这是怎么回事？

public class Solution {
    public static void main(String[] args) {
    }

    static class MyException extends IOException {
    }

    static class MyException2 extends FileNotFoundException {
    }

    static class MyException3 extends NumberFormatException{
    }

    static class MyException4 extends ArithmeticException{
    }
}

package zh.codegym.task.task09.task0918;
import java.io.FileNotFoundException;
import java.net.URISyntaxException;
import java.io.IOException;
/*
相识是缘分，连异常都不例外
*/

public class Solution {
    public static void main(String[] args) {
    }

    static class MyException extends NumberFormatException{
        public MyException() throws Exception{
            throw new NumberFormatException();
        }
    }

    static class MyException2 extends NullPointerException{
        public MyException2() throws Exception{
            throw new NullPointerException();
        }
    }

    static class MyException3 extends FileNotFoundException{
        public MyException3() throws Exception{
            throw new FileNotFoundException();
        }
    }

    static class MyException4 extends IOException{
        public MyException4() throws Exception{
            throw new IOException();
        }
    }
}

//
   static class MyException extends NullPointerException{
        public void method1() {
            throw new NullPointerException();
        }
    }

    static class MyException2 extends ClassNotFoundException {
        public void method1() throws ClassNotFoundException {
            throw new ClassNotFoundException();
        }
    }

    static class MyException3 extends ArithmeticException {
        public void method1() {
            throw new ArithmeticException();
        }
    }

    static class MyException4 extends NoSuchFieldException {
        public void method1() throws NoSuchFieldException {
            throw new NoSuchFieldException();
        }
    }