static keyword in java

Filed Under: Java

static keyword in Java is used a lot in java programming. Java static keyword is used to create a Class level variable in java. static variables and methods are part of the class, not the instances of the class.

static keyword in java

java static, static keyword in java

Java static keyword can be used in five cases as shown in below image.

static keyword in java

We will discuss four of them here, the fifth one was introduced in Java 8 and that has been discussed at Java 8 interface changes.

  1. Java static variable

    We can use static keyword with a class level variable. A static variable is a class variable and doesn’t belong to Object/instance of the class.

    Since static variables are shared across all the instances of Object, they are not thread safe.

    Usually, static variables are used with the final keyword for common resources or constants that can be used by all the objects. If the static variable is not private, we can access it with ClassName.variableName

    
        //static variable example
        private static int count;
        public static String str;
        public static final String DB_USER = "myuser";
    
  2. Java static method

    Same as static variable, static method belong to class and not to class instances.

    A static method can access only static variables of class and invoke only static methods of the class.

    Usually, static methods are utility methods that we want to expose to be used by other classes without the need of creating an instance. For example Collections class.

    Java Wrapper classes and utility classes contains a lot of static methods. The main() method that is the entry point of a java program itself is a static method.

    
        //static method example
        public static void setCount(int count) {
            if(count > 0)
            StaticExample.count = count;
        }
        
        //static util method
        public static int addInts(int i, int...js){
            int sum=i;
            for(int x : js) sum+=x;
            return sum;
        }
    

    From Java 8 onwards, we can have static methods in interfaces too. For more details please read Java 8 interface changes.

  3. Java static block

    Java static block is the group of statements that gets executed when the class is loaded into memory by Java ClassLoader.

    Static block is used to initialize the static variables of the class. Mostly it’s used to create static resources when the class is loaded.

    We can’t access non-static variables in the static block. We can have multiple static blocks in a class, although it doesn’t make much sense. Static block code is executed only once when the class is loaded into memory.

    
        static{
            //can be used to initialize resources when class is loaded
            System.out.println("StaticExample static block");
            //can access only static variables and methods
            str="Test";
            setCount(2);
        }
    
  4. Java Static Class

    We can use static keyword with nested classes. static keyword can’t be used with top-level classes.

    A static nested class is same as any other top-level class and is nested for only packaging convenience.

    Read: Java Nested Classes

Let’s see all the static keyword in java usage in a sample program.

StaticExample.java


package com.journaldev.misc;

public class StaticExample {

    //static block
    static{
        //can be used to initialize resources when class is loaded
        System.out.println("StaticExample static block");
        //can access only static variables and methods
        str="Test";
        setCount(2);
    }
    
    //multiple static blocks in same class
    static{
        System.out.println("StaticExample static block2");
    }
    
    //static variable example
    private static int count; //kept private to control its value through setter
    public static String str;
    
    public int getCount() {
        return count;
    }

    //static method example
    public static void setCount(int count) {
        if(count > 0)
        StaticExample.count = count;
    }
    
    //static util method
    public static int addInts(int i, int...js){
        int sum=i;
        for(int x : js) sum+=x;
        return sum;
    }

    //static class example - used for packaging convenience only
    public static class MyStaticClass{
        public int count;
        
    }

}

Let’s see how to use static variable, method and static class in a test program.

TestStatic.java


package com.journaldev.misc;

public class TestStatic {

    public static void main(String[] args) {
        StaticExample.setCount(5);
        
        //non-private static variables can be accessed with class name
        StaticExample.str = "abc";
        StaticExample se = new StaticExample();
        System.out.println(se.getCount());
        //class and instance static variables are same
        System.out.println(StaticExample.str +" is same as "+se.str);
        System.out.println(StaticExample.str == se.str);
        
        //static nested classes are like normal top-level classes
        StaticExample.MyStaticClass myStaticClass = new StaticExample.MyStaticClass();
        myStaticClass.count=10;
        
        StaticExample.MyStaticClass myStaticClass1 = new StaticExample.MyStaticClass();
        myStaticClass1.count=20;
        
        System.out.println(myStaticClass.count);
        System.out.println(myStaticClass1.count);
    }
    
}

The output of the above static keyword in java example program is:


StaticExample static block
StaticExample static block2
5
abc is same as abc
true
10
20

Notice that static block code is executed first and only once as soon as class is loaded into memory. Other outputs are self-explanatory.

