Is there more to an interface than having the correct methods

Question

So lets say I have this interface:

public interface IBox
{
   public void setSize(int size);
   public int getSize();
   public int getArea();
  //...and so          


        

Answer 1

One of the many uses I have read is where its difficult without multiple-inheritance-using-interfaces in Java :

class Animal
{
void walk() { } 
....
.... //other methods and finally
void chew() { } //concentrate on this
} 

Now, Imagine a case where:

class Reptile extends Animal 
{ 
//reptile specific code here
} //not a problem here

but,

class Bird extends Animal
{
...... //other Bird specific code
} //now Birds cannot chew so this would a problem in the sense Bird classes can also call chew() method which is unwanted

Better design would be:

class Animal
{
void walk() { } 
....
.... //other methods 
} 

Animal does not have the chew() method and instead is put in an interface as :

interface Chewable {
void chew();
}

and have Reptile class implement this and not Birds (since Birds cannot chew) :

class Reptile extends Animal implements Chewable { } 

and incase of Birds simply:

class Bird extends Animal { }