Variables and data types

In Java, variables are used to store data values that can be manipulated and accessed throughout a program. Java is a statically-typed language, which means that each variable must have a declared data type. Here are some common data types and variable declarations in Java:

Primitive Data Types:
1. byte: 8-bit integer (range: -128 to 127)
2. short: 16-bit integer (range: -32,768 to 32,767)
3. int: 32-bit integer (range: -2,147,483,648 to 2,147,483,647)
4. long: 64-bit integer (range: -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807)
5. float: 32-bit floating-point number (e.g., 3.14f)
6. double: 64-bit floating-point number (e.g., 3.14)
7. boolean: true or false
8. char: single 16-bit Unicode character (e.g., ‘a’, ‘1’, ‘$’)
Example declarations:
```
int age = 25;
double pi = 3.1416;
boolean isTrue = true;
char grade = 'A';
```

2. Reference Data Types:

String: a sequence of characters (e.g., “Hello, World!”)
Arrays: a collection of elements of the same type
Classes: user-defined types

Example declarations:

String name = "John Doe";
int[] numbers = {1, 2, 3, 4, 5};
MyClass obj = new MyClass();

3. Other Data Types:

Enums: a special data type that represents a group of constants
Wrapper classes: provide a way to use primitive types as objects (e.g., Integer, Boolean, Character)

Example declarations:

enum Day {MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY}
Integer count = 10;
Boolean isValid = false;

It’s important to note that Java is a strongly-typed language, meaning that variables must be declared with their appropriate types and cannot be used interchangeably without explicit conversions.