Java static import

Normally we access static members using Class reference, from Java 1.5 we can use java static import to avoid class reference. Below is a simple example of Java static import.


package com.journaldev.test;

public class A {

	public static int MAX = 1000;
	
	public static void foo(){
		System.out.println("foo static method");
	}
}

package com.journaldev.test;

import static com.journaldev.test.A.MAX;
import static com.journaldev.test.A.foo;

public class B {

	public static void main(String args[]){
		System.out.println(MAX); //normally A.MAX
		foo(); // normally A.foo()
	}
}

Notice the import statements, for static import we have to use import static followed by the fully classified static member of a class. For importing all the static members of a class, we can use * as in import static com.journaldev.test.A.*;. We should use it only when we are using the static variable of a class multiple times, it’s not good for readability.

Update: I have recently created a video to explain static keyword in java, you should watch it below.

Comments

  1. Rohit Tandon says:

    Hi Pankaj,

    I use journaldev for preparation of interviews. It covers a lot and make it easy to crack interview.
    One thing I would like to include in core java interview questons is
    Can we invoke this keyword in static method or block?
    Can we invoke super keyword in static method or block

    1. Sangita says:

      we can’t use this and super keyword with static members

  2. Newton says:

    Hello Pankaj Sir,

    Thank you so this blog. I just have 1 question after reading static block .

    JDK 6 allows to run java program without main method using static block as static block execute first at the time of class loading . and jdk 7 and jdk 8 will compile program but will give error as Main method not found at runtime . So In Java 7 or 8 , why it is not executing static block and print statement from static block first and then print error for main method not found. It directly throws error for main .

    Please clear my doubt or if I am wrong.

    1. LUCIFER says:

      Always when static block is made ……It is always initialized before main() function …..this is the special thing about it.

  3. Venkat says:

    Very nice blog.
    Static is a keyword is applied to the following in Java.
    1) Variable
    2) Method
    3) Class
    4) Static blocks

    More details on java-w3schools.blogspot.in/2013/08/static-in-java.html

  4. milad says:

    hi Pankaj, thanks for great article T i have question please answer me
    if we have static gloabal object, where store in memory??? heap or ….????
    generaly static object different place stored than another object ???
    please cleear me

    1. Pratiksha Sawant says:

      It will be stored on heap.

      1. mahesh says:

        Method Area

    2. sadiqullah says:

      in heap but static variable is alive at the end of program

  5. Anurag Singh says:

    this article is very useful.
    it clear all the doubts about static keyword in java.
    thank you

  6. mossama says:

    Hi, I need to ask you a question after reading How do you gain your knowledge as you answered from video tutorials and some e-books so If I depend on your blog to gain my knowledge it will be not enough to be a good java developer and crack java interviews,
    Thanks in Advance

  7. Emanuele says:

    Hi,

    I know that, for the sake of your example, you created a StaticExample() object (se) to compare its .count with StaticExample class .count object .
    My question is: don’t you need a constructor to create an object?

    Thanks

    1. Gooner8 says:

      for creating a object constructor is requried but if programmer dosenot specify any construstor then Compiler itself adds default constructor . This constructor is not visible to us.
      Hope u got it!!!

  8. Sonia says:

    Static Variable and Static Block are executed before Main method gets executed,but Static Variable and Static Block are executed based on the order we are declare from top to bottom

  9. Gurpreet Singh says:

    Why static block is needed , though we can intialize static variables at the time of declaring static variable

  10. Davinder says:

    Is it possible to implement static methods in interface before Java 8 ? What is the improvement that would be brought in with this change in Java 8 static mehtods in interface.?

  11. Sahdev says:

    Hello Pnakaj Sir,

    First i would like to thank you for very good blog in every concept in java.

    Here i have one question related static block and static variable.

    what is the load first static block else static variable.

    “My understanding is first static variable will be loaded”.

    But i am confused for below code please help me.

    In the below program i have declared static block first.

    public class StaticRud {

    static {
    a = 20;
    }

    private static int a = 10;

    public static void main(String[] args) {
    System.err.println(StaticRud.a);

    }

    }

    Out put is : 10

    =============================

    and in the below code i have declared static variable first

    public class StaticRud {

    private static int a = 10;

    static {
    a = 20;
    }

    public static void main(String[] args) {
    System.err.println(a);

    }

    }

    Out put is : 20

    Is there any way to declare static variable before or after static block?

    Please help me sir.

    1. Pankaj says:

      That’s an interesting observation, it might be happening because of the way of execution of the code. Ideally if you are initialize a variable in the static block, you should not initialize it at the time of assignment.

  12. Sumit hazra says:

    Pankaj Sir, your blog help me lot to understand the basic concept of JAVA. Thank you sir .

  13. Arya says:

    Can you please explain me difference between singleton pattern and object created through Static keyword.What are the advantages of singleton pattern.

    1. Pankaj says:

      object created through static keyword

      I didn’t understood what you are trying to say here.

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