Why can't we instantiate an interface or an abstract class in java without an anonymous class method?

问题

I know, we can not instantiate either an interface or an abstract class in java except using anonymous class method but what is the reason behind it?

回答1:

You can't instantiate an interface or an abstract class because it would defy the object oriented model.

Interfaces represent contracts - the promise that the implementer of an interface will be able to do all these things, fulfill the contract.

Abstract classes are a similar idea, in that they represent an unfulfilled contract, a promise to be able to do things, except unlike interfaces they have some of their functions or fields defined but need filling in before they can used.

Simply, in a good object oriented program, you should never want to instantiate an abstract class or interface. If you do, the design is probably wrong.

(Anonymous classes are actually non-abstract instantiations, just that they don't need to be given a name, so they appear to be 'raw interfaces' but they're actually an implementation of the interface that has no name. That's my understanding, at least.)

回答2:

Here is a basic explanation without deeper concept.

• Interface has no method implemented, so there is no purpose to instantiate it as 'nothing' will happen when invoke a method
• Abstract class can have abstract method declaration, which is like a interface method with no implementation.

回答3:

You can't instantiate interfaces or abstract classes because some of their methods might not have any definitions.

来源：https://stackoverflow.com/questions/17077547/why-cant-we-instantiate-an-interface-or-an-abstract-class-in-java-without-an-an