Private Class Data

• Control write access to class attributes

• Separate data from methods that use it

• Encapsulate class data initialization

• Providing new type of final - final after constructor

Example - Hide attributes by keeping them elsewhere

In this example, we want to hide user's password variable.

class User {
    private UserData userData;

    public User(String name, String password) {
        userData = new UserData(name, password);
    }

    public String getName() {
        return userData.getName();
    }
}

class UserData {
    private String name;
    private String password;

    public UserData(String name, String password) {
        this.name = name;
        this.password = password;
    }

    public String getName() {
        return name;
    }
}

User of the code does not have to know how is data internally stored in class, he uses the code as normally. But he can't access getPassword() method anywhere.

User john = new User("John", "secret");

This is simplified example, we could be hiding variables, methods or some functionality at initialization of an object.