Inheritance vs Composition

When to use which?

1. If there is an IS-A relation, and a class wants to expose all the interface to another class, inheritance is likely to be preferred.
2. If there is a HAS-A relationship, composition is preferred. 

Some cases we required some properties of one class not fully we cannot use interfaces, here is one simple example

public class InheritanceTest {
   public static void main(String args[]) {
      Person p = new Employee();
      System.out.println(p.getAge());
      System.out.println(p.getPersonCountry());
   }
}

class Person {
   public int getAge() {
      return 1;
   }

   public String getPersonCountry() {
      return "INDIA";
   }
}

class Employee extends Person {
   public int getAge() {
      return 2;
   }
}

Here is if we want to change Employee getAge return type it will not allow, see below error.

In this type of situation we can use the composition e.g.:

public class CompositionTest {
   public static void main(String args[]) {
      Employee1 p = new Employee1();
      System.out.println(p.getAge());
      System.out.println(p.getPersonCountry());
   }
}

class Person1 {
   public int getAge() {
      return 1;
   }
   public String getPersonCountry() {
      return "INDIA";
   }
}

class Employee1 {

   public String getAge() {
      return "2";
   }

   public String getPersonCountry() {
      Person1 p1 = new Person1();
      return p1.getPersonCountry();
   }
}