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
Related
It is said that Java has an alternative to achieve multiple inheritance by implementing number of interfaces. According to the Java documentation about inheritance found here:
A subclass inherits all the members (fields, methods, and nested classes) from its superclass . The idea of inheritance is simple but powerful: When you want to create a new class and there is already a class that includes some of the code that you want, you can derive your new class from the existing class. In doing this, you can reuse the fields and methods of the existing class without having to write (and debug!) them yourself.
and about interfaces here, it says that interfaces don't have methods implemented. They just contain method declarations. Also, as far as I know, interfaces don't contain nested classes either. One more point, not all the interfaces have constant fields, many a time the interfaces (which I use for my work) contain just method declarations.
In that case, what do we really inherit from them, if we have to define all the methods ourselves? I know the other uses of interfaces very well. But I am not getting what it has to do with inheritance.
In that case, what do we really inherit from them, if we have to define all the methods ourselves?
In classic Java (7 and earlier), implementing an interface gives you:
an "is-a" relationship with an abstraction,
a set of method signatures that have to be implemented (to satisfy the "is-a" relationship), and
an implied "contract" that says what the methods are supposed to do.
In Java 8, you also inherit default methods declared in the interface.
It is true that a class the implements an interface doesn't inherent any state declarations, or methods that can access the state declarations, as it would if it extend a class. But the flip-side is that a class can implement multiple interfaces ... which you can't do with a extend.
In short, implements allows you to do things you can't do with extends and vice versa.
Interfaces offer a contract, meaning that when a class implements an interface, then, by definition it provides a set of methods. How these are implemented is left to the class implementing the interface.
Consider:
public interface IPrint
{
public void print(String str);
}
public interface IClean
{
public void clean();
}
public class CanonPrint implements IPrint, IClean //Prints to a physical canon printer
{
public void print(String str)
{
//Send data to canon printer
}
public void clean()
{
//Clean printer buffer, etc
}
}
public class Console implements IPrint, IClean //Prints to a console
{
public void print(String str)
{
//Send data to console
}
public void clean()
{
//Clean console buffer, etc
}
}
The above example, you know that the CanonPrint and ConsolePrint classes can print and clean after themselves, they have the logic to do it, the fact that they implement the IPrint and IClean interfaces guarantees it. Nothing is inherited, there is no functionality being extended, just a guaranteed a set of functions. The logic, etc, is delegated.
For example a real estate builder is constructing an apartment with many flats. All the rooms in the flats have the same design, except the bedroom. The bedroom design is left for the people who would own the flats i.e; the bed Rooms can be of different designs for different flats.
I can achieve this through an abstract class like below:
public abstract class Flat
{
//some properties
public void livingRoom(){
//some code
}
public void kitchen(){
//some code
}
public abstract void bedRoom();
}
}
An implementation class would be as follows:
public class Flat101 extends Flat
{
public void bedRoom() {
System.out.println("This flat has a customized bedroom");
}
}
Alternatively I can use an interface instead of an abstract class to achieve the same purpose like follows:
class Flat
{
public void livingRoom(){
System.out.println("This flat has a living room");
}
public void kitchen(){
System.out.println("This flat has a kitchen");
}
}
interface BedRoomInterface
{
public abstract void bedRoom();
}
public class Flat101 extends Flat implements BedRoomInterface
{
public void bedRoom() {
System.out.println("This flat has a customized bedroom");
}
}
Now the question is : For this why should choose to use an interface (or) why should I choose to use an abstract class?
It depends on your intention or use case. But in general, you should prefer interface over abstract classes (Item 18 in Bloch's Effective Java). Abstract classes are more fragile, because someone may modify the abstract class changing the behavior of other classes extending from it (this is a general statement).
It's more flexible to work with interfaces, because if you have BedroomInterface and LivingRoomInterface, then you can have FlatInterface implementing both interfaces, then Flat101 implementation class implements FlatInterface (instead of extending from Flat then implementing an interface). This seems clearer, and later on you can have ExecutiveFlatInterface which not only have bedroom and living room but also guess room, then Flat102 can implement from it.
Option 2 is to have Flat101 extend from Flat, then Flat implements BedroomInterface and LivingRoomInterface. This really depends on what you want to do and what methods are likely needed.
