Variables in Java - GeeksforGeeks

A variable is a name given to a memory location. It is the basic unit of storage in a program.

The value stored in a variable can be changed during program execution.
A variable is only a name given to a memory location, all the operations done on the variable effects that memory location.
In Java, all the variables must be declared before use.

How to declare variables?

We can declare variables in java as follows:

type: Type of data that can be stored in this variable.
name: Name given to the variable.
In this way, a name can only be given to a memory location. It can be assigned values in two ways:

Variable Initialization
Assigning value by taking input

How to initialize variables?

datatype: Type of data that can be stored in this variable.
variable_name: Name given to the variable.
value: It is the initial value stored in the variable.

Examples:

float simpleInterest; //Declaring float variable
int time = 10, speed = 20; //Declaring and Initializing integer variable
char var = 'h'; // Declaring and Initializing character variable

Types of variables

There are three types of variables in Java:

Local Variables
Instance Variables
Static Variables

Let us now learn about each one of these variables in detail.

Local Variables: A variable defined within a block or method or constructor is called local variable.

These variable are created when the block in entered or the function is called and destroyed after exiting from the block or when the call returns from the function.
The scope of these variables exists only within the block in which the variable is declared. i.e. we can access these variable only within that block.
Initilisation of Local Variable is Mandatory.

Sample Program 1:

public class StudentDetails { 
    public void StudentAge() 
    { 
        // local variable age 
        int age = 0; 
        age = age + 5; 
        System.out.println("Student age is : " + age); 
    } 
  
    public static void main(String args[]) 
    { 
        StudentDetails obj = new StudentDetails(); 
        obj.StudentAge(); 
    } 
} 

Output:

Student age is : 5

In the above program, the variable age is a local variable to the function StudentAge(). If we use the variable age outside StudentAge() function, the compiler will produce an error as shown in below program.

Sample Program 2:

public class StudentDetails { 
    public void StudentAge() 
    { // local variable age 
        int age = 0; 
        age = age + 5; 
    } 
  
    public static void main(String args[]) 
    { 
        // using local variable age outside it's scope 
        System.out.println("Student age is : " + age); 
    } 
} 

Output:

Compilation Error in java code :- 
prog.java:12: error: cannot find symbol
        System.out.println("Student age is : " + age);
                                                 ^
  symbol:   variable age
  location: class StudentDetails
1 error

Instance Variables: Instance variables are non-static variables and are declared in a class outside any method, constructor or block.

As instance variables are declared in a class, these variables are created when an object of the class is created and destroyed when the object is destroyed.
Unlike local variables, we may use access specifiers for instance variables. If we do not specify any access specifier then the default access specifier will be used.
Initilisation of Instance Variable is not Mandatory. Its default value is 0
Instance Variable can be accessed only by creating objects.

Sample Program:

import java.io.*; 
class Marks { 
    // These variables are instance variables. 
    // These variables are in a class 
    // and are not inside any function 
    int engMarks; 
    int mathsMarks; 
    int phyMarks; 
} 
  
class MarksDemo { 
    public static void main(String args[]) 
    { 
        // first object 
        Marks obj1 = new Marks(); 
        obj1.engMarks = 50; 
        obj1.mathsMarks = 80; 
        obj1.phyMarks = 90; 
  
        // second object 
        Marks obj2 = new Marks(); 
        obj2.engMarks = 80; 
        obj2.mathsMarks = 60; 
        obj2.phyMarks = 85; 
  
        // displaying marks for first object 
        System.out.println("Marks for first object:"); 
        System.out.println(obj1.engMarks); 
        System.out.println(obj1.mathsMarks); 
        System.out.println(obj1.phyMarks); 
  
        // displaying marks for second object 
        System.out.println("Marks for second object:"); 
        System.out.println(obj2.engMarks); 
        System.out.println(obj2.mathsMarks); 
        System.out.println(obj2.phyMarks); 
    } 
} 

Output:

Marks for first object:
50
80
90
Marks for second object:
80
60
85

As you can see in the above program the variables, engMarks , mathsMarks , phyMarksare instance variables. In case we have multiple objects as in the above program, each object will have its own copies of instance variables. It is clear from the above output that each object will have its own copy of instance variable.

Static Variables: Static variables are also known as Class variables.

These variables are declared similarly as instance variables, the difference is that static variables are declared using the static keyword within a class outside any method constructor or block.
Unlike instance variables, we can only have one copy of a static variable per class irrespective of how many objects we create.
Static variables are created at the start of program execution and destroyed automatically when execution ends.
Initilisation of Static Variable is not Mandatory. Its default value is 0
If we access the static variable like Instance variable (through an object), the compiler will show the warning message and it won’t halt the program. The compiler will replace the object name to class name automatically.
If we access the static variable without the class name, Compiler will automatically append the class name.

To access static variables, we need not create an object of that class, we can simply access the variable as

class_name.variable_name;

Sample Program:

import java.io.*; 
class Emp { 
  
    // static variable salary 
    public static double salary; 
    public static String name = "Harsh"; 
} 
  
public class EmpDemo { 
    public static void main(String args[]) 
    { 
  
        // accessing static variable without object 
        Emp.salary = 1000; 
        System.out.println(Emp.name + "'s average salary:"
                           + Emp.salary); 
    } 
} 

Output:

Harsh's average salary:1000.0

Instance variable Vs Static variable

Each object will have its own copy of instance variable whereas We can only have one copy of a static variable per class irrespective of how many objects we create.
Changes made in an instance variable using one object will not be reflected in other objects as each object has its own copy of instance variable. In case of static, changes will be reflected in other objects as static variables are common to all object of a class.
We can access instance variables through object references and Static Variables can be accessed directly using class name.

Syntax for static and instance variables:

class Example
    {
        static int a; //static variable
        int b;        //instance variable
    }

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