Interfaces in Java

Introduction to Object-Oriented Programming in Java

Sani Yusuf

Lead Software Engineering Content Developer

Limitations of inheritance

With inheritance, all members are inherited
You cannot selectively inherit members


  // Car class   
  class Car {
    // Battery capacity is only applicable
    // to electric cars
    public int batteryCapacity;

    void drive() {

    }  
  }


  // Toyata class
  class Toyota extends Car {  
    void drive(){

    }           
  }


  // Man class
  public static class Main {  

    public static void main(String[] args) {      
      Toyota myToyota = new Toyota();
      // All cars will inherit the "batteryCapacity"
      // even if they are not electric cars
      System.out.println(
        myToyota.batteryCapacity);    // Not applicable
    }
  }

Creating interfaces

Interfaces provide a way to selectively inherit properties and methods
Classes use interfaces using the implements keyword


  // ElectricCar Interface 
  interface ElectricCar {

  }


  // Tesla implementing the
  // ElectricCar interface
  class Tesla implements ElectricCar {

  }

Adding properties to interfaces

Properties use UPPER_SNAKE_CASE by convention
Value must be set immediately and cannot be changed
Properties are public static final behind the scenes


  interface ElectricCar {
    // Implictly public static final
    // Cannot be changed
    int BATTERY_CAPACITY = 310;

  }

Adding interface methods

Interfaces are typically used to house methods
The methods are abstract by default without code implementation
Interfaces can have concrete methods that have code implementation
All abstract methods must be implemented in the subclass

  interface ElectricCar {
    // Implictly public static final
    // Cannot be changed
    int BATTERY_CAPACITY = 310;


    // This is an abstract method by default
    void charge();


    // This is a concrete method
    void autoPark() {

    }

  }  


  class Tesla implements ElectricCar {
    // Must be implemented
    public void charge() {

    }
  }

Let's practice!

Introduction to Object-Oriented Programming in Java