If you're designing an API that is going to be widely used, you'd use both: an interface to express the contract to be fulfilled by implementing classes, and an abstract class which partially implements that interface and thus permits code re-use.
As an example, consider Java's List: methods in the Collections framework (eg Collections.sort()) are written in terms of the List interface, which is partially implemented by the abstract class AbstractList, which in turn is extended into the concrete implementations LinkedList and ArrayList. LinkedList and ArrayList re-use code from AbstractList, but that does not prevent someone from writing their own completely separate implementation of List and then sorting it using Collections.sort().
That said, in a lot of circumstances this approach can be overkill. If the type hierarchy you're building is only used within a relatively small scope, its generally fine to just use abstract classes. If you decide later on that you want an interface later, its a pretty painless refactoring task to change things.
Abstract classes do have a few advantages:
they allow you to specify abstract methods with package/protected modifiers
they facilitate code re-use
via the use of abstract methods and final methods on the super class they allow you to restrict the manner in which your class is subclassed, which can be useful in a wide variety of circumstances (see also: the Template pattern)
code that references classes is generally easier to follow in an IDE (clicking "open declaration" on an abstract class type parameter is usually more useful than on an interface type parameter)
If you have a class which provides some of the functionality required by derived classes, but each derived class additionally requires differing implementation of other functionality, then an abstract class provides a means of defining the common implementation, while leaving the specific behaviors required by derived classes to be made specific to each derived class.
I feel it is generalization means; an abstract class is most useful if the property and the behaviors of the class are common among a given package or module. One good example is drum brake; as all drum brake works same way holding brakes inside wheel drum so this behavior can be inherited in all class of cars which uses drum brake.
For interface; it is more like specification or contract which force you to implement its speciation. Let’s take an example of model of a building it has all speciation like doors, window, lift ….. But while you implement the model into actual building you we need to keep the window but the internal behavior is decided by (as the widow could be a simple widow or a slider window, the color and material …)
Hope this helps!!
I feel when we need to implement some common functionality and some abstract functionality for multiple class then we should use abstract class. If we see the example of Flat, where we have some common design and some custom design, in such use case it is better to use abstract rather then use interface again to implement custom function and use of abstract as derived class doesn't create an extra instance as normal derived class.
You can not extends more than one class but you can implements more than one interface
If you need to change frequently of your design then abstract class is better because any change happen in abstract class , no force implementation need in sub class. But If any change in interface you have to implement of the implementation class.
This question already has answers here:
Closed 12 years ago.
Possible Duplicates:
Abstract class and Interface class?
Java: interface / abstract classes / abstract method
In Java, whatever use of interface is fulfilled by abstract class. I know one advantage of interfaces is that if we implement an interface then we can also extend another class. Is there any other use or advantage of interface in Java?
Interfaces allow you to use classes in different hierarchies, polymorphically.
For example, say you have the following interface:
public interface Movable {
void move();
}
Any number of classes, across class hierarchies could implement Movable in their own specific way, yet still be used by some caller in a uniform way.
So if you have the following two classes:
public class Car extends Vehicle implements Movable {
public void move() {
//implement move, vroom, vroom!
}
}
public class Horse extends Animal implements Movable {
public void move() {
//implement move, neigh!
}
}
From the perspective of the caller, it's just a Movable
Movable movable = ...;
movable.move(); //who am I?
I hope this helps.
What you like : thousands of abstract methods in one Abstract Class and inherit this class OR make as many interfaces for specific abstract methods and use those only you want by inheriting as many interfaces as needed...
abstract class A
{
//thousands of abstract method();
abstract methodA();
abstract methodB();
abstract methodC();
}
//OR
interface ForOnlymethodA
{
void methodA();
}
interface FormethodBandmethodC
{
void methodB();
void methodC();
}
So, use that method only what you just need by inheriting particular interface, if you are inheriting Abstract classes then you are unnecessarily inheriting all methods that you don't need in one class and may be needed in some other classes..
Multiple interfaces can be implemented, but only one class can be extended. A completely abstract class is a lot like an interface, except that an abstract class can contain variables.
