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About Petros Koulianos

Petros Koulianos
Petros studied Computer Science in Hellenic Open University . He is an experienced administrator with a demonstrated history of working in the health information technology systems and health services industry. Skilled in Oracle Databases, HL7 Standards, Health Information Systems (HIS), Laboratory Information Systems (LIS) . In his free time designs and develop apps in Java , Android and node.js .

Static Variable Java Example

1. Introduction

This is a static variable Java Example. Static variables are declared with the static word before the type of variable. The main difference among regular variables is that static variables are not bounded by any object instances, and are shared among all instances.

static datatype Variable
//static variable example
static int count = 0; 

2. Simple Static Variable Java Example

Static variables are useful when we need to declare a specific value among all instances. We can access Static variables in 2 ways:

static variables java
  1. By Class name
  2. By Instance name

Create a class with name Laptop and paste the following code :

Laptop.java
package com.javacodegeeks;
/**
 * @author Petros Koulianos
 *
 */
public class Laptop {

	//static variable
	public static String motherboard = "ASUS";
	
	public String cpu ;
	
	public String monitor_dimension ;
	
	public String storage ;
	
	

}

Create a second class with name StaticVariableExample1 and paste the following code :

StaticVariableExample1.java
package com.javacodegeeks;
 
/**
 * @author Petros Koulianos
 *
 */
public class StaticVariableExample1 {
     
 
    public static void main(String[] args) {
           	
        Laptop laptop1 = new Laptop();
         
        laptop1.cpu = "i5";
         
        laptop1.monitor_dimension = "15 inch";
         
        laptop1.storage = "500 gb";
         
        Laptop laptop2 = new Laptop();
         
        laptop2.cpu = "i3";
         
        laptop2.monitor_dimension = "17 inch";
         
        laptop2.storage = "1 tb";
         
        //static variables can be access with Class name or with instance name
        //preferred way is to refer with Class name
        System.out.println("laptop1  motherboard:"+Laptop.motherboard+" cpu:"+laptop1.cpu+" monitor_dimension:"+laptop1.monitor_dimension+" storage:"+laptop1.storage);
         
        System.out.println("laptop2  motherboard:"+laptop2.motherboard+" cpu:"+laptop2.cpu+" monitor_dimension:"+laptop2.monitor_dimension+" storage:"+laptop2.storage);
         
        // The company has change the motherboard
        Laptop.motherboard = "GIGABYTE";
         
         
        System.out.println("laptop1  motherboard:"+Laptop.motherboard+" cpu:"+laptop1.cpu+" monitor_dimension:"+laptop1.monitor_dimension+" storage:"+laptop1.storage);
         
        System.out.println("laptop2  motherboard:"+laptop2.motherboard+" cpu:"+laptop2.cpu+" monitor_dimension:"+laptop2.monitor_dimension+" storage:"+laptop2.storage);
 
    }
     
 
}
Example Output
laptop1  motherboard:ASUS cpu:i5 monitor_dimension:15 inch storage:500 gb
laptop2  motherboard:ASUS cpu:i3 monitor_dimension:17 inch storage:1 tb
laptop1  motherboard:GIGABYTE cpu:i5 monitor_dimension:15 inch storage:500 gb
laptop2  motherboard:GIGABYTE cpu:i3 monitor_dimension:17 inch storage:1 tb

The above example shows how to declare a static variable and how to access it.

3. Static Variable in Static Blocks

Static blocks can access static variables since static variables are initialized before the execution of a static block.

Update Laptop.java Class and add the following code:

   // static block  can access static variables  
    static {
    	
    	System.out.println("motherboard: "+motherboard);
    	
    }

Now run the StaticVariableExample1.java Class

Example Output
motherboard : ASUS
laptop1  motherboard:ASUS cpu:i5 monitor_dimension:15 inch storage:500 gb
laptop2  motherboard:ASUS cpu:i3 monitor_dimension:17 inch storage:1 tb
laptop1  motherboard:GIGABYTE cpu:i5 monitor_dimension:15 inch storage:500 gb
laptop2  motherboard:GIGABYTE cpu:i3 monitor_dimension:17 inch storage:1 tb

In the above example, we saw that when the static block runs can access and print the static variable.

4. Static Methods

The static variables can also be accessed inside static methods.

