Java is a robust language and is used to build many programs.

Variables are the building blocks of any language. Variables in Java have a huge role to play in making it a widely accepted language.

What is a variable? A variable can be defined as a name given to a certain memory location, which is a basic unit in a program. All languages use variables to store data and the value of a variable can be changed during the execution of a program. Any operation done on the variable will affect the memory location.

It's critical that you understand how variables work if you're learning to become a Java programmer. In this tutorial, you will learn how to write Java variables.

Table of Contents

You can skip to a specific section of this Java tutorial using the table of contents below:

• Java Variables

• Declaring and Initializing Java Variables

  • Number
  • Character and Strings
  • Boolean

• Type of Variables in Java

  • 1. Local Variables
  • 2. Static Variables
  • 3. Instance Variables
  • 4. Parameters
• Java Variables Naming Conventions
• Java Local-Variable Type Inference
• The Bottom Line

Java Variables

In Java, variables are declared before use. I will discuss more about this in the next section.

A variable contains a data value and thus, it has a data type. It can be anything ranging from text to code to numbers.

Despite Java being object-oriented, not all variable types are objects. It is built on basic variable types called primitives which include the following:

• byte (number, 1 byte)
• short (number, 2 bytes)
• int (number, 4 bytes)
• long (number, 8 bytes)
• float (float number, 4 bytes)
• double (float number, 8 bytes)
• char (a character, 2 bytes)
• boolean (true or false, 1 byte)

Everything in Java is either a primitive or an object.

Declaring and Initializing Java Variables

Java is a strong-typed language and therefore, variables have to be defined before they are used. Said differently, a variable's type must be specified before you can assign a value to it.

Not all languages are like this. For example, Python is dynamically-typed, which means that any Python variable can hold any data type.

Here is how to declare and initialize different variables in Java.

Number

int myNum;

To initialize myNum, you can use following two methods:

int myNum;
myNum = 40;
// declaring and initializing the variable separately

Or

int myNum = 5;  
// declaring and initializing the variable together

To declare and initialize double floating-point number:

double d = 4.5;
d = 3.0;

To use float, you need to cast:

float f = (float) 4.5;

Or you can do it as follows (a shorter way of casting):

float f = 4.5f;

Character and Strings

It is not common to put an ASCII value in a character. It has special syntax as follows:

Char c = ‘g’;

Strings also have special treatment in Java. Here are some ways of creating string variables in Java:

String s = new String (“This is a string created using constructor”);
String s1 = “This is a string created without a constructor”;
String sAdd = s + s1;

Operator overloading is supported only in the String class in Java. The + operator is used to concat two strings.

int myNum = 7;
String s = “I have “ + myNum + “pens”;

Boolean

Boolean variables can accepts only true and false as values. They are very useful when building conditional logic into a Java application.

Here is how you can create boolean variables in Java:

public class Program {
    public static void main(String[] args) {

        // Test true and false booleans.
        boolean value = true;
        if (value) {
            System.out.println("A");
        }
        value = false;
        if (!value) {
            System.out.println("B");
        }
    }
}

Output

A
B

Type of Variables in Java

1. Local Variables

A variable that is defined within a block or a method is called a local variable. Their scope is limited to the block or method in which they are declared. Said differently, local variables can only be accessed within the block where they are created. Initializing these variables is mandatory.

Here is an example of how you could create a local variable in Java:

public class StudentData {
public void StuAge()
{
    	// local variable age
    	int age = 0;
    age = age + 5;
    	System.out.println("The age of the student is : " + age);
}
 	public static void main(String args[])
{
    	StudentData obj = new StudentData();
    	obj.StuAge();
}
}

Output:

The age of the student is : 5

2. Static Variables

Static variables are also referred to as class variables.

Static variables are declared using the static keyword within a class outside a block, or within a method constructor. There is only one copy of a static variable per class irrespective of the number of objects we create.

Static variables are created when the program execution begins and are destroyed when execution ends. Initialization is not mandatory in static variables as they have a default value of 0.

Accessing static variables through an object will not halt the program. Instead, the compiler will replace the object name to the class name automatically.

