What's the use of interface and abstract methods in java while they dont have no body?

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the interface doesn't apear to reduce any code length or making anything easier in the programming

That's either because you don't understand it, or you just aren't experienced enough yet.

An interface isn't a class at all - it just looks like one. What it is is a promise, or a contract: "if you do this thing, I'll let you join my club" And joining the club gives you advantages that all members share, and non members don't.
For example, you use foreach in your code, yes? That works because each of the collections you can use with foreach implement the Iterable interface[^] - it you don't implement it, you can't iterate over it! If your class implements Iterable, it "joins the club" and an instance of your class can be used as a target in a foreach. If it doesn't, the compiler "door staff" / "bouncers" won't let it in! :laugh:

That's an obvious example, but there are many, many places where Interfaces make a developers life easier, just as Inheritance itself does. As Richard suggests - read the documentation!

Although I agree with Richard and Griff, I must add a critical note:
Too many times I have seen needless interface implementations from people that think that every class must have an interface.
See article here: Best Practices in Software Development: Interface Overuse - DZone Java[^]

Interfaces and abstract classes are "contracts." If you fail to implement everything spelled out in those "contracts" in your code that uses those interfaces and classes, your code doesn't compile.

It's both something that the code you're writing against expects of your code, and a handy reminder to implement everything spelled out in those contracts.