Converting Between Types

Automatic conversion

Java happily puts a smaller type into a bigger box, because nothing can be lost:

public class Main {
    public static void main(String[] args) {
        int whole = 7;
        double decimal = whole;
        System.out.println(decimal);
    }
}

Output

7.0

Think of it like this: Pouring a small cup of water into a big bucket is always safe. Pouring a bucket into a cup might spill.

Casting: forcing a conversion

Going from a bigger type to a smaller one could lose information, so Java makes you say it on purpose. Write the new type in brackets. This is called a cast:

public class Main {
    public static void main(String[] args) {
        double price = 9.99;
        int rupees = (int) price;
        System.out.println(rupees);
    }
}

Output

9

A cast to int simply chops off the decimal part. It does not round: 9.99 becomes 9.

Without the cast, the compiler stops you:

public class Main {
    public static void main(String[] args) {
        double price = 9.99;
        int rupees = price;
    }
}

Output (the program does not compile)

Main.java:4: error: incompatible types: possible lossy conversion from double to int
        int rupees = price;
                     ^

Text to numbers

Text like "25" is not a number, even if it looks like one. Convert it like this:

public class Main {
    public static void main(String[] args) {
        String text = "25";
        int number = Integer.parseInt(text);
        System.out.println(number + 5);

        double d = Double.parseDouble("3.5");
        System.out.println(d * 2);
    }
}

Output

30
7.0

Numbers to text

public class Main {
    public static void main(String[] args) {
        int age = 30;
        String s = String.valueOf(age);
        System.out.println(s + "5");
        System.out.println("" + age + 1);
    }
}

Output

305
301

When one side of + is text, Java joins instead of adding. That's why "" + 30 + 1 gives 301, not 31.

Remember: (int) 9.99 chops to 9. Integer.parseInt("9") turns text into a number.