Here is an example of how you could create a static variable in Java:

import java.io.*;
class Employee {
 
	// static variable salary
	public static double salary;
	public static String name = "Nick";
}
 
public class EmpDemo {
	public static void main(String args[])
	{
 
    	// accessing static variable without object
    	Emp.salary = 10,000;
   	 System.out.println(Emp.name + "'s average salary is:"
                       	+ Emp.salary);
	}
}

Output

Nick’s average salary is: 10,000

3. Instance Variables

Also known as non-static variables, instance variables are declared in a class outside any method, block, or constructor. Instance variables are created when an object of a class is created. They are destroyed when the instance is destroyed.

Access specifiers may be used for instance variables. Initialization is not mandatory, and the default value is set to 0. They can be accessed only by creating objects.

Here is an example of how you could create instance variables in Java:

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 sciMarks;
	int comMarks;
}
 
class MarksDemo {
	public static void main(String args[])
	{
    	// first object
    	Marks obj1 = new Marks();
    	obj1.engMarks = 90;
            obj1.sciMarks = 85;
    	obj1.comMarks = 80;
 
    	// second object
    	Marks obj2 = new Marks();
    	obj2.engMarks = 70;
    	obj2.sciMarks = 75;
    	obj2.comMarks = 85;
 
    	// displaying marks for the first object
    	System.out.println("Marks for the first object:");
    	System.out.println(obj1.engMarks);
    	System.out.println(obj1.sciMarks);
    	System.out.println(obj1.comMarks);
 
    	// displaying marks for the second object
    	System.out.println("Marks for the second object:");
    	System.out.println(obj2.engMarks);
    	System.out.println(obj2.sciMarks);
    	System.out.println(obj2.comMarks);
	}
}

The output will be:

Marks for the first object:
90
85
80
Marks for the second object:
70
75
85

4. Parameters

A parameter can be defined as a variable that is passed into a method when the method is called. They are only accessible inside the method where they are declared and a value is assigned to them when the method is invoked.

Java Variables Naming Conventions

When naming the variables in Java, there are certain rules that should be followed to keep the code easy to understand and error-free. Here are some basic conventions:

1. Variables in Java are case sensitive. As an example, student, Student, and STUDENT are three different variables.
2. The name of a variable must start with a letter, an underscore (_), or the $ character.
3. After the first character, a variable name can also contain numbers (although a variable name cannot start with the number).
4. The reserved keywords in Java can not be used as variable names. This includes the keywords like int, char, Boolean, and more that have a specific meaning in the Java code.

Here are a few examples of valid variable names in Java:

myint
myInt
MYINT
_myInt
$myInt
myInt1
myInt_1

There are also other rules that are generally popular in the developer community. That means these conventions are to make the code more readable although if ignored these will not cause any error. Java developers follow these conventions so that they can understand the code and to make it easier for other developers to follow it who have not contributed to writing it. These are:

1. The names of variables are written in lowercase. For example, student, emp, etc.
2. If there are multiple words in the name of the variable, the first letter after each word is written in uppercase. For instance, studentMarks, empSalary, etc.
3. Even though the code technically allows it, variable names generally shouldn't being with \_ or $.
4. The static final fields (constants) are named in all uppercase and for a multiple word variable, they are separated by an underscore. For example, EXCHANGE_RATE.

Java Local-Variable Type Inference

Since Java 10, it is now possible for the compiler to infer the type of a local variable by looking at what actual type that is assigned to the variable at the time of declaration. The enhancement is specific to the local variables, indexes in for-each loops, and local variables declared in for loops.

Let's see an example to help you understand this. Before Java 10, you had to specify variable types like this:

String myVar = "An example of a string!";

However, from Java 10 onwards, you need not specify the type of variable during declaration if it can be inferred from the value assigned to it. Here is how it works:

var myVar = "An example of a string!";

The keyword var is used before the variable name here instead of String. The value assigned to the variable tells the compiler about the type of the variable and therefore, it needs not to be written explicitly in the code.

Here are a few more examples to drive this point home:

var list = new ArrayList();
var myNum = new Integer(123);
var myClassObj = new MyClass();

The Bottom Line

Variables form the basic blocks on which a programming language is built.

Choosing variables and specifying their names are very important when it comes to the functionality and readability of your code. The names must be chosen carefully so that they do not compromise either of these factors. Other developers will rely on your variables when making modifications to your code in the future.

This tutorial explained how to write Java variables the right way. Keep these guidelines in mind as you continue down your path as a Java developer.