Why interface methods have no body

Question

To achieve multiple inheritance, we must use interfaces, but why don't interface methods have bodies and why do they have to be overridden in the derived class?

I really want a lucid answer , not involving too much computer jargon , i cant seem to understand this , i have referred various references

Answer 1

Because Java, in contrast to languages like C++ or Eiffel, only has multiple inheritance of types (i.e. interfaces as well as one class), not multiple inheritance of state and behaviour. The latter of which add enormous complexity (especially state).

The Java designers (and C#, for that matter) opted to not include it as it presented C++ programmers often with very hard to debug issues. You can solve pretty much most problems that require true multiple inheritance with implementing multiple interfaces, so the tradeoff was deemed worth it.

Note that multiple inheritance of behaviour (not state) might come to Java 8 (unless they postpone it again like one of the many other things) in form of virtual extension methods where an interface can declare a method that delegates to one in another class, which then exists on all types that implement that interface.

Answer 2

Interfaces declare WHAT services the implementing class provides, not HOW (that's the job of the implementing class). Multiple inheritance is regarded bad, as it leads to complicated code and class hierarchies.

Answer 3

Interfaces only have constant variables(public + static + final) and abstract methods(public & abstract). These are meant to be used by the classes which implement the interfaces.

Interfaces simply say 'Am a contract', which if you wish to use, should stick to some rules(give implementation to all abstract methods).

Multiple inheritance is omitted in Java by making sure that a class can extend only 1 class, in order to avoid the diamond problem. You can anyways have multiple inheritance of types in Java by using interfaces.

Answer 4

A Java interface contains a list of methods that must be implemented by the class that implements the interface. Thus, the methods have no body: the body of each method is in the implementing class(es).