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Why implement interfaces 'A' and 'B', when 'B' extends 'A'

Suppose I have the following code...
interface A{
void a();
}
interface B extends A{
void b();
}
class ImplementOne implements B{
public void a(){};
public void b(){};
}
class ImplementTwo implements B, A{
public void a(){};
public void b(){};
}
Regardless of whether class ImplementTwo implements both B and A, or just B, it would still need to implement method a() in interface A, since interface B extends interface A. Is there any reason one would explicitly do
...implements B, A
instead of just
...implements B
?

There is no difference between the two approaches in terms of behavior. In terms of bytecode information, there is a small difference when it comes to information about implemented interfaces. For example:
Class<?>[] interfaces = ImplementTwo.class.getInterfaces();
for (int i = 0; i < interfaces.length; i++) {
System.out.println(interfaces[i]);
}
would return two class instances when implements B, A is used, whereas when using implements B it would return one instance.
Still, the following returns true using both approaches:
A.class.isAssignableFrom(ImplementTwo.class)

IMO the only reason you would want to specify it explicitly like that is if you were attempting to make the code more easily readable by others who needed to interact with it. That even being said, really, a two-step indirection like this is not so abstract that it's difficult to follow, so I don't really think this would ever have a need to happen.

The most famous example is of the use of the interface Serializable.
This is often repeated for the purpose of: Should the super interface suddenly gets detached from Serializable interface, it's sub-interface will still remain Serializable since it's already defined as Serializable.
This often occurs doing code refactoring. Other than that, there is no difference.

Both variants are exactly the same semantically. You might prefer one over the other for stylistic reasons (for example, to make it immediately clear to the reader of the class that it implements both A and B).

There is no difference in how the code will behave (or compile).
Some people prefer explicitly listing all implemented interfaces even if they are enforced by another interface implemented. It's purely a matter of personal/code style preferences.

Both approaches are equal. You might choose implements A, B instead of implements B to specify whole list of types for object ithout knowledge about A-B hierarchy

In this case it doesn't make difference. But technically you could have two interfaces which are not related one to other with same method declaration. And this implementation would implement method for both interfaces.

Why doesn't interfaces implement methods and override them?

I already read the post of research effort required to post a SO question. I am ashamed again to post this question to a pile of million questions. But I still don't get the idea of interfaces in java. They have unimplemented methods and then defined for every class in which they are implemented. I searched about it. Interfaces were used to support multiple inheritance in java and also to avoid (Deadly) Diamond Death of inheritance. I also came across Composition vs Inheritance and that inheritance is not for code reuse and its for polymorphism. So when I have a common code as a class to extend it will not be supported due to multiple inheritance which gives the option to use Interfaces(Correct me if I am wrong). I also came across that its not possible in most cases to define a generic implementation. So what is the problem in having a common definition (not a perfect generic implementation) of the interface method and then Override it wherever necessary and why doesn't java support it. Eg. When I have 100 classes that implements an interface 70 of them have a common implementation while others have different implementation. Why do I have to define the common method in interface over 70 classes and why can't I define them in Interface and then override them in other 30 classes which saves me from using same code in 70 classes. Is my understanding of interfaces wrong?

