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:
Cat.java
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+"."; } }
CatsTheMusical.java
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
andfriend
. Each field has a type, which ensures that we can only store aString
inname
, anint
inage
and anotherCat
infriend
. The wordprivate
before each field indicates that the field can only be accessed from within the classCat
.- Within the class the field (e.g.
name
) can be accessed simply by callingname
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).
- Within the class the field (e.g.
- 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 variablesname
andage
. The arguments of the constructor create variables with the same name as the the fields of the class, shadowing the fields. By using the keywordthis
, 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.
- Because the constructor is public, it can be invoked from other classes. An example of the invoking the constructor can be seen in
- Methods: The class
Cat
has two methods,getName(){...
} andsayHello(...){..
}. 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
.