Java Variables

A variable is a named box in the memory where you keep a value. In Java every variable has a type, which says what kind of value it can keep.

Declare and assign

Syntax: type name = value;

public class Main {
    public static void main(String[] args) {
        int age = 21;
        double price = 99.5;
        String name = "Riya";
        boolean active = true;

        System.out.println(name + " is " + age + " years old");
        System.out.println("Price: " + price + ", active: " + active);
    }
}

Declare first, assign later

public class Later {
    public static void main(String[] args) {
        int score;          // declared
        score = 50;         // assigned
        score = score + 10; // changed
        System.out.println(score);
    }
}

A local variable must have a value before you use it. If not, the compiler gives an error:

public class Empty {
    public static void main(String[] args) {
        int x;
        System.out.println(x);
    }
}

Many variables of one type

public class Many {
    public static void main(String[] args) {
        int a = 1, b = 2, c = 3;
        int x, y;
        x = y = 10;
        System.out.println(a + b + c + " " + x + " " + y);
    }
}

Constants with final

Add final and the value can not be changed. Constants are written in UPPER_CASE.

public class Const {
    public static void main(String[] args) {
        final double PI = 3.14159;
        PI = 3;
    }
}

The var keyword

Since Java 10 you can write var for local variables, and Java finds the type from the value. The variable is still strongly typed.

public class Var {
    public static void main(String[] args) {
        var count = 10;          // int
        var title = "Java";      // String
        var rate = 4.5;          // double
        System.out.println(count + " " + title + " " + rate);
    }
}

Naming rules

• Start with a letter, _ or $, not with a digit.
• Use camelCase: totalPrice, studentName.
• Use meaningful names: age is better than a.
• You cannot use a keyword as a name.

Scope

A variable lives only inside the block { } where you declared it.

public class Scope {
    public static void main(String[] args) {
        if (true) {
            int inside = 5;
        }
        System.out.println(inside);
    }
}

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