It really depends on what you need. C++ allows you to extend as many classes you want, and it turns into a bit of a disaster. The nice thing about having only one superclass is that there's only ever one other set of implementations that you have to worry about (even if the parent has a parent, the parent's particular combination becomes your parent...)
Interfaces allow one object to play many roles, but they don't allow code reuse.
It's really to simplify thinking about inheritance. On the balance, I think they got it right.
Advantages over an abstract class? except the fact you can implement multiple interfaces but extend only one (abstract or not) class, it's the same as an abstract class that all of it's methods are abstract and public
Interfaces allow the nominative typing in Java to work across disjoint class hierarchies.
This is due to the "single inheritance" limitation on a class hierarchy and "closed types". I will hold my tongue on subtype polymorphism and Java's implementation of it ;-)
There are other solutions to this problem such as dynamic typing, structural typing, multiple inheritance, and traits, etc. Each approach has advantages and dis-advantages. Interfaces were just the approach that Java took.
Java interface
- provides the data encapsulation which means, implementation of the methods can not be seen. The class which extends this interface must implement all the methods declared in it.
more info: wiki answer
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());
}
}
HI
I have a question If interface has got four methods,and I like to implement only two methods, how this could be achieved?
Can any explain is that possible or should I go for implementing all the methods.
You can't "partially" implement an interface without declaring the implementing class abstract, thereby requiring that some subclass provide the remaining implementation. The reason for this is that an interface is a contract, and implementing it declares "I provide the behavior specified by the interface". Some other code is going to use your class via the declared interface and will expect the methods to be there.
If you know the use case does not use the other two methods you can implement them by throwing OperationNotSupported. Whether this is valid or not very much depends on the interface and the user. If the interface can legitimately be partially implemented this way it would be a code smell that the interface is poorly designed and perhaps should have been two interfaces.
You may also be able "implement" the interface by doing nothing, though this is usually only proper for "listener" or "callback" implementations.
In short, it all depends.
If you control the design of the interface, simply split it in two. One interface specifies the two only some implementations implement, and one interface specifies the other two (or inherits the first two and adds more, your choice)
You can make the implementing class abstract and implement two of the 4 methods from the interface.
I think #sateesh 's answer is the one closer to solving your problem. Let me elaborate on it. First of all, it is imperative that any class implementing an interface should provide definitions for all of its methods. But in some cases the user may find no use for a majority of the methods in the interface save for one or two. Consider the following interface having 6 abstract methods:
public interface HugeInterface {
void a();
void b();
void c();
void d();
void e();
void f();
}
Suppose your code finds use for the method 'c()' only and you wish to provide implementation details for only the method 'c()'. You can create a new class HugeInterfaceAdapter in a separate file which implements all the methods of the interface HugeInterface like shown below:
public class HugeInterfaceAdapter implements HugeInterface {
public void a() {}
public void b() {}
public void c() {}
public void d() {}
public void e() {}
public void f() {}
}
Note that you need not provide any actual implementation code for any of the methods. Now comes the interesting part. Yes, your class in which the need to implement a huge interface arose in the first place.
public class MyClass {
HugeInterfaceAdapter mySmallInterface = new HugeInterfaceAdapter() {
#Override
public void c() {
//Your class-specific interface implementation code here.
}
};
}
Now you can use the reference variable mySmallInterface in all the places where a HugeInterface is expected. This may seem a little hackish but I may say that it is endorsed officially by Java and classes like MouseAdapter bears testimony to this fact.
It's not possible.
You can implement all four methods, but the two you don't need should throw an UnsupportedOperationException.
If you want a concrete class which is implementing this interface, then it is not possible to have unimplemented methods, but if you make have abstract class implementing this interface then you can leave any number of methods as you want to be unimplemented.
As other answers mention you cannot have a concrete class implementing only some of the methods of the interface it implements. If you have no control over the interface your class is extending, you can think of having Adapter classes.
The abstract Adapter class can provide dummy implementation for the methods of an interface and the client classes can
extend the Adapter class. (Of course the disadvantage is that you cannot extend more than one class)
This is common practice with GUI programming (using Swing) where the event listener class
might not be interested in implementing all methods specified by the EventListener interface. For example
take a look at the java.awt.event.MouseListener interface and and the corresponding adapter class java.awt.event.MouseAdapter.