Variable Types in Java with Example

This tutorial explains different types of variables in java, differences between them and their allocations in memory. The tutorial will help the beginners to use the correct variable type for their need in programs. In java there are three types of variables :

• Local variable
• Instance variable
• Static or Class variable

Java Local variable

Variables defined inside methods, constructors or blocks are called local variables. These variables are created in memory(stack memory) when execution of that method, constructor or block starts in which they are declared. These variables are accessible only within the method, constructor or block in which they are created. Local variables must be initialized with some value before their use otherwise compiler throws an error.

How to declare and access local variable in java

The declaration of local variable is same as normal variable, the only thing is that they are declared in method, constructor or block. In other way if you have declared variables in method, constructor or block they are basically the local variables in java. To access a local variable, you just have to use it's name. The program below shows how to create and access local variables in java.

Local variable program in java

 class LocalVariable {
   public LocalVariable() {
     int a = 10; // local variable a, created inside constructor
     System.out.println("a = "+a);
   }
   {
     int b = 30; // local variable b, created inside block
     System.out.println("b = "+b);
   }   
   public static void main(String [] args) {
     int c = 20; // local variable c, created inside method     
     System.out.println("c = "+c);
     // Constructor and block will be executed by below line
     LocalVariable lv = new LocalVariable();
     int d; // local variable d, created inside method
     // Below code will throw error if uncommented, as d is not initialized.
     // System.out.println("d = "+d);     
   } 
 }

Output:

Java Instance variable

Variables defined inside a class but outside any method, constructor or block are known as instance variable. Initialization of instance variable is not compulsory, if not initialized, these variables will have default values as per their data type

Instance variables, as the name suggests belongs to the instance or object of the class. Instance variables are created in memory(heap memory) as soon as the object of the class containing the instance variable is created. Every object of a class has it's own copy of instance variables, changes made to the variable value of one object doesn't reflect in other object.

How to declare and access instance variable in java

The declaration of instance variable is same as normal variable, the only thing is that they are declared inside class directly. In other way if you have declared variables inside a class directly, they are basically the instance variables in java

The Instance variables are accessed using object name like objName.instanceVarName. You can also access the instance variable directly as well, in this case java internally applies the object name for you. The good practice is to always use the object name for accessing the instance variable. The program below shows how to declare and access instance variables in java.

Instance variable program in java

 class InstanceVariable {
    int age = 20; // instance variable age, created inside class
    String name = "Rahul"; // instance variable name, created inside class
    public static void main(String [] args) {
      InstanceVariable iv = new InstanceVariable();
      System.out.println("Age = " +iv.age);
      System.out.println("Name = " +iv.name);
    }    
 }

Output:

Class variable in Java

Variables defined inside a class(not inside method, constructor or block) using static keyword are known as static or class variables. Initialization of class variable is also not compulsory, if not initialized, these variables will have default values as per their data type. These variables are known as class variables because they belong to class, not objects.

Class variables as the name suggests, belongs to class and is common to all objects of that class. These variables are created in memory as soon as the class containing the variable is loaded inside memory. You can refer static keyword tutorial to get complete detail about static or class variable in java.

How to declare and access class variable in java

The declaration of static or class variable is same as instance variable, except that they are declared along with static keyword. Static variable can be accessed using class name as className.staticVarName or using any of the object instances as well like objName.staticVarName. The program below shows how to declare and access class variable in java.

Class variable program in java

 class ClassVariable {
    static String classVar = "Class variable Test"; // class variable classVar
    public static void main(String [ ] args) {
      System.out.println(ClassVariable.classVar);
    }    
 }

Output:

Difference between local, instance and static variable in java

Java Program of local, instance and static variable

 class Student 
   {	
      static String college = "SUMVS"; // static or class variable college
      int rollNo;  // instance variable rollNo
      String name; // instance variable name     
   
      public static void main(String [] args)
        {  
           Student s1 = new Student(); // local variable s1
           s1.rollNo = 1;
           s1.name = "Ram";
                   
           Student s2 = new Student(); // local variable s2
           s2.rollNo = 2;
           s2.name = "Shyam";          
         
           s1.printDetail();
           s2.printDetail();
         
           int count = 2; // local variable count
           System.out.println("Total students = "+ count);
        }       
      void printDetail()
        {
           int count2 = 2; // local variable count2
           System.out.println(rollNo +", "+ name+", "+ Student.college+", "+ count2);
        }
   } 

Output:

Memory allocation for local, instance and static variables in Java

The image below shows where the memory is allocated for different types of java variables used in above program. The variable count and count2 are local variables, rollNo and Name are instance variables whereas college is a class variable.