JAVA #05 – Variables in Java

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Variables: A temporary holding area of memory in which data can be store (also called operand).

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 aliAge(){
int age = 0;
age = age + 7;
System.out.println("Ali age is : " + age);
}

public static void main(String args[]){
Test test = new Test();
test.aliAge();
}
}

Some basic datatypes in Java and its values declaration.

  1. float 10.6756567f
  2. int   16,123
  3. String   “Ali Khan”
  4. byte   10101010111
  5. long 12379873547912
  6. short 0-255
  7. boolean true,false
  8. char ‘A’
  9. double 45.5675675687345

 

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 keyword ‘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 throughout 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) should be called using the fully qualified name .VariableName.

Example:

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(“Ransika”);
empOne.setSalary(1000);
empOne.printEmp();
}
}

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