In Java, variables are containers for storing data values. Each variable must be declared with a data type before you can use it. The data type specifies the size and type of values that can be stored in the variable.

Declaring Variables

To declare a variable in Java, you specify the data type followed by the variable name. For example:

  int age;

Here, int is the data type and age is the variable name. You can also initialize a variable at the time of declaration:

  int age = 25;

Data Types

Java has different types of variables, which can be categorized into two groups:

  1. Primitive Data Types: These include byte, short, int, long, float, double, char, and boolean.
  2. **Reference Data Types`: These include objects, arrays, and strings.

Examples

Integer Variable 

  public class Main {
    public static void main(String[] args) {
        int age = 30; // Declaring and initializing an integer variable
        System.out.println("Age: " + age);
    }
}

Double Variable 

  public class Main {
    public static void main(String[] args) {
        double price = 19.99; // Declaring and initializing a double variable
        System.out.println("Price: " + price);
    }
}

Character Variable 

  public class Main {
    public static void main(String[] args) {
        char grade = 'A'; // Declaring and initializing a character variable
        System.out.println("Grade: " + grade);
    }
}

Boolean Variable 

  public class Main {
    public static void main(String[] args) {
        boolean isJavaFun = true; // Declaring and initializing a boolean variable
        System.out.println("Is Java fun? " + isJavaFun);
    }
}

String Variable 

  public class Main {
    public static void main(String[] args) {
        String greeting = "Hello, World!"; // Declaring and initializing a string variable
        System.out.println(greeting);
    }
}

Multiple Variables

You can declare multiple variables of the same type in a single statement, separated by commas:

  public class Main {
    public static void main(String[] args) {
        int a = 5, b = 10, c = 15;
        System.out.println(a + " " + b + " " + c);
    }
}

Final Variables

If you want to declare a constant variable, you can use the final keyword. The value of a final variable cannot be changed once it is initialized.

  public class Main {
    public static void main(String[] args) {
        final int YEAR = 2024;
        System.out.println("Year: " + YEAR);
    }
}

Variable Scope

The scope of a variable is the part of the program where the variable is accessible. In Java, there are three types of variables based on their scope:

  1. Local Variables: Declared inside a method or block.
  2. Instance Variables: Declared inside a class but outside any method, with no static keyword.
  3. Class/Static Variables: Declared with the static keyword inside a class, outside any method.

Local Variable Example 

  public class Main {
    public static void main(String[] args) {
        int x = 5; // Local variable
        System.out.println(x);
    }
}

Instance Variable Example 

  public class Main {
    int instanceVar = 10; // Instance variable

    public static void main(String[] args) {
        Main obj = new Main();
        System.out.println(obj.instanceVar);
    }
}

Static Variable Example 

  public class Main {
    static int staticVar = 20; // Static variable

    public static void main(String[] args) {
        System.out.println(Main.staticVar);
    }
}