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public class Cat {
  private int age;
  private String name;
  private Cat friend;

  public Cat(String name, int age) {
    this.name = name;
    this.age = age;
    this.friend = null;
  }

  public String getName() {
    return name;
  }

  public Cat getFriend() {
    return friend;
  }

  public void setFriend(Cat friend) {
    this.friend = friend;
  }

  public String sayHello(String aboutMe) {
    return "Hello World!  My name is " + 
      name + " and I am " + age + 
      " years old." + " I am " + aboutMe + ".";
  }
}

public class CatsTheMusical {

  public static void main(String[] args) {
    Cat mist = new Cat("Mr. Mistoffelees", 8);
    System.out.println(mist.sayHello("magical"));
    Cat garf = new Cat("Garfield", 12);
    System.out.println(garf.sayHello("tired/hungry"));
    mist.setFriend(garf);
    System.out.println(mist.getName() + " and " + garf.getName()
        + " are friends? " + CatsTheMusical.areFriends(mist, garf));
    garf.setFriend(mist);
    System.out.println(mist.getName() + " and " + garf.getName()
        + " are friends? " + CatsTheMusical.areFriends(mist, garf));
  }

  public static boolean areFriends(Cat cat1, Cat cat2) {
    if (cat1.getFriend() != null && cat2.getFriend() != null) {
      return cat1.getFriend() == cat2 && cat2.getFriend() == cat1;
    }
    return false;
  }
}

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• Fields: The class Cat has three fields, name, age and friend. Each field has a type, which ensures that we can only store a String in name, an int in age and another Cat in friend. The word private before each field indicates that the field can only be accessed from within the class Cat.
• Constructors: The class Cat has one constructor public Cat(String name, int age){...}. In the constructor, the fields name and age are initialized with the values given in the arguments, while friend is simply set to null.
• Methods: The class Cat has two four methods, getName(){...}, getFriend(){...}, setFriend(...){...} and sayHello(...){..}. The keyword{{public}} indicates that the methods can be called outside of the class Cat. The first is an example of a getter method. Note that while there is a getter method, there is no setter method, so once a Cat is created, the name cannot be changedsecond word in each method is the return type, or the type of the result of the method. Notice that setFriend(...) has a return type of void, indicating that nothing is returned. The first two methods are examples of getter methods, as they simply retrieve the value of a private field. The third method is a setter method, which allows the field friend to be changed from outside the class Cat.

Now that we have the basic terminology down, lets examine some of the finer points:

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