Variables in Java

In Java, a variable is a container that holds a value. The value of a variable can be changed during the execution of a program. Here are some examples of variables in Java:

1. Integer variable:

int age = 20;

In this example, age is an integer variable that holds the value 20.

2. Double variable:#

double height = 1.75;

In this example, height is a double variable that holds the value 1.75.

3. Boolean variable:

boolean isStudent = true;

In this example, isStudent is a boolean variable that holds the value true.

4. String variable:

String name = "John";

In this example, name is a string variable that holds the value "John".

5. Array variable:

int[] numbers = {1, 2, 3, 4, 5};

In this example, numbers is an array variable that holds the values {1, 2, 3, 4, 5}.

6. Object variable:

Scanner scanner = new Scanner(System.in);

In this example, scanner is an object variable that holds an instance of the Scanner class.

Note that in Java, you must declare the type of the variable before you can use it. You can also assign an initial value to the variable when you declare it, as shown in the examples above.

In Java, there are rules that you must follow when naming a variable. These rules ensure that your code is readable and easy to understand. Here are the rules for naming a variable in Java:

1. The name must begin with a letter (a to z or A to Z), or the dollar sign ($), or an underscore (_).
2. After the first character, the name can contain letters, numbers, dollar signs, or underscores.
3. Variable names are case-sensitive. For example, name and Name are two different variables.
4. You cannot use a Java keyword as a variable name. For example, int is a keyword and cannot be used as a variable name.
5. Variable names should be descriptive and meaningful. For example, instead of using a variable name like x, use a name like age or height that reflects the meaning of the variable.
6. Variable names should not be too long. A good practice is to use a name that is short and descriptive, and represents the purpose of the variable.

Here are some examples of valid and invalid variable names in Java:

Valid variable names:

int age;
double height;
String firstName;

Invalid variable names:

int 2years;  // variable name cannot start with a number
double my-height;  // variable name cannot contain a hyphen (-)
boolean public;  // variable name cannot be a keyword
String _address;  // variable name can start with an underscore, but it's not a recommended practice

By following these rules, you can create variable names that are easy to understand and help make your code more readable and maintainable.