Example of a class definition

We want to realize a Java class to represent persons. The properties of interest for a person object are the name, defined once and for all, and the residence, which may change.

Let us define a Java class Person to represent persons.

public class Person {
  //instance variables (data fields)
  private String name;
  private String residence;

  //methods (operation fields)
  public String getName() {
    return name;
  }
  public String getResidence() {
    return residence;
  }
  public void setResidence(String newResidence) {
    residence = newResidence;
  }
}

The definition of the class Person consists of the following elements:

• the name of the class, i.e., Person;
• two private data fields (or instance variables) of type String, namely name and residence;
• three public fields, each of which is a method definition, namely getName(), getResidence() and setResidence().

The keywords public and private specify which fields are public and which are private (see later).

• Name and residence of persons are represented each by an instance variable of type String in the class Person. Since these variables are private, they cannot be accessed from outside the class.
• Two of the methods defined in the class, getName() and getResidence(), return respectively the name and the residence of a person.
• The method setResidence(), instead, allows us to change the residence of the person represented by the invocation object for the method.

When a method, such as setResidence(), modifies the object on which it is called we say that it has a side-effect (see, e.g., the methods of the class StringBuffer). In general, the decision whether a method of a class should have a side-effect or not is a design choice that has important consequences on the way in which the class must be used by its clients.

Note: The definition of a class has to be saved in a file with the same name as the class and extension .java. For example, the definition of the class Person must be saved in a file called Person.java.

Note: In the definition of a class, the order of the fields (instance variables and methods) is irrelevant.