Subinterfaces and Superinterfaces Tutorial Java 8+

As you might guess from the title, interfaces can extend other interfaces. The interface to the left of the extends keyword is the subinterface and the interface(s) to the right of the extends keyword is the superinterface(s). Yes, you are seeing that correctly, a subinterface can extend more than just one superinterface. Not so with a subclass/superclass relationship, a subclass can only extend a single superclass. The basic structure looks like this:
(interface) (InterfaceName) (extends) (Superinterface one, Superinterface two, ...)
Since I am on a technical tangent, when a class implements an interface(s), that interface(s) is a superinterface and the class is a subclass of the superinterface.

Open the command prompt (CMD - see the Getting Started ) and type in the following commands.

C:\Windows\System32>cd \
C:\>md Java
C:\>cd Java
C:\Java>md Tester
C:\Java>cd Tester

Copy and Paste, or type the following code into Notepad and be sure to save the file when you are done.

class Tester implements InterfaceOne {
    public static void main(String args[]) {

        new Tester().youWillInheritMeAndYouBetterEnjoyIt();

    public void youWillInheritMeAndYouBetterEnjoyIt() {
        System.out.println("Look at all this beautiful inheritance ... sniff, sniff.");

interface InterfaceOne extends InterfaceTwo, InterfaceThree, GoodOldGrandpa {
    public static final String inOne = "Hello from interface ONE!";  // remember all interface variables are public static final implicitly   

interface InterfaceTwo {
    String inTwo = "Hello from interface TWO!";    

interface InterfaceThree {
    String inThree = "Hello from interface THREE!";  

interface GoodOldGrandpa {
    void youWillInheritMeAndYouBetterEnjoyIt();

Now switch back to the command prompt (CMD) and type in javac and press Enter.
Now type in java Tester and press Enter.

C:\Java\Tester>java Tester
    Hello from interface ONE!";    
    Hello from interface TWO!";    
    Hello from interface THREE!";    
    Look at all this beautiful inheritance ... sniff, sniff.

Final thoughts