Variables in Java are used to store values. Variables have a type, a name, and a value. There are several types of variables in Java, including:

1. 1. Primitive Variables: These are the basic data types in Java, including int (integer), float (floating-point number), double (double-precision floating-point number), char (character), boolean (true or false), and others.

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3. 2. Reference Variables: These variables store references to objects. Reference variables are declared using a class name, and their type determines the kind of objects they can refer to.

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5. 3. Array Variables: Arrays are objects that store multiple values of the same type. Arrays are declared using square brackets ([]).

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7. 4. Static Variables: These variables are associated with a class, not an object. They are declared using the "static" keyword and are also called class variables.

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9. 5. Instance Variables: These variables are associated with objects and are also called non-static variables. Instance variables are declared outside of any method and have a default value (null for objects and 0 for numeric types).

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To declare a variable in Java, you need to specify the type, the name, and an optional value:

int age = 20;
String name = "John";
char gender = 'M';

Variable names must start with a letter, dollar sign ($), or underscore (_) and can contain letters, digits, dollar signs, and underscores. They are case-sensitive and cannot be a keyword or reserved word in Java.

It's important to initialize variables before using them, otherwise, you may get unexpected results. The value of a variable can be changed later in the program by assigning a new value to the variable.

In Java, you can use variables to perform various operations and make decisions based on their values. Variables play a critical role in Java programming, and it's important to understand their usage and scope in your programs.