Declaring Member Variables

Member variables in a class are also called fields or instance variables. The member variables are used to keep the state of the created object. Member variable are declared in the same way as normal variables, the same rules and conventions apply.

Field declaration are composed of three components, in order:

1. Zero, or more modifiers, such as public or private access modifier.
2. The field's type
3. The field's name

Example

Here is a sample Message class from a chat application. The member variables are id, author, createdAt, channelId, and text. Notice we had to import the Date class.

import java.util.Date;

public class Message {
    String id;
    String author;
    Date createdAt;
    String channelId;
    String text;
}

Here is another example of how we could model a User class in our chat application.

public class User {
    String id;
    String email;
    String nickname;
    String profilePictureUrl;
}

And a model Room class for the chat application could be represented as follows:

public class Room {
    String channelID;
    String[] owners;
    String[] members;
    String title;
}

Notice in the Room class, owners is an array of Strings. The members field is also an array of Strings.

Controlling Access to Member Variables

Access level modifiers determine whether other classes can use use a particular field or method of a class.

Class Fields Access Modifiers

Example Access Modifiers

public modifier

public class User {
   public String profilePictureUrl;
}

The profilePictureUrl field variable will be accessible from within a class, package, subclass and the world.

protected modifier

public class User {
   protected String profilePictureUrl;
}

With the protected keyword, the profilePictureUrl field is accesible from the class, package, subclass but not the world.

default, no modifier

public class User {
   String profilePictureUrl;
}

With no modifier, the field is a assigned a default modifier. The field is accesbile from the class and package, but not from the subclass and the world.

private modifier

public class User {
  private String profilePictureUrl;
}

The field is only accesbile from within the class and not accesible anywhere else.