Java static keyword
last modified July 15, 2024
In this article we cover the static keyword in Java. We cover
static variables, methods, nested classes, blocks, and imports.
Definition
The static keyword is a non-access modifier. The type that has
static modifier does not belong to the instance of a class; it
belongs to the class. In addition to this, static can be used
to create class initializers, constants, and have imports of static variables
without class qualification.
Usage of static keyword
The static keyword can be applied to:
- variables
- methods
- blocks
- nested classes
- imports
Java static variable
Static variables are also known as class variables. All instances of a class
share the same copy of a static variable. They are initialized only once, at
the start of the execution. A class variable can be accessed directly by the
class name, without the need to create a instance. One common use of
static is to create a constant value that is attached to a class.
Static variable example
import java.util.ArrayList;
import java.util.List;
class Being {
public static int count;
}
class Cat extends Being {
public Cat() {
count++;
}
}
class Dog extends Being {
public Dog() {
count++;
}
}
class Donkey extends Being {
public Donkey() {
count++;
}
}
void main(String[] args) {
List<Being> beings = new ArrayList<>();
beings.add(new Cat());
beings.add(new Cat());
beings.add(new Cat());
beings.add(new Dog());
beings.add(new Donkey());
int nOfBeings = Being.count;
System.out.format("There are %d beings %n", nOfBeings);
}
In the code example, we keep track of beings created with a static variable.
class Being {
public static int count;
}
A static variable is defined. The variable belongs to the Being
class and is shared by all instances of Being, including
descendants.
class Cat extends Being {
public Cat() {
count++;
}
}
The Cat class inherits from Being. It increments the
count variable.
class Dog extends Being {
public Dog() {
count++;
}
}
The Dog class increments the same class variable. So Dog
and Cat refer to the same class variable.
int nOfBeings = Being.count;
We get the number of all beings created. We refer to the class variable by its class name followed by the dot operator and the variable name.
Java static variable properties
- Static variables have default values.
- Static variables can be accessed directly in static and non-static methods.
- Static variables are called class variables or static fields.
- Static variables are associated with the class, rather than with any object.
Java static method
Static methods are called without an instance of the object. To call a static
method, we use the name of the class, the dot operator, and the name of the
method. Static methods can only work with static variables. Static methods are
often used to represent data or calculations that do not change in response to
object state. For instance, java.lang.Math contains static methods
for various calculations.
We use the static keyword to declare a static method. When no
static modifier is present, the method is said to be an instance
method.
Static method restrictions
Static method can only call other static methods. They can only access static
data and cannot refer to this and super.
Static method example
package com.zetcode;
class Basic {
static int id = 2321;
public static void showInfo() {
System.out.println("This is Basic class");
System.out.format("The Id is: %d%n", id);
}
}
public class Main {
public static void main(String[] args) {
Basic.showInfo();
}
}
In our code example, we define a static ShowInfo method.
static int id = 2321;
A static method can only work with static variables. Static variables are not available to instance methods.
public static void showInfo() {
System.out.println("This is Basic class");
System.out.format("The Id is: %d%n", id);
}
This is our static ShowInfo method. It works with a
static id member.
Basic.showInfo();
To invoke a static method, we do not need an object instance. We call the method by using the name of the class and the dot operator.
The static main method
In Java console and GUI applications, the entry point has the following singnature:
public static void main(String[] args)
By declaring the main method static, it can be invoked
by the runtime engine without having to create an instance of the main class. Since the
primary reason of main is to bootstrap the application, there is no
need to have an instance of the main class.
In addition, if the main method was not static,
it would require additional contracts such as a default constructor or a requirement
of the main class not to be abstract. So having a static main
method is a less complex solution.
Java static block
A code block with the static modifier is called a class initializer. A code block without the static modifier is an instance initializer. Class initializers are executed in the order they are defined, top down, when the class is loaded.
A static block executes once in the life cycle of any program, and there is no other way to invoke it.
Static block example
package com.zetcode;
public class Main {
private static final int i;
static {
System.out.println("Class initializer called");
i = 6;
}
public static void main(String[] args) {
System.out.println(i);
}
}
This is an example of a static initializer.
static {
System.out.println("Class initializer called");
i = 6;
}
In the static initializer, we print a message to the console and initialize a static variable.
Static nested classes
A static nested class is a nested class that can be created without the instance of the enclosing class. It has access to the static variables and methods of the enclosing class.
Static nested classes can logically group classes that are only used in one place. They increase encapsulation and provide more readable and maintainable code.
Static nested classes restrictions
Static nested classes cannot invoke non-static methods or access non-static fields of an instance of the enclosing class.
Static nested class example
package com.zetcode;
public class Main {
private static int x = 5;
static class Nested {
@Override
public String toString() {
return "This is a static nested class; x:" + x;
}
}
public static void main(String[] args) {
Main.Nested sn = new Main.Nested();
System.out.println(sn);
}
}
The example presents a static nested class.
private static int x = 5;
This is a private static variable of the JavaStaticNestedClass
class. It can be accessed by a static nested class.
static class Nested {
@Override
public String toString() {
return "This is a static nested class; x:" + x;
}
}
A static nested class is defined. It has one method which prints a message
and refers to the static x variable.
Main.Nested sn = new Main.Nested();
The dot operator is used to refer to the nested class.
Java static import
Static imports allow members (fields and methods) defined in a class as
public static to be used in Java code without specifying the class
in which the field is defined.
Static import disadvantages
The overuse the static import feature can make our program unreadable and unmaintainable, polluting its namespace with all the static members we import.
Static import example
package com.zetcode;
import static java.lang.Math.PI;
public class Main {
public static void main(String[] args) {
System.out.println(PI);
}
}
In the example, we use the PI constant without its class.
Java constants
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.
public static final double PI = 3.14159265358979323846;
For example, in java.lang.Math we have a constant named
PI, whose value is an approximation of pi (the ratio of the
circumference of a circle to its diameter).
Singleton pattern
Singleton design pattern ensures that one and only one object of a particular class is ever constructed during the lifetime of the application.
public class Singleton {
private static final Singleton INSTANCE = new Singleton();
private Singleton() {}
public static Singleton getInstance() {
return INSTANCE;
}
}
In this simple code excerpt, we have an internal static reference to the single allowed object instance. We access the object via a static method.
Source
In this article we have presented the Java static keyword.
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