Controlling the access to the fields of a class

The access modifiers public and private have the following meaning:

• public indicates that the method / instance variable is visible outside the class, i.e., by the clients of the class.
• private indicates that the method / instance variable is not visible outside the class, and hence can only be used inside the class itself and not by its clients.

Example:

public class ClientClassPerson2 {
  public static void main(String[] args) {
    Person p1;
    p1 = new Person();
    p1.setResidence("Roma");
                 //OK! the field setResidence is public
    System.out.println(p1.getResidence());
                 //OK! the field getResidence is public
    System.out.println(p1.residence);
                 //ERROR! the field residence is private
  }
}

This example shows a client that tries to access the public and private fields of the class Person. While the access to the public fields, setResidence and getResidence, is allowed, the access to the private field, residence, causes an error at compile time.