[java basics] understanding of interfaces

I have just initially learned the interface of java, and I will write down my gains here:

package com.funyoo.about_interface;

public interface MyFirstInterface {
	public static final int red = 1;
	int blue = 2;
	
	public abstract void func();
	int fun();
}

Define the first interface: MyFirstInterface

Note: The interface has the following "iron law": the interface will be jointly decorated with public static final to its members;

                                                Its methods will be decorated with public abstract union

Therefore, the above member blue is also modified by public, static, final, and the method fun() is also modified by public, abstract.

package com.funyoo.about_interface;

public interface MySecondInterface extends MyFirstInterface {
	public static final int red = 2;
	
	public int fun2();
}

This is the second interface defined, MySecondInterface, which inherits the first interface MyfirstInterface;

package com.funyoo.about_interface;

public interface MyThirdInterface extends MyFirstInterface {
	int red = 3;
	
}

Define the third interface, MyThirdInterface, which also inherits the first interface MyfirstInterface;

Now we create a new class to implement the first interface: ImplementensMyfirstInterface

package com.funyoo.about_interface;

public class ImplemensMyfirstInterface implements MyFirstInterface {

	@Override
	public void func() {

	}

	@Override
	public int fun() {
		return 0;

	}

}

Because the methods created by the interface are abstract methods, when they are implemented, the abstract methods should also be implemented.

Next we create a new class to implement all the interfaces: ImplementensMyInterface

package com.funyoo.about_interface;

public class ImplemensMyInterface implements MyFirstInterface,
		MySecondInterface, MyThirdInterface {

	@Override
	public int fun2() {
		return MyFirstInterface.red;
	}

	@Override
	public void func() {
		// TODO auto-generated method stub

	}

	@Override
	public int fun() {
		return MyFirstInterface.red;
	}

}

Let's create a main class to conduct an experiment:

package com.funyoo.about_interface.test;

import com.funyoo.about_interface.ImplemensMyInterface;
import com.funyoo.about_interface.ImplemensMyfirstInterface;
import com.funyoo.about_interface.MyFirstInterface;
import com.funyoo.about_interface.MySecondInterface;
import com.funyoo.about_interface.MyThirdInterface;

public class Test {

	public static void main(String[] args) {
		ImplemensMyInterface imi = new ImplemensMyInterface();
		ImplemensMyfirstInterface imf = new ImplemensMyfirstInterface();
		
// Equivalent to
//		MyFirstInterface imi = new ImplemensMyInterface();
//		MyFirstInterface imf = new ImplemensMyfirstInterface();
		
		System.out.println(imi instanceof MyFirstInterface);         //true    //***
		System.out.println(imi instanceof MySecondInterface);        //true
		System.out.println(imi instanceof MyThirdInterface);         //true
		System.out.println(imi instanceof ImplemensMyInterface);     //true
		
		System.out.println("--------------------------------");
		
		System.out.println(imf instanceof MyFirstInterface);         //true   //***
		System.out.println(imf instanceof MySecondInterface);        //false
		System.out.println(imf instanceof MyThirdInterface);         //false
		
		System.out.println("---------------------------------");
		System.out.println(imi.getClass());     //class com.funyoo.about_interface.ImplemensMyInterface
		System.out.println(imf.getClass());     //class com.funyoo.about_interface.ImplemensMyfirstInterface
	}

}

Judging from the above program and running results, imi and imf belong to two different classes, but are linked because of an interface;

Look at the running results of lines 19 and 26

We can boldly change both ImplemensMyInterface and ImplemensMyfirstInterface on lines 12 and 13 to:

MyFirstInterface

The result of running the program remains unchanged!

So in the future programming, we can "link" two completely different classes with one interface!