Java Type Casting

Type casting means changing a value from one data type to another. Java has two kinds: widening (automatic) and narrowing (manual).

Widening casting (automatic)

A smaller type goes into a bigger type without any problem, so Java does it by itself: byte → short → int → long → float → double.

public class Main {
    public static void main(String[] args) {
        int n = 10;
        long l = n;          // int to long
        double d = n;        // int to double
        System.out.println(n + " " + l + " " + d);
    }
}

Narrowing casting (manual)

A bigger type going into a smaller one can lose data, so you must write the cast in brackets. Without it, the code does not compile:

public class Bad {
    public static void main(String[] args) {
        double d = 9.78;
        int n = d;
    }
}

public class Narrow {
    public static void main(String[] args) {
        double d = 9.78;
        int n = (int) d;        // the decimal part is cut, not rounded
        System.out.println(d + " -> " + n);

        long big = 3_000_000_000L;
        int small = (int) big;  // too big for int, the value is wrong
        System.out.println(big + " -> " + small);