static Keyword in Java

The static keyword in Java is mainly used for memory management, allowing variables and methods to belong to the class itself rather than individual instances. The static keyword is used to share the same variable or method of a given class. The users can apply static keywords with variables, methods, blocks, and nested classes. The static keyword belongs to the class rather than an instance of the class. The static keyword is used for a constant variable or a method that is the same for every instance of a class.

What is a static Keyword in Java?

The static keyword is a non-access modifier used for:

• Variables (class-level, shared across instances)
• Methods (called without object creation)
• Blocks (executed once when the class loads)
• Nested Classes (static inner classes)

Note: To create a static member(block, variable, method, nested class), precede its declaration with the keyword static. 

Characteristics of the "static" Keyword

Here are some characteristics of the static keyword in Java:

• Static variables and methods use memory only once when the program runs, and this memory is shared by all the objects of the class.
• We do not need to create objects of the class to use static methods
• We can call static members using the class name directly.
• Static members belong to the class, not to any specified object.
• Static members can not access non-static members.
• Static methods cannot be overridden in subclasses because they belong to the class, not to an object.

When a member is declared static, it can be accessed before any objects of its class are created, and without reference to any object. For example, in the below java program, we are accessing the static method m1() without creating any object of the Test class. 

Example: This example demonstrates that a static method can be called without creating an instance of the class.

// Java program to demonstrate that a static member
// can be accessed before instantiating a class
class Geeks
{
    // static method
    static void m1()
    {
        System.out.println("from m1");
    }

    public static void main(String[] args)
    {
          // calling m1 without creating
          // any object of class Test
           m1();
    }
}

from m1

Static Blocks

If you need to do the computation in order to initialize your static variables, you can declare a static block that gets executed exactly once, when the class is first loaded. 

Example: This example demonstrates the use of a static block to initialize static variables before the main method is executed.

// Java program to demonstrate use of static blocks
class Geeks
{
    // static variable
    static int a = 10;
    static int b;
    
    // static block
    static {
        System.out.println("Static block initialized.");
        b = a * 4;
    }

    public static void main(String[] args)
    {
       System.out.println("from main");
       System.out.println("Value of a : "+a);
       System.out.println("Value of b : "+b);
    }
}

Static block initialized.
from main
Value of a : 10
Value of b : 40

Static Variables

When a variable is declared as static, then a single copy of the variable is created and shared among all objects at the class level. Static variables are, essentially, global variables. All instances of the class share the same static variable.

Important points for static variables:

• We can create static variables at the class level only.
• static block and static variables are executed in the order they are present in a program.

Example: This example demonstrates that static variables are initialized by calling static methods before the static block is executed.

// Java program to demonstrate execution
// of static blocks and variables
class Geeks
{
    // static variable
    static int a = m1();
    
    // static block
    static {
        System.out.println("Inside static block");
    }
    
    // static method
    static int m1() {
        System.out.println("from m1");
        return 20;
    }
    
    // static method(main !!)
    public static void main(String[] args)
    {
       System.out.println("Value of a : "+a);
       System.out.println("from main");
    }
}

from m1
Inside static block
Value of a : 20
from main

Static Methods

When a method is declared with the static keyword, it is known as the static method. The most common example of a static method is the main() method. As discussed above, Any static member can be accessed before any objects of its class are created, and without reference to any object. Methods declared as static have several restrictions: 

• They can only directly call other static methods.
• They can only directly access static data.
• They cannot refer to this or super in any way.

Example: This example demonstrates the restriction that static methods cannot access instance variables, instance methods, or use super in a static context.

// Java program to demonstrate restriction on static methods
class Geeks
{
    // static variable
    static int a = 10;
    
    // instance variable
    int b = 20;
    
    // static method
    static void m1()
    {
        a = 20;
        System.out.println("from m1");
        
         // Cannot make a static reference to the non-static field b
         b = 10; // compilation error
                
         // Cannot make a static reference to the 
                 // non-static method m2() from the type Test
         m2();  // compilation error
         
         //  Cannot use super in a static context
         System.out.println(super.a); // compiler error 
    }
    
    // instance method
    void m2()
    {    
        System.out.println("from m2");
    }
    
    
    
    public static void main(String[] args)
    {
        // main method 
    }
}

Output:

prog.java:18: error: non-static variable b cannot be referenced from a static context
         b = 10; // compilation error
         ^
prog.java:22: error: non-static method m2() cannot be referenced from a static context
         m2();  // compilation error
         ^
prog.java:25: error: non-static variable super cannot be referenced from a static context
         System.out.println(super.a); // compiler error 
                            ^
prog.java:25: error: cannot find symbol
         System.out.println(super.a); // compiler error 
                                 ^
  symbol: variable a
4 errors

When to Use Static Variables and Methods?

