Java Variables

Java Variables

Variables in Java are indeed containers for storing data values, and there are various types of variables available to accommodate different data types and values. Let's briefly discuss each type mentioned:

1. String:

   • The String type is used to store text data. String values are sequences of characters enclosed within double quotes (").
   • Example: "Hello", "Java is awesome"

2. int:

   • The int type is used to store integer values, which are whole numbers without any decimal points.
   • Example: 123, -456

3. float:

   • The float type is used to store floating-point numbers, which are numbers with decimal points.
   • Example: 19.99f, -3.14f

4. char:

   • The char type is used to store single characters, enclosed within single quotes (').
   • Example: 'a', 'B'

5. boolean:

   • The boolean type is used to store values representing two states: true or false.
   • Example: true, false

In addition to these primitive data types, Java also supports other data types such as double, long, byte, short, and more, each with its specific range and usage.

Here's a simple example demonstrating the declaration and initialization of variables of different types:

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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);

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);

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

Example

int myNum;
myNum = 15;
System.out.println(myNum);

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";

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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);

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);

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

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

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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;

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

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