When to use Single method Interfaces in Java

Question

I have seen in many libraries like Spring which use a lot of interfaces with single methods in them like BeanNameAware, etc.

Answer 1

Favor Composition over Inheritance tutorial of Head First Design Pattern book recommends this approach to add functionality dynamically to a class. Let's take below case:

public interface Quackable {
   public void quack();
} 

public class Quacks implements Quackable {
   public void quack(){
       //quack behavior
   }
}

public class DontQuack implements Quackable {
     public void quack(){
         //dont quack
    }
}

public class QuackableDuck{
    Quackable quack;  //add behavior dynamicall

}

So QuackableDuck class can add feature dynamically.

   quack = new Quacks();
   //or 
   quack = new DontQuack();

So similarly you can add multiple behavior to the class dynamically.