Question

I am new to java and learning interfaces. I did not understand the use of interfaces other than just standardizing the expectation.

If there is any other uses please help me to understand that.

Answer 1

Interfaces have a few good uses:

Polymorphism

They allow you to treat several different, but similar, objects in the same way. For example:

public interface Flyer {
  public void fly();
}   

public class Bird implements Flyer {
  @Override
  public void fly() {
    System.out.println("I'm flying");
  }   
}   

public class Airplane implements Flyer {
  @Override
  public void fly() {
    System.out.println("I'm inanimate and flying");
  }   
}   

// then you can have a method like so:
public void makeFly(Flyer flyer) {
  flyer.fly();
} 

// or make a list like so:
List<Flyer> fliers = Arrays.asList(new Airplane(), new Bird());

They also allow Objects to fit into several different boxes. For example, a Bird not only flies, but it is an Animal. Interfaces allow you to treat Bird as either one:

public interface Animal {
  public void move();
}

public class Bird implements Flyer, Animal {
  @Override
  public void fly() {
    System.out.println("I'm flying");
  }   

  @Override
  public void move() {
    fly();
  }
}

// now I can do:
Bird bird = new Bird();
Flyer flyer = new Bird();
Animal animal = new Bird();

Decoupled Design

Additionally, they give you flexibility in choosing your implementation. (I’m not sure what you mean by “Standardizing the Expectation”, but this use is probably most related to standardization. i.e. This works because it creates a standard.) For example, say I have a method that returns a List<Flyer>. Since List is an interface, I can choose use ArrayList, LinkedList, etc. as my implementation:

public List<Flyer> makeFlyerList() {
  return new ArrayList<Flyer>(Arrays.asList(new Airplane(), new Bird()));
}

// works just the same
public List<Flyer> makeFlyerList() {
  return new LinkedList<Flyer>(Arrays.asList(new Airplane(), new Bird()));
}