VARIABLES , DATATYPES , OPERATORS

Java Variables

Variables are containers for storing data values.

In Java, there are different types of variables, for example:

String - stores text, such as "Hello". String values are surrounded by double quotes
int - stores integers (whole numbers), without decimals, such as 123 or -123
float - stores floating point numbers, with decimals, such as 19.99 or -19.99
char - stores single characters, such as 'a' or 'B'. Char values are surrounded by single quotes
boolean - stores values with two states: true or false

Declaring (Creating) Variables

To create a variable, you must specify the type and assign it a value:

Syntax

type variable = value;

Where type is one of Java's types (such as int or String), and variable is the name of the variable (such as x or name). The equal sign is used to assign values to the variable.

To create a variable that should store text, look at the following example:

Example

Create a variable called name of type String and assign it the value "John":

String name = "John";

System.out.println(name);

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To create a variable that should store a number, look at the following example:

Example

Create a variable called myNum of type int and assign it the value 15:

int myNum = 15;

System.out.println(myNum);

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You can also declare a variable without assigning the value, and assign the value later:

Example

int myNum;

myNum = 15;

System.out.println(myNum);

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Note that if you assign a new value to an existing variable, it will overwrite the previous value:

Example

Change the value of myNum from 15 to 20:

int myNum = 15;

myNum = 20; // myNum is now 20

System.out.println(myNum);

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Final Variables

However, you can add the final keyword if you don't want others (or yourself) to overwrite existing values (this will declare the variable as "final" or "constant", which means unchangeable and read-only):

Example

final int myNum = 15;

myNum = 20; // will generate an error: cannot assign a value to a final variable

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Other Types

A demonstration of how to declare variables of other types:

Example

int myNum = 5;

float myFloatNum = 5.99f;

char myLetter = 'D';

boolean myBool = true;

String myText = "Hello";

You will learn more about data types in the next chapter.

Display Variables

The println() method is often used to display variables.

To combine both text and a variable, use the + character:

Example

String name = "John";

System.out.println("Hello " + name);

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You can also use the + character to add a variable to another variable:

Example

String firstName = "John ";

String lastName = "Doe";

String fullName = firstName + lastName;

System.out.println(fullName);

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For numeric values, the + character works as a mathematical operator (notice that we use int (integer) variables here):

Example

int x = 5;

int y = 6;

System.out.println(x + y); // Print the value of x + y

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From the example above, you can expect:

x stores the value 5
y stores the value 6
Then we use the println() method to display the value of x + y, which is 11

Declare Many Variables

To declare more than one variable of the same type, use a comma-separated list:

Example

int x = 5, y = 6, z = 50;

System.out.println(x + y + z);

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Java Identifiers

All Java variables must be identified with unique names.

These unique names are called identifiers.

Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).

Note: It is recommended to use descriptive names in order to create understandable and maintainable code:

Example

// Good

int minutesPerHour = 60;

// OK, but not so easy to understand what m actually is

int m = 60;

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The general rules for constructing names for variables (unique identifiers) are:

Names can contain letters, digits, underscores, and dollar signs
Names must begin with a letter
Names should start with a lowercase letter and it cannot contain whitespace
Names can also begin with $ and _ (but we will not use it in this tutorial)
Names are case sensitive ("myVar" and "myvar" are different variables)
Reserved words (like Java keywords, such as int or boolean) cannot be used as names

Java Data Types

As explained in the previous chapter, a variable in Java must be a specified data type:

Example

int myNum = 5; // Integer (whole number)

float myFloatNum = 5.99f; // Floating point number

char myLetter = 'D'; // Character

boolean myBool = true; // Boolean

String myText = "Hello"; // String

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Data types are divided into two groups:

Primitive data types - includes byte, short, int, long, float, double, boolean and char
Non-primitive data types - such as String, Arrays and Classes (you will learn more about these in a later chapter)

Primitive Data Types

A primitive data type specifies the size and type of variable values, and it has no additional methods.

There are eight primitive data types in Java:

Data Type

Size

Description

byte

1 byte

Stores whole numbers from -128 to 127

short

2 bytes

Stores whole numbers from -32,768 to 32,767

int

4 bytes

Stores whole numbers from -2,147,483,648 to 2,147,483,647

long

8 bytes

Stores whole numbers from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807

float

4 bytes

Stores fractional numbers. Sufficient for storing 6 to 7 decimal digits

double

8 bytes

Stores fractional numbers. Sufficient for storing 15 decimal digits

boolean

1 bit

Stores true or false values

char

2 bytes

Stores a single character/letter or ASCII values

Numbers

Primitive number types are divided into two groups:

Integer types stores whole numbers, positive or negative (such as 123 or -456), without decimals. Valid types are byte, short, int and long. Which type you should use, depends on the numeric value.

boolean isJavaFun = true;

boolean isFishTasty = false;

String greeting = "Hello World";

System.out.println(greeting);

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The String type is so much used and integrated in Java, that some call it "the special ninth type".

A String in Java is actually a non-primitive data type, because it refers to an object. The String object has methods that are used to perform certain operations on strings. Don't worry if you don't understand the term "object" just yet. We will learn more about strings and objects in a later chapter.

Non-Primitive Data Types

Non-primitive data types are called reference types because they refer to objects.

The main difference between primitive and non-primitive data types are:

Primitive types are predefined (already defined) in Java. Non-primitive types are created by the programmer and is not defined by Java (except for String).
Non-primitive types can be used to call methods to perform certain operations, while primitive types cannot.
A primitive type has always a value, while non-primitive types can be null.
A primitive type starts with a lowercase letter, while non-primitive types starts with an uppercase letter.
The size of a primitive type depends on the data type, while non-primitive types have all the same size.

Examples of non-primitive types are Strings, Arrays, Classes, Interface, etc. You will learn more about these in a later chapter.

Java Type Casting

Type casting is when you assign a value of one primitive data type to another type.

In Java, there are two types of casting:

Widening Casting (automatically) - converting a smaller type to a larger type size
byte -> short -> char -> int -> long -> float -> double
Narrowing Casting (manually) - converting a larger type to a smaller size type
double -> float -> long -> int -> char -> short -> byte

Widening Casting

Widening casting is done automatically when passing a smaller size type to a larger size type:

Example

public class Main {

public static void main(String[] args) {

int myInt = 9;

double myDouble = myInt; // Automatic casting: int to double

System.out.println(myInt); // Outputs 9

System.out.println(myDouble); // Outputs 9.0

}

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Narrowing Casting

Narrowing casting must be done manually by placing the type in parentheses in front of the value:

Example

public class Main {

public static void main(String[] args) {

double myDouble = 9.78;

int myInt = (int) myDouble; // Manual casting: double to int

System.out.println(myDouble); // Outputs 9.78

System.out.println(myInt); // Outputs 9

}

Java Operators

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Java Operators

Operators are used to perform operations on variables and values.

In the example below, we use the + operator to add together two values:

Example

int x = 100 + 50;

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Although the + operator is often used to add together two values, like in the example above, it can also be used to add together a variable and a value, or a variable and another variable: