Java variables – Types of Variables

A variable in Java can be thought of as a box. This “box” has a certain size: the memory that is allocated for it. How much memory will be allocated depends on the type of the variable, but we are going to talk about this a little bit later. At the same time, the size of the box itself cannot be changed after its creation, but the contents can. The box may be empty. You can “insert” some value into it, and then extract and put some other value.

Thus, a variable is a field that stores data values during the execution of a Java program and also we can say Variable is a name of memory location. It is the basic unit of storage in a program.

Types of Variables

1) local variable

2 ) instance variable

3) class/static variable

Variable Declaration and Initialization

You must declare all variables before they can be used.

1) Declaration: Before using a variable, you must declare it. This involves specifying the variable’s data type and giving it a name.

For Example:

int age; // Declaration of an integer variable named age

2) Initialization:- After declaring a variable, you can optionally initialize it, which means assigning it an initial value. Uninitialized variables have a default value depending on their type (e.g., 0 for numeric types, false for boolean, null for reference types).

For Example:

int age = 25; // Initialization of the age variable with a value of 25

3) Data Types: Java supports various data types for variables, including primitive types (such as int, double, boolean, char) and reference types (such as objects and arrays). Each data type has different sizes and ranges.

1. Java 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 1: Variable’s local scope with initialization

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();
   }
}

Output

Puppy age is: 7

Example 2: Variable’s local scope without initialization—

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();
   }
}

Output

Test.java:4:variable number might not have been initialized
age = age + 7;
         ^
1 error

2. Java 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 (when instance variables are given accessibility), they should be called using the fully qualified name. ObjectReference.VariableName.

Example of Java Instance Variables

 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();
   }
}

Output

name  : Ransika
salary :1000.0

3. Java 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 the 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 variable 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 of Java Class/Static 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 = 1000;
      System.out.println(DEPARTMENT + "average salary:" + salary);
   }
}

Output

Development average salary:1000

Conclusion

Variables in Java are important objects that store data for manipulation and use throughout the program. There are three main types of variables: local, instance, and class (static) variables. Local variables are declared in methods, constructors, and blocks and are accessible only in those scopes. Instance variables, also known as volatile fields, are declared within the class but outside of each method and unique to each instance of the class. The static keyword is used to declare class variables, or static fields, within a class and is shared across all instances of that class. Understanding these variables is important for effective Java programming, as they determine the size, lifetime, and availability of the variables, thus influencing program design and behavior

Frequently Asked Questions