Human.java

public class Human
{
    private String name = "unnamed"; // the name of this human
    private Human friend = null; // the human's friend
 
    /**
      * This "creates" a new Human
      */
    public Human(String name, Human friend) {
        this.name = name;
        this.friend = friend;
    }
    public Human(String name) {
        this.name = name;
        this.friend = null;
    }
    public Human() {
        this.name = "unnamed";
        this.friend = null;
    }
    public void sayName() {
        System.out.println("My name is " + this.name);
    }
    public void sayGoodnight() {
        if (friend == null)
            System.out.println("Good night nobody.");
        else
            System.out.println("Good night " + friend.name);
    }
}

Main.java

public class Main
{
    public static void main(String[] args) {
        //create a new human object named stephen
        Human stephen = new Human("Stephen");
        //create a human object named joe with stephen as a friend
        Human joe = new Human("Joe", stephen);
        stephen.sayName(); //shows 'My name is Stephen'
        stephen.sayGoodnight(); //shows 'Good night nobody.'
        joe.sayName(); // shows 'My name is Joe'
        joe.sayGoodnight(); //shows 'Good night Stephen'
    }
}