Inheritance and interfaces

extends, implements, overriding and polymorphism.

A class can extend one parent, and can implement many interfaces. An interface lists what a class must be able to do.

Java

interface Payable {
  int pay();
}

abstract class Staff implements Payable {
  protected final String name;

  Staff(String name) {
    this.name = name;
  }
}

class Developer extends Staff {
  Developer(String name) {
    super(name);
  }

  @Override
  public int pay() {
    return 30000;
  }
}

class Intern extends Staff {
  Intern(String name) {
    super(name);
  }

  @Override
  public int pay() {
    return 5000;
  }
}

public class Main {
  public static void main(String[] args) {
    Staff[] team = {new Developer("Anu"), new Intern("Bala")};
    for (Staff s : team) {
      System.out.println(s.name + " " + s.pay());
    }
  }
}

Output

Anu 30000
Bala 5000

The loop calls pay() on a Staff variable, and each object runs its own version. This is polymorphism, the most commonly asked object-oriented idea in interviews.

Use	When
`Abstract class`	Related classes share code and fields
`Interface`	Unrelated classes share a capability

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