How interfaces achieve multiple inheritance

Question

In many books it is written that interfaces are a replacement of multiple inheritance, but I don't find any similarity between both of them.

Inheritance is mostly important for re-usability of code and functionality and multiple inheritance was helping to re-use code from more than one class, but in interface I didn't find any such feature except that a class can inherit from more than one interface.

Interface is just declaration of functions/methods and it didn't contain any implementation part by itself, so class which are inheriting this interface should have to write their own implementation code.

So I don't feel any re-usability of code in case of interface.

Is any document or link which will clear my doubts with you answer please share.

Answer 1

Regarding reusability of code, you are right. In that respect, multiple interfaces are no replacement of multiple inheritance.

However, there's another aspect of inheritance: it establishes an is-a relasionship between the base- and sub-class. So a class inheriting multiple super-classes can act as any of them. In this respect, interfaces serve as a valid replacement, e.g. a method accepting an interface will also accept any class implementing that interface, in the same way as a method will accept any class derived from the excpected class. But as you stated, each class has to implement the interface methods by their own.

Example:

public interface Foo {
    int doFoo();
}

public interface Bar {
    long doBar();
}

public class Baz {
    String doBaz() {
        return "This is baz";
    }
}

public class FooBar extends Baz implements Foo, Bar {
    public long doBar() {
        return 123;
    }
    public int doFoo() {
        return 456;
    }
}

// Accepts interface Bar implementing objects
public void doSomething(Bar b) {
    System.out.println(b.doBar() * 10);
}

// Accepts interface Foo implementing objects
public void doSomethingOther(Foo f) {
    System.out.println(f.doFoo() / 10);
}

// Accepts objects of class Baz and subclasses
public void doMore(Baz b) {
    System.out.println(b.doBaz());
}

void bla() {
    FooBar fb = new FooBar();

    // FooBar can act as Foo, Bar, and Baz
    doSomething(fb);
    doSomethingOther(fb);
    doMore(fb);
}