Quick introduction to Java Programming language - Part 1

Variable is a name of memory location. In Java each variable has a specific type, which determines the size and layout of the variable's memory.

Types of Variable:

There are three types of java variables

• Local variable
• Global/static/class variable
• Instance variable

Local Variables:

• Local variable declared within a method are known as local variable.
• In java local variables must be initialized before using them or else there will be compilation error.
• These variables are not members of class; hence there is no concept of static or non-static here.
• Local variables are available only for a specific method. We cannot use the local variable outside the method.

Example:

public lass RunLocalVar
{
  public void setValue()
  {
      int age =10;     //Here "age" is a local variable.
      System.out.println("set value method");
   }
}
 

Global Variable:

• Global variable declared within a class directly not within any method are known as Global variable.
• It can be used by any method of the class.
• If we declare a global variables as public static final, then those variables are constants, it should be in upper case.
• Are created when the program starts executed and destroyed when the execution ends.
• Only one copy of global variables in a class, but can any numbers of objects are created from it.

Example:

public class RunGlobalVar
{
 int temp; //Here “temp” is a Global variable
 float sum=3.2;// Here “sum” is a Global variable
 public void setValue()
 {
 int age =10;     //Here “age” is a local variable.
 System.out.println(“set value method”);
    }
}

Data Members are two types:

Static: Member declared with a keyword “static”.

Non-static: Member declared without static key word (blank).

public class A
{

// data members

  Variable 1; // non static
  static Variable 2;  // static

//Member functions 

  Method 1;
  Method 2; 
}

Instance variable:

• These variables are created at the time of object is created by using “new” keyword.
• These variables can be declared in class before or after use.
• Instance variables have default values:
  Default values 
  For numbers the default value is 0
  For Booleans it is false and
  For object references it is null.
• These are declared in a class, but outside a method, constructor.
  

  import java.io.*;
  public class InstanceVar {
  	public String name;
  	double salary;
  	
  	public void subName(String subjectName){
  	      name = subjectName;  	   
  	}
  	// To assing a value for salary variable.
  	public void setSalary(doubleempSal){
  	      salary = empSal; 
  	}
  	// Method to print the employee details.
  	public void printDetails()
         {
  		System.out.println("Subject name    : " + name );
  		System.out.println("Assigned salary : " + salary);
  	}
  	public static void main(String args[]){
  		InstanceVar instance = newInstanceVar();
  		instance.subName("Java Instance Variable");
  		instance.setSalary(30000);
  		instance.printDetails();
  	   }	
  }      
  

  
  The output: