The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
In this section, we discuss the use of the static
keyword to create fields and methods that belong to the class, rather than to an instance of the class.
When a number of objects are created from the same class blueprint, they each have their own distinct copies of instance variables. In the case of the Bicycle
class, the instance variables are cadence
, gear
, and speed
. Each Bicycle
object has its own values for these variables, stored in different memory locations.
Sometimes, you want to have variables that are common to all objects. This is accomplished with the static
modifier. Fields that have the static
modifier in their declaration are called static fields or class variables. They are associated with the class, rather than with any object. Every instance of the class shares a class variable, which is in one fixed location in memory. Any object can change the value of a class variable, but class variables can also be manipulated without creating an instance of the class.
For example, suppose you want to create a number of Bicycle
objects and assign each a serial number, beginning with 1 for the first object. This ID number is unique to each object and is therefore an instance variable. At the same time, you need a field to keep track of how many Bicycle
objects have been created so that you know what ID to assign to the next one. Such a field is not related to any individual object, but to the class as a whole. For this you need a class variable, numberOfBicycles
, as follows:
public class Bicycle { private int cadence; private int gear; private int speed; // add an instance variable for the object ID private int id; // add a class variable for the // number of Bicycle objects instantiated private static int numberOfBicycles = 0; ... }
Class variables are referenced by the class name itself, as in
Bicycle.numberOfBicycles
This makes it clear that they are class variables.
myBike.numberOfBicycles
You can use the Bicycle
constructor to set the id
instance variable and increment the numberOfBicycles
class variable:
public class Bicycle { private int cadence; private int gear; private int speed; private int id; private static int numberOfBicycles = 0; public Bicycle(int startCadence, int startSpeed, int startGear){ gear = startGear; cadence = startCadence; speed = startSpeed; // increment number of Bicycles // and assign ID number id = ++numberOfBicycles; } // new method to return the ID instance variable public int getID() { return id; } ... }
The Java programming language supports static methods as well as static variables. Static methods, which have the static
modifier in their declarations, should be invoked with the class name, without the need for creating an instance of the class, as in
ClassName.methodName(args)
instanceName.methodName(args)
A common use for static methods is to access static fields. For example, we could add a static method to the Bicycle
class to access the numberOfBicycles
static field:
public static int getNumberOfBicycles() { return numberOfBicycles; }
Not all combinations of instance and class variables and methods are allowed:
this
keyword as there is no instance for this
to refer to.The static
modifier, in combination with the final
modifier, is also used to define constants. The final
modifier indicates that the value of this field cannot change.
For example, the following variable declaration defines a constant named PI
, whose value is an approximation of pi (the ratio of the circumference of a circle to its diameter):
static final double PI = 3.141592653589793;
Constants defined in this way cannot be reassigned, and it is a compile-time error if your program tries to do so. By convention, the names of constant values are spelled in uppercase letters. If the name is composed of more than one word, the words are separated by an underscore (_).
Bicycle
ClassAfter all the modifications made in this section, the Bicycle
class is now:
public class Bicycle { private int cadence; private int gear; private int speed; private int id; private static int numberOfBicycles = 0; public Bicycle(int startCadence, int startSpeed, int startGear) { gear = startGear; cadence = startCadence; speed = startSpeed; id = ++numberOfBicycles; } public int getID() { return id; } public static int getNumberOfBicycles() { return numberOfBicycles; } public int getCadence() { return cadence; } public void setCadence(int newValue) { cadence = newValue; } public int getGear(){ return gear; } public void setGear(int newValue) { gear = newValue; } public int getSpeed() { return speed; } public void applyBrake(int decrement) { speed -= decrement; } public void speedUp(int increment) { speed += increment; } }