Classes

index
BASIC

A class is a description of objects; the objects are called the instances of that class. (The terms "object" and "instance" are often used interchangeably.)

Here is an example of a simple class Person:

public class Person {
    // instance variables
    String name;
    int age;
 
    // constructor
    public Person(String name) {
        this.name = name;
    }
 
    // method
    public void birthday() {
        age = age + 1;
    }
}

This class itself does not represent a person; rather, it describes the characteristics that each person object must have:

New instances of this class are created using the keyword new, for example,

Person candidate = new Person("Hillary Clinton");
Person favoriteAuthor = new Person("Terry Pratchett")

The fields and methods of a class can be accessed by "dot notation," for example,

candidate.birthday();
System.out.println(candidate.name + " is now " + candidate.age;

Java requires that each public class be in a separate file, and that the file name is the class name with a .java extension. For example, the Person class must be in a file named Person.java. (For platform independence, capitalization must be correct.) It follows that there can be only one public class in a file. It is conventional--and good style--to put every class in a separate file, whether public or not.

Also see Objects.

INTERMEDIATE