Hi, I was wondering why th class RussianTranslator has to be Static ? thanks.

public class Solution {

    public static void main(String[] args) throws Exception {
        //to create an object of an inner class you first need an object of the outer class
        Solution s = new Solution();
        //then you can create an inner class object using the reference of the outer class object
        InnerClass t = s.new InnerClass();
        //now you can use the object of the inner class as usual
        t.testMethod();
        //extending static nested classes is no problem.
        t.methodOfAbstractClass();

        //a static nested class does not need a reference of an outer class' object.
        //It's comparable to a static method, that one also doesn't need a reference to an object
        new StaticNestedClass().testMethod();

    }

    //inner class
    public class InnerClass extends AbstractClass {
        public void testMethod() {
            System.out.println("Inner class");
        }

        @Override
        void methodOfAbstractClass() {
            System.out.println("Extending static nested classes is no problem, Sir!");
        }
    }

    public static class StaticNestedClass {
        public void testMethod() {
            System.out.println("Static nested class");
        }
    }

    public static abstract class AbstractClass {
        abstract void methodOfAbstractClass();
    }
}