Variables and Data Types in Java

In this unit, we will learn some basics of Java, like storing information (integers, characters, sequence of characters, etc) and the data types that are supported by Java.

1. Variables

Variables are like containers for storing information.

1.1 Declaring a Variable

A Variable is created when it is declared, and to declare a variable, we must specify the type of data first, followed by the variable name.

Syntax

dataType variableName;

 

  • dataType: It specifies the type of information we are storing in the variable.

  • variableName: It will say about the name given to the variable for storing that information.

 

The Java programming language is statically-typed, which means that all variables must first be declared before they can be used. For example, to store information of an integer we need to declare a variable using the keyword int

Later in this module, we will learn different data types that Java supports to store information

Naming Convention: In Java, the programmers generally follow camel case notation. Whereas in Python, snake case notation is followed.

 

1.2 Initializing a Variable

Assigning a value to a variable for the first time is called Initialization. Memory is allocated when a variable is initialized.

Example

class Main {

    public static void main(String[] args) {

       int points;  // Declaration

       points = 90;  // Initialization, information is assigned to the variable 'points'

    }

}

 

A variable is only a name given to a memory location. All the operations done on the variable affect that memory location

 

1.3 Defining a Variable

Declaring and initializing a variable in a single statement is called Defining a variable.

Example

class Main {

    public static void main(String[] args) {

       int points;  // Declaration

       points = 90;  // Initialization

       int totalPoints = 100;  // Definition

 

       System.out.println(points);

       System.out.println(totalPoints);

    }

}

 

Output

90

100

 

 Note: Java executes the code line-by-line

Example

class Main {

    public static void main(String[] args) {

        System.out.println(points);

        int points = 90;

    }

}

 

Output

file.java:3: error: cannot find symbol

      System.out.println(points);

                    ^

 symbol:   variable points

 location: class Main

1 error

  •  

 

 

  • Variable points is not created by the time we tried to print.
  • As Java programming language is a statically typed language. So before using the points variable we must declare it.

 

2. Data Types

In programming languages, every value or data has an associated type known as data type. Java supports various data types. These data types are categorized into,

  1. Primitive Data Types

  2. Non-Primitive Data Types

 

2.1 Primitive Data Types

Primitive data types are those that are predefined by the programming language (Java) Below are the primitive data types in Java,

  • boolean

  • char

  • byte

  • short

  • int

  • long

  • float

  • double

Data types determine how the value or data can be used in the program. For example, mathematical operations can be done on integers (i.e.,int, float, etc).

 

2.1.1 boolean

In a general, anything that can take one of two possible values is considered a Boolean. Examples include the data that can take values like

  • True or False

  • Yes or No

  • 0 or 1

  • On or Off, etc.

As per the Java Syntax, true and false are considered Boolean values.

 

Example

class Main {

    public static void main(String[] args) {

        boolean canLearn = true;

        System.out.println(canLearn);

    }

}

 

Output

true

Note: In Java, every program begins with the main method of a class. Use the above code template to practice declaring various types of variables.

 

2.1.2 char

The char data type is used to store a single character. A char value must be represented using single quotes, like 'A'.

Example

char alphabet = 'A';

 

2.1.3 byte

The byte data type is used to store integers (positive, negative and zero) without any fractional part. It is used to store small values. The range of values is -128 to 127.

Example

 

byte points = 100;

 

2.1.4 short

The short data type is used to store integers without any fractional part. It is used to store values that are bigger than a byte. The range of values is -32,768 to 32,767.

Example

short number = 28745;

 

2.1.5 int

The int data type is used to store integers without any fractional part. It is used to store values that are bigger than short. The range of values is -231 to 231-1.

Example

int distance = 2036372;

 

2.1.6 long

The long data type is used to store large integers without any fractional part. It is used to store values that are bigger than int. The range of values is -263 to 263-1

The long values should contain the suffix 'l' or 'L'

Example

long area = 203637254L;

 

2.1.7 float

The float data type is used to store any number with a decimal point. The float data type stores a value up to 7 point precision (ex: 12.1234567). Hence, it is used when less precision is required. The float values should contain the suffix 'f' or 'F'

Example

float height = 5.10f;

 

2.1.8 double

The double data type is used to store any number with a decimal point. The double data type stores a value up to 16 point precision. Hence, it is used when more precision is required. The double values may contain the suffix 'd' or 'D'

Example  

double length = 5.112;

double breadth = 9.2345

 

Data Type

Size (in bits)

Range of Values

Example

boolean

1

true, false

true, false

byte

8

-128 to 127

100, -126

char

16

ASCII values from 0 to 255

'a', '7'

short

16

-215 to 215-1

-24456, 4532

int

32

-231 to 231-1

-2036372537, 4500

long

64

-263 to 263-1

3847539L, 8589934592L

float

32

up to 7 decimal points

1.23f, 5.67F

double

64

up to 16 decimal points

1.23456d, 5.6789D

 

2.2 Non-Primitive Data Types

These data types are used to store multiple values. non-primitive data types are defined by the programmer.

In Java programming, all non-primitive data types are simply called objects. Some commonly used non-primitive data types are,

  • String (String is the keyword)

  • Class (class is the keyword)

  • Array

 

2.2.1 String

The String data type is simply a sequence of characters. For example, "Rahul" is a string which consists of the characters 'R', 'a', 'h', 'u', and 'l'. In Java, double quotes (") are used to represent a string.

Example

String name = "Rahul";

 

Possible Mistake

Double quotes (") are used to represent a string in Java. Using single quotes (') for strings results in an error.

Error code

class Main {

   public static void main(String[] args) {

      String name = 'Rahul';

      System.out.println(name);

   }

}

 

Output

file.java:3: error: unclosed character literal

      String name = 'Rahul';

                ^

 

Working Example Code

class Main {

    public static void main(String[] args) {

        String name = "Rahul";

        System.out.println(name);

    }

}

 

Output

Rahul

We will learn more about String, Class and Array later in the course

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