MainClass.java

class OuterClass {
    void printMessage(){
        System.out.println("This is the outer class!");
    }
    class InnerClass {
        void print(){
            System.out.println("This is the inner class!");
        }
    }
}

public class MainClass {
    public static void main(String[] args) {
        OuterClass outerClass = new OuterClass();
        OuterClass.InnerClass innerClass = outerClass.new InnerClass();

        outerClass.printMessage();
        innerClass.print();
    }
}