Java Sub-Interfaces.

Java Sub-Interfaces

  • An interface can extend another interface, just as a class can extend another class.
  • Interface is allowed to extend as many interfaces as it wants.
    Sub Interface example:
    interface Pricing {
      public String getPrice();
      public String getDiscount();
      public String DECIMALPATTERN="#";
      public static String getFormatted(double value) {
        DecimalFormat formatter = new DecimalFormat(DECIMALPATTERN);
        return formatter.format(value);
    // Pricing is a sub interface of Reputation
    interface Reputation extends Pricing{
      public String getInfo();
    class Car implements Reputation {
      private final double price, discount;
      private final List<String> reputations= new ArrayList<>();
      public void addReputation(String reputation) {
      public Car(double price, double discount) {
        this.price = price; = discount;
      public String getInfo() {
        String returns ="";
        for (String ret : reputations) {
          returns +="\n"+ret;
        return returns;   
      public String getPrice() {
        return "$ "+Pricing.getFormatted(price);
      public String getDiscount() {
        return Pricing.getFormatted(discount)+"%";
    public class CarReputationApp {
      public static void main(String[] args) {
        Car myCar = new Car(100000,15); // instantiated object of class car
        System.out.println("Ordinary car price is " + myCar.getPrice());
        System.out.println("You've got a car discount of " + myCar.getDiscount());
        myCar.addReputation("\tComfortable to drive");
        myCar.addReputation("\tAdditional winter wheels");
        myCar.addReputation("\t2016 modell");
        System.out.println("Car reputations:" + myCar.getInfo());
    The result of this is:
    Ordinary car price is $ 100000
    You've got a car discount of 15%
    Car reputations:
    	Comfortable to drive
    	Additional winter wheels
    	2016 modell
    You can download this example here (needed tools can be found in the right menu on this page).
  • A class that implements Reputation must also implement methods of the sub interface Pricing.
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