inheritance , multipath and hybrid inheritance because of the following ambiguity
problem.
Scenario for multiple inheritance: Let us take class A , class B , class C. class A has alphabet(); method , class B has also alphabet(); method. Now class C extends A, B and we are creating object to the subclass i.e., class C , so C ob = new C(); Then if you want call those methods ob.alphabet(); which class method takes ? is class A method or class B method ? So in the JVM level ambiguity problem occurred. Thus Java does not support multiple inheritance.
multiple inheritance
Reference Link: https://plus.google.com/u/0/communities/102217496457095083679
Take for example the case where Class A has a getSomething method and class B has a getSomething method and class C extends A and B. What would happen if someone called C.getSomething? There is no way to determine which method to call.
Interfaces basically just specify what methods a implementing class needs to contain. A class that implements multiple interfaces just means that class has to implement the methods from all those interfaces. Whci would not lead to any issues as described above.
the image explaining the problem with multiple inheritances.
What is the inherited member of the derived class? it is still private or publically available in the derived class?
For not getting this type of problem in Java they removed multiple inheritance. This image is a simple example of an object-oriented programming problem.
* This is a simple answer since I'm a beginner in Java *
Consider there are three classes X,Y and Z.
So we are inheriting like X extends Y, Z
And both Y and Z is having a method alphabet() with same return type and arguments. This method alphabet() in Y says to display first alphabet and method alphabet in Z says display last alphabet.
So here comes ambiguity when alphabet() is called by X. Whether it says to display first or last alphabet???
So java is not supporting multiple inheritance.
In case of Interfaces, consider Y and Z as interfaces. So both will contain the declaration of method alphabet() but not the definition. It won't tell whether to display first alphabet or last alphabet or anything but just will declare a method alphabet(). So there is no reason to raise the ambiguity. We can define the method with anything we want inside class X.
So in a word, in Interfaces definition is done after implementation so no confusion.
It is a decision to keep the complexity low.
With hybrid inheritance, things would have been more complicated to implement, and anyways what is achievable by multiple inheritances is also with other ways.