Thursday, 1 December 2011

Java Variable Types


In Java, all variables must be declared before they can be used. The basic form of a variable declaration is shown here:
type identifier [ = value][, identifier [= value] ...] ;
The type is one of Java's datatypes. The identifier is the name of the variable. To declare more than one variable of the specified type, use a comma-separated list.
Here are several examples of variable declarations of various types. Note that some include an initialization.
int a, b, c;         // declares three ints, a, b, and c.
int d = 3, e, f = 5; // declares three more ints, initializing
                     // d and f.
byte z = 22;         // initializes z.
double pi = 3.14159; // declares an approximation of pi.
char x = 'x';        // the variable x has the value 'x'.
This chapter will explain various variable types available in Java Language. There are three kinds of variables in Java:
  1. Local variables
  2. Instance variables
  3. Class/static variables

Local variables :

  • 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.

Example:

Here age is a local variable. This is defined inside pupAge() method and its scope is limited to this method only.
public class Test{ 
   public void pupAge(){
      int age = 0;
      age = age + 7;
      System.out.println("Puppy age is : " + age)
   }
   
   public static void main(String args[]){
      Test test = new Test();
      Test.pupAge();
   }
}
This would produce following result:
Puppy age is: 7

Example:

Following example uses age without initializing it, so it would give an error at the time of compilation.
public class Test{ 
   public void pupAge(){
      int age;
      age = age + 7;
      System.out.println("Puppy age is : " + age)
   }
   
   public static void main(String args[]){
      Test test = new Test();
      Test.pupAge();
   }
}
This would produce following error while compiling it:
Test.java:4:variable number might not have been initialized
age = age + 7;
         ^
1 error

Instance variables :

  • 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.

Example:

import java.io.*;

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("Ransika");
      empOne.setSalary(1000);
      empOne.printEmp();
   }
}
This would produce following result:
name  : Ransika
salary :1000.0

Class/static variables :

  • 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.

Example:

import java.io.*;

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 = 1000;
      System.out.println(DEPARTMENT+"average salary:"+salary);
   }
}
This would produce following result:
Development average salary:1000
Note: If the variables are access from an outside class the constant should be accessed as Employee.DEPARTMENT