Variables, thesarfo

To declare a variable in Java, you need the datatype of the variable and the name of the variable. Declaring a variable means that the compiler will know that the variable exists.

class HelloWorld {
    public static void main(String[] args) {
        int age; // we have declared a variable

        age = 21; // we have defined a variable
    }
}

Aside from declaring a variable, you can also define a variable — this simply means assigning a value to the variable.

public class Main {
    public static void main(String[] args) {
        int age; // variable declaration
        age = 21; // variable definition

        int another_age = 22; // declaring and defining on the same line

        System.out.println("I am " + age + " years old");
    }
}

You can change the value of a variable as many times as you want, but you cannot declare the same variable twice.

There are two kinds of variables:

Instance variables — declared within a class but not within a method
Local variables — declared in a method, and must be initialized before use

Scopes

Any variable you declare inside a method is only accessible inside that particular method. This is called a local variable. However, you can also declare variables outside the method.

public class Main {
    int another_age = 22; // accessible everywhere within the class
    public static void main(String[] args) {
        int age = 21; // only accessible in the main method

        System.out.println("I am " + age + " years old");
    }
}

However, to use the “global” variable in the Main class inside the main method, you will have to declare that variable as static.

public class Main {
    static int another_age = 22; // made it static
    public static void main(String[] args) {
        int age = 21;

        System.out.println("I am " + age + " years old");
    }
}

Remember that static methods can only work with static variables.

Note that when you declare a variable at the class level, the Java compiler assigns a default value to it. For integers, it is 0.

public class Main {
    static int age;
    public static void main(String[] args) {
        System.out.println("I am " + age + " years old"); // prints 0, unless you define the variable with your preferred value
    }
}