Dinesh on Java

int var = 10;//Here var is variable

Types of Variable:
There are three types of variables in java

1. local variable
2. instance variable
3. static variable

Local Variable
A variable that is declared inside the method is called local variable.

• Local variables are declared in methods, constructors, or blocks.
• Local variables are created when the method, constructor or block is entered and the variable will be destroyed once it exits the method, constructor or block.
• Access modifiers cannot be used for local variables.
• Local variables are visible only within the declared method, constructor or block.
• Local variables are implemented at stack level internally.
• There is no default value for local variables so local variables should be declared and an initial value should be assigned before the first use.

public class TestDemo{ 
   public void calc(){
      int sum = 0;
      sum = sum + 17;
      System.out.println("Total sum is : " + sum);
   }
   
   public static void main(String args[]){
      TestDemo test = new TestDemo();
      test.calc();
   }
}

This would produce following result:

Following example uses sum without initializing it, so it would give an error at the time of compilation.

public class TestDemo{ 
   public void calc(){
      int sum;
      sum = sum + 17;
      System.out.println("Total sum is : " + sum);
   }
   
   public static void main(String args[]){
      TestDemo test = new TestDemo();
      test.calc();
   }
}

This would produce following result:

Instance Variable
A variable that is declared inside the class but outside the method is called instance variable . It is not declared as static.

• Instance variables are declared in a class, but outside a method, constructor or any block.
• When a space is allocated for an object in the heap a slot for each instance variable value is created.
• Instance variables are created when an object is created with the use of the key word ‘new’ and destroyed when the object is destroyed.
• Instance variables hold values that must be referenced by more than one method, constructor or block, or essential parts of an object.s state that must be present through out the class.
• Instance variables can be declared in class level before or after use.
• Access modifiers can be given for instance variables.
• The instance variables are visible for all methods, constructors and block in the class. Normally it is recommended to make these variables private (access level).However visibility for subclasses can be given for these variables with the use of access modifiers.
• Instance variables have default values. For numbers the default value is 0, for Booleans it is false and for object references it is null. Values can be assigned during the declaration or within the constructor.
• Instance variables can be accessed directly by calling the variable name inside the class. However within static methods and different class ( when instance variables are given accessibility) the should be called using the fully qualified name . ObjectReference.VariableName.

import java.io.*;

public class Employee{
   // this instance variable is visible for any child class.
   public String name;
   
   // salary  variable is visible in Employee class only.
   private double salary;
   
   // The name variable is assigned in the constructor. 
   public Employee (String empName){
      name = empName;
   }

   // The salary variable is assigned a value.
   public void setSalary(double empSal){
      salary = empSal;
   }
   
   // This method prints the employee details.
   public void printEmp(){
      System.out.println("name  : " + name );
      System.out.println("salary :" + salary);
   }

   public static void main(String args[]){
      Employee empOne = new Employee("Dinesh Rajput");
      empOne.setSalary(70000);
      empOne.printEmp();
   }
}

This would produce following result:

Static variable
A variable that is declared as static is called static variable. It cannot be local.

• Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.
• There would only be one copy of each class variable per class, regardless of how many objects are created from it.
• Static variables are rarely used other than being declared as constants. Constants are variables that are declared as public/private, final and static. Constant variables never change from their initial value.
• Static variables are stored in static memory. It is rare to use static variables other than declared final and used as either public or private constants.
• Static variables are created when the program starts and destroyed when the program stops.
• Visibility is similar to instance variables. However, most static variables are declared public since they must be available for users of the class.
• Default values are same as instance variables. For numbers the default value is 0, for Booleans it is false and for object references it is null. Values can be assigned during the declaration or within the constructor. Additionally values can be assigned in special static initializer blocks.
• Static variables can be accessed by calling with the class name . ClassName.VariableName.
• When declaring class variables as public static final, then variables names (constants) are all in upper case. If the static variables are not public and final the naming syntax is the same as instance and local variables.

import java.io.*;

public class Employee{
   // salary  variable is a private static variable
   private static double salary;

   // DEPARTMENT is a constant
   public static final String DEPARTMENT = "Development ";

   public static void main(String args[]){
      salary = 70000;
      System.out.println(DEPARTMENT+"average salary:"+salary);
   }
}

This would produce following result:

Note: If the variables are access from an outside class the constant should be accessed as Employee.DEPARTMENT

In Java, all variables must be declared before they can be used. The basic form of a variable declaration is shown here: