why interface and abstract methods can't be instantiated?

I am unable to figure out why instantiation of interface and abstract class is restricted in java. I know reference for implementation of interface and abstract class can be created. I am clear about that, but 开发者_开发知识库why it can't be instantiated? Anyone please help me

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The point of both an interface and an abstract class is to provide an API which has to be implemented in a concrete class.

For example, suppose I declare this interface:

public interface Foo
{
    int bar();
}

And imagine if this were valid code:

Foo foo = new Foo();
int x = foo.bar();

What could the value of x be? We haven't specified an implementation of bar anywhere. It's a meaningless call, without a real implementation to back it up.

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the If you think of a class as blueprints for creating (instantiating) an instance, much like the blueprints for a house tell you how to build a house. Think of an interface as a floorplan for the house - its an incomplete view (specification) of the house. There isn't enough detail to build the house from it - its only an outline of the rooms. An abstract method is worse - its just the outline of one room.

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Interfaces and Abstract classes are not concrete classes. They are deamed to be incomplete and not to be created. You can use a subclass or implementing class.

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An Abstract class is a class that is not fully implemented. You want to force the developer to implement all the abstract parts of the class BEFORE he/she can instanciate it.

An Interface is a contract that a class must respect. As such, it cannot be instanciated. It can be important to define an Interface that a set of classes must respect in the case of a plugin system for example : All you plugins will share the same interface and thus will be inter-changeable.