First, an interface in Java (as of Java 7) has no code. It's a mere definition, a contract a class must fulfill.
So what is the problem in having a common definition (not a perfect
generic implementation) of the interface method and then Override it
wherever necessary and why doesn't java support it
Yes you can do that in Java, just not with interfaces only. Let's suppose I want from this Example interface to have a default implementation for method1 but leave method2 unimplemented:
interface Example {
public void method1();
public String method2(final int parameter);
}
abstract class AbstractExampleImpl implements Example {
#Override
public void method1() {
// Implement
}
}
Now classes that want to use this method1 default implementation can just extend AbstractExampleImpl. This is more flexible than implementing code in the interface because if you do so, then all classes are bound to that implementation which you might not want. This is the advantage of interfaces: being able to reference a certain behavior (contract) without having to know how the class actually implements this, for example:
List<String> aList = MyListFactory.getNewList();
MyListFactory.getNewList() can return any object implementing List, our code manipulating aList doesn't care at all because it's based on the interface.
What if the class that uses interface already is a Sub-class. Then we
can't use Abstract class as multiple inheritance is not supported
I guess you mean this situation:
class AnotherClass extends AnotherBaseClass
and you want to extend AbstractExampleImpl as well. Yes, in this case, it's not possible to make AnotherClass extend AbstractExampleImpl, but you can write a wrapped inner-class that does this, for example:
class AnotherClass extends AnotherBaseClass implements Example {
private class InnerExampleImpl extends AbstractExampleImpl {
// Here you have AbstractExampleImpl's implementation of method1
}
}
Then you can just internally make all Example methods being actually implemented by InnerExampleImpl by calling its methods.
Is it necessary to have the interface in AnotherClass?
I guess you mean AnotherClass implements Example. Well, this is what you wanted: have AnotherClass implement Example with some default implementation as well as extend another class, or I understood you wrong. Since you cannot extend more than one class, you have to implement the interface so you can do
final Example anotherClass = new AnotherClass();
Otherwise this will not be possible.
Also for every class that implements an interface do I have to design
an inner class?
No, it doesn't have to be an inner class, that was just an example. If you want multiple other classes have this default Example implementation, you can just write a separate class and wrap it inside all the classes you want.
class DefaultExampleImpl implements Example {
// Implements the methods
}
class YourClass extends YetAnotherClass implements Example {
private Example example = new DefaultClassImpl();
#Override
public void method1() {
this.example.method1();
}
#Override
public String method2(final int parameter) {
return this.example.method2(parameter);
}
}

You can create an abstract class to implement that interface, and make your those classes inherit that abstract class, that should be what you want.

A non abstract class that implements and interface needs to implement all the methods from the interface. A abstract class doesn't have to implement all the methods but cannot initiated. If you create abstract class in your example that implements all the interface methods except one. The classes that extend from these abstract class just have to implement the one not already implemented method.

The Java interfaces could have been called contracts instead to better convey their intent. The declarer promise to provide some functionality, and the using code is guaranteed that the object provides that functionality.
This is a powerful concept and is decoupled from how that functionality is provided where Java is a bit limited and you are not the first to notice that. I have personally found that it is hard to provide "perfect" implementations which just need a subclass or two to be usable in a given situation. Swing uses adapters to provide empty implementations which can then be overrides as needed and that may be the technique you are looking for.

The idea of the interface is to create a series of methods that are abstract enough to be used by different classes that implement them. The concept is based on the DRY principle (Don't repeat yourself) the interface allows you to have methods like run() that are abstract enough to be usuable for a game loop, a players ability to run,

You should understand the funda of interface first. Which is
It is use to provide tight coupling means tight encapsulation
It helps us to hide our code from the external environment i.e. from other class
Interface should have only definition and data which is constant
It provide facility to class open for extension. Hence it cannot be replace by the any other class in java otherwise that class will become close for extension. which means class will not be able to extend any other class.

I think you are struggling with the concept of Object Oriented Design more than anything. In your example above where you state you have 100 classes and 70 of them have the same method implementation (which I would be stunned by). So given an interface like this:
public interface Printable
{
void print();
}
and two classes that have the "same" implementation of print
public class First implements Printable
{
public void print()
{
System.out.println("Hi");
}
}
public class Second implements Printable
{
public void print()
{
System.out.println("Hi");
}
}
you would instead want to do this:
public abstract class DefaultPrinter implements Printable
{
public void print()
{
System.out.println("Hi");
}
}
now for First and Second
public class First extends DefaultPrinter
{
}
public class Second extends DefaultPrinter
{
}
Now both of these are still Printable . Now this is where it gets very important to understand how to properly design object hierarchies. If something IS NOT a DefaultPrinter YOU CANNOT AND SHOULD NOT make the new class extend DefaultPrinter

Why interface methods have no body

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.

How do I implement multiple inheritence in Java

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.

How do Java Interfaces simulate multiple inheritance?

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());
}
}