What is the advantage of using interfaces [duplicate]

Answer 1

Why is this better than just having individual methods in your classes, and not implementing an interface?

Because if a class C implements an interface I, you can use a C whenever an I is expected. If you do not implement the interface, you could not do this (even if you provided all of the appropriate methods as mandated by the interface):

interface I {
    void foo();
}

class C1 implements I {
    public void foo() {
        System.out.println("C1");
    }
}

class C2 {  // C2 has a 'foo' method, but does not implement I
    public void foo() {
        System.out.println("C2");
    }
}

...

class Test {
    public static void main(String[] args) {
        I eye1 = new C1();  // works
        I eye2 = new C2();  // error!
    }
}