What is difference between interface and abstract in java?

Answer 1

While an abstract class can define both abstract and non-abstract methods, an interface can have only abstract methods. Another way interfaces differ from abstract classes is that interfaces have very little flexibility in how the methods and variables defined in the interface are declared. These rules are strict:

• All interface methods are implicitly public and abstract. In other words, you do not need to actually type the public or abstract modifiers in the method declaration, but the method is still always public and abstract. (You can use any kind of modifiers in the Abstract class)

• All variables defined in an interface must be public, static, and final-in other words, interfaces can declare only constants, not instance variables.

• Interface methods must not be static.

• Because interface methods are abstract, they cannot be marked final, strictfp, or native. (More on these modifiers later.)

• An interface can extend one or more other interfaces.

• An interface cannot extend anything but another interface.

• An interface cannot implement another interface or class.

• An interface must be declared with the keyword interface.

You must remember that all interface methods are public and abstract regardless of what you see in the interface definition.