Skip to main content

Java - Variable Types

A variable provides us with named storage that our programs can manipulate. Each variable in Java has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.
You must declare all variables before they can be used. Following is the basic form of a variable declaration −
data type variable [ = value][, variable [ = value] ...] ;
Here data type is one of Java's datatypes and variable is the name of the variable. To declare more than one variable of the specified type, you can use a comma-separated list.
Following are valid examples of variable declaration and initialization in Java −

Example

int a, b, c;         // Declares three ints, a, b, and c.
int a = 10, b = 10;  // Example of initialization
byte B = 22;         // initializes a byte type variable B.
double pi = 3.14159; // declares and assigns a value of PI.
char a = 'a';        // the char variable a iis initialized with value 'a'
This chapter will explain various variable types available in Java Language. There are three kinds of variables in Java −
  • Local variables
  • Instance variables
  • 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 only this method.

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 will produce the following result −

Output

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 will produce the following error while compiling it −

Output

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 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


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();
   }
}
This will produce the following result −

Output

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 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


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);
   }
}
This will produce the following result −

Output

Development average salary:1000

Comments

Post a Comment

Popular posts from this blog

Spring Security with JWT for REST API

Spring is considered a trusted framework in the Java ecosystem and is widely used. It’s no longer valid to refer to Spring as a framework, as it’s more of an umbrella term that covers various frameworks. One of these frameworks is Spring Security , which is a powerful and customizable authentication and authorization framework. It is considered the de facto standard for securing Spring-based applications. Despite its popularity, I must admit that when it comes to single-page applications , it’s not simple and straightforward to configure. I suspect the reason is that it started more as an MVC application -oriented framework, where webpage rendering happens on the server-side and communication is session-based. If the back end is based on Java and Spring, it makes sense to use Spring Security for authentication/authorization and configure it for stateless communication. While there are a lot of articles explaining how this is done, for me, it was still frustrating to set it up for the f...
Post a Comment
Read more

Java Functional Interfaces

  The term   Java functional interface   was introduced in Java 8. A   functional interface   in Java is an interface that contains only a single abstract (unimplemented) method. A functional interface can contain default and static methods which do have an implementation, in addition to the single unimplemented method. Here is a Java functional interface example: public interface MyFunctionalInterface { public void execute(); } The above counts as a functional interface in Java because it only contains a single method, and that method has no implementation. Normally a Java interface does not contain implementations of the methods it declares, but it can contain implementations in default methods, or in static methods. Below is another example of a Java functional interface, with implementations of some of the methods: public interface MyFunctionalInterface2{ public void execute(); public default void print(String text) { System.out.println(t...
Post a Comment
Read more

Java Logger

In Java, logging is an important feature that helps developers to trace out the errors. Java is the programming language that comes with the logging approach. It provides a Logging API that was introduced in Java 1.4 version. It provides the ability to capture the log file. In this section, we are going to deep dive into the Java Logger API. Also, we will cover logging level, components, Logging handlers or appenders, logging formatters or layouts, Java Logger class, What is logging in Java? In Java, Logging is an API that provides the ability to trace out the errors of the applications. When an application generates the logging call, the Logger records the event in the LogRecord. After that, it sends to the corresponding handlers or appenders. Before sending it to the console or file, the appenders format that log record by using the formatter or layouts. Need for Logging It provides the complete tracing information of the application. It records the critical failure if any occur in ...
Post a Comment
Read more