Update Laptop.java Class and add the following code:

Laptop.java
package com.javacodegeeks;
/**
 * @author Petros Koulianos
 *
 */
public class Laptop {
       
    //static variable 
    public static String motherboard = "ASUS";
     
    public String cpu ;
     
    public String monitor_dimension ;
     
    public String storage ;
     
   // static block  can access static variables 
	
	  static {
	  
	  System.out.println("static block motherboard : "+motherboard);
	  
	  }
	 
    
    //This is a Static Methods access static variables 
    static void motherboardname(){
    	
        System.out.println("static method motherboard : "+motherboard);
        
    }
    

 
}

Update StaticVariableExample1.java Class and add the following code:

StaticVariableExample1 .java
package com.javacodegeeks;
 
/**
 * @author Petros Koulianos
 *
 */
public class StaticVariableExample1 {
     
 
    public static void main(String[] args) {
         
    	Laptop.motherboardname();
    	
        Laptop laptop1 = new Laptop();
         
        laptop1.cpu = "i5";
         
        laptop1.monitor_dimension = "15 inch";
         
        laptop1.storage = "500 gb";
         
        Laptop laptop2 = new Laptop();
         
        laptop2.cpu = "i3";
         
        laptop2.monitor_dimension = "17 inch";
         
        laptop2.storage = "1 tb";
         
        //static variables can be access with Class name or with instance name
        //preferred way is to refer with Class name
        System.out.println("laptop1  motherboard:"+Laptop.motherboard+" cpu:"+laptop1.cpu+" monitor_dimension:"+laptop1.monitor_dimension+" storage:"+laptop1.storage);
         
        System.out.println("laptop2  motherboard:"+laptop2.motherboard+" cpu:"+laptop2.cpu+" monitor_dimension:"+laptop2.monitor_dimension+" storage:"+laptop2.storage);
         
        // The company has change the motherboard
        Laptop.motherboard = "GIGABYTE";
         
         
        System.out.println("laptop1  motherboard:"+Laptop.motherboard+" cpu:"+laptop1.cpu+" monitor_dimension:"+laptop1.monitor_dimension+" storage:"+laptop1.storage);
         
        System.out.println("laptop2  motherboard:"+laptop2.motherboard+" cpu:"+laptop2.cpu+" monitor_dimension:"+laptop2.monitor_dimension+" storage:"+laptop2.storage);
 
    }
     
 
}

Now run the StaticVariableExample1.java Class

Example Output
static block motherboard : ASUS
static method motherboard : ASUS
laptop1  motherboard:ASUS cpu:i5 monitor_dimension:15 inch storage:500 gb
laptop2  motherboard:ASUS cpu:i3 monitor_dimension:17 inch storage:1 tb
laptop1  motherboard:GIGABYTE cpu:i5 monitor_dimension:15 inch storage:500 gb
laptop2  motherboard:GIGABYTE cpu:i3 monitor_dimension:17 inch storage:1 tb

5. Static final variables

It is very common to combine static variables with the final modifier to declare constants. Keep in mind that by convention final static values are spelled in uppercase letters .

    //final static variable 
    public final static String COMPANY ="FOO Company";

6. Static variables and Multithreading

Ιt is very important to know that static variables are not Thread-Safe. This means if 2 or more threads modify the same static variable, unexpected and multiple malfunctions can be caused (ex. dead locks etc) .

Create a class with name StaticVariableExample2 and paste the following code :

StaticVariableExample2.java
package com.javacodegeeks;

/**
 * @author Petros Koulianos
 *
 */

public class StaticVariableExample2 {

	static int counter = 10;
	
	public static void main(String[] args) {
		
		//launch first thread with step= -1
		newThread("first thread",500,1);
		
		
		//launch second thread with step= -3
		newThread("second thread",250,3);
		
	}
	
	public static void newThread(String name , int sleep , int step) {
		
		Thread th = new Thread() {
			@Override
			public void run() {
				
				while(counter != 0) {
					
					System.out.println("**"+name+"** counter:"+counter);
					
					counter = counter - step;
					
					try {
						//sleep the thread to slow down the execution
						Thread.sleep(sleep);
						
					} catch (InterruptedException e) {
						
						e.printStackTrace();
					}
				}
				
			}
			
		};
		
		th.start();
	}

}

Example Output
**second thread** counter:10
**first thread** counter:10
**second thread** counter:6
**second thread** counter:3
**first thread** counter:3
**second thread** counter:-1
**first thread** counter:-4
**second thread** counter:-4
**second thread** counter:-8
**first thread** counter:-11
**second thread** counter:-11
**second thread** counter:-15
**first thread** counter:-18
**second thread** counter:-19
**second thread** counter:-22
**first thread** counter:-25
**second thread** counter:-26
**second thread** counter:-29
**first thread** counter:-32
**second thread** counter:-33
............................

The above example runs 2 separate threads that modify the same static variable differently and the result is to fall both threads into a deadlock.

7. Download the Source Code

This was a Static variable Java example.

Download
You can download the full source code of this example here: Static Variable Java Example

Last updated on Sept. 10, 2019

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