How to solve this "non-static variable" issue in Java?

Question

public class InterfaceTest {
    interface  InterfaceA {
         int  len =  1 ;
         void  output();
    }

    interface  InterfaceB {
           int  len =  2 ;
           void  output();
    }

    interface  InterfaceSub  extends  InterfaceA, InterfaceB {            }

    public class Xyz implements  InterfaceSub {

         public   void  output() {
            System.out.println( "output in class Xyz." );
        }

          public   void  outputLen(int  type) {
              switch (type) {
                      case  InterfaceA.len:
                             System.out.println( "len of InterfaceA=." +type);
                              break ;
                      case  InterfaceB.len:
                             System.out.println( "len of InterfaceB=." +type);
                              break ;
             }
        }
    }

    public   static   void  main(String[] args) {
           Xyz xyz = new Xyz();
           xyz.output();
           xyz.outputLen(1);
   }
}

Hi, I want to learn Java's interface and multiple inheritance concept. I found above code and try to compile it, but below error occurs. I don't know how to make the code work, who could help? Thanks!

test$ javac InterfaceTest.java 
InterfaceTest.java:33: error: non-static variable this cannot be referenced from a static context
           Xyz xyz = new Xyz();
                     ^
1 error

Answer 1

This is because a non-static inner class cannot be instantiated in a static method because it does not have an instance of the enclosing class to work with.

If you define Xyz as a static inner class it should work:

public static class Xyz implements InterfaceSub {
  ....
}

Alternatively, you can create Xyz within an instance of the enclosing class - this is not needed here but this would be required if Xyz needed to access some member variables of the enclosing class.