Use the static variable for the property that is common to all objects. For example, in class Student, all students share the same college name. Use static methods for changing static variables.

Example: This example demonstrates the use of static variables and methods to share data (like cllgName and counter) across all instances of a class.

// A java program to demonstrate use of
// static keyword with methods and variables

// Student class
class Student {
    String name;
    int rollNo;

    // static variable
    static String cllgName;

    // static counter to set unique roll no
    static int counter = 0;

    public Student(String name)
    {
        this.name = name;

        this.rollNo = setRollNo();
    }

    // getting unique rollNo
    // through static variable(counter)
    static int setRollNo()
    {
        counter++;
        return counter;
    }

    // static method
    static void setCllg(String name) { cllgName = name; }

    // instance method
    void getStudentInfo()
    {
        System.out.println("name : " + this.name);
        System.out.println("rollNo : " + this.rollNo);

        // accessing static variable
        System.out.println("cllgName : " + cllgName);
    }
}

// Driver class
public class StaticDemo {
    public static void main(String[] args)
    {
        // calling static method
        // without instantiating Student class
        Student.setCllg("XYZ");

        Student s1 = new Student("Geek1");
        Student s2 = new Student("Geek2");

        s1.getStudentInfo();
        s2.getStudentInfo();
    }
}

name : Geek1
rollNo : 1
cllgName : XYZ
name : Geek2
rollNo : 2
cllgName : XYZ

Below is the diagrammatic illustration:

Static Classes 

A class can be made static only if it is a nested class. We cannot declare a top-level class with a static modifier but can declare nested classes as static. Such types of classes are called Nested static classes. Nested static class doesn’t need a reference of Outer class. In this case, a static class cannot access non-static members of the Outer class. 

Note: For static nested class, see a static nested class in java

Example: This example demonstrates the use of a static nested class that can be instantiated without needing an instance of the outer class.

// A java program to demonstrate use
// of static keyword with Classes
import java.io.*;

public class Geeks {

    private static String str = "GeeksforGeeks";

    // Static class
    static class MyNestedClass {
      
        // non-static method
        public void disp(){ 
          System.out.println(str); 
        }
    }
  
    public static void main(String args[])
    {
        Geeks.MyNestedClass obj
            = new Geeks.MyNestedClass();
        obj.disp();
    }
}

GeeksforGeeks

Example: This example demonstrates how static variables are shared across all instances of a class, while instance variables are unique to each object.

public class Geeks {
    public static int count = 0;
    public int id;

    public Geeks() {
        count++;
        id = count;
    }

    public static void printCount() {
        System.out.println("Number of instances: " + count);
    }

    public void printId() {
        System.out.println("Instance ID: " + id);
    }

    public static void main(String[] args) {
        Geeks g1 = new Geeks();
        Geeks g2 = new Geeks();
        Geeks g3 = new Geeks();

        g1.printId();
        g2.printId();
        g3.printId();

        Geeks.printCount();
    }
}

Instance ID: 1
Instance ID: 2
Instance ID: 3
Number of instances: 3

Static vs Non-Static

The table below demonstrates the difference between Static and Non-Static

Advantages of Static Keyword

• Static members use the memory only once and this helps save memory when we have to deal with big programs.
• Static members provide fast access because static members belong to the class not to an object and that's why they can be access faster than regular member.
• We can access static members from anywhere, whether an object of the class has been created or not.
• We can use static final variables to create constant that stays the same throughout the program.

Disadvantages of Static Keyword

• Static members can't be overridden or dynamically bound like instance members.
• Static methods and variables make unit testing difficult due to tight coupling.
• Static variables create a global state, which can lead to unwanted side effects across different parts of the program.
• Static variables stay in memory as long as the program runs, which might cause memory to be used longer than needed.
• Using too many static members can reduce the benefits of object-oriented programming, like hiding data and using inheritance.