What is a class?

In Java, all code is associated with a class. Classes have fields and methods. Here is an example of two simple classes:

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

  public String getName(){
    return name;
  }

  public String getFriend(){
    if(friend == null){
      return "I am have no friends :(";
    }
    else{
      return "My friend is " + friend.getName();
    }
  }

  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"));
  }
}

Lets make sure we understand how this works. First, lets talk about the contents of the files Cat.java.

• 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.
  • Within the class the field (e.g. name) can be accessed simply by calling name or {this.name}}. In Java, by convention most fields are private, and if you want to be able to access them from outside the class, you add getter and setter methods (we will get to these shortly).
• Constructors: The class Cat has two constructors {{public Cat(String name, int age){...} }}, {{public Cat(String name, int age, Cat friend){...} }}. Constructors are used to create instances of the class. Notice that:
  • Because the constructor is public, it can be invoked from other classes. An example of the invoking the constructor can be seen in CatsTheMusical.java.
  • We use the keyword this to set the variables name and age. The arguments of the constructor create variables with the same name as the the fields of the class, shadowing the fields. By using the keyword this, we can access methods and fields associated with the current instance of the class (being built in the constructor). This pattern, while a little confusing, is conventional.
• Methods: The class Cat has two methods, getName(){...} and sayHello(...){..}. 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 changed.

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

Here, we included a method getName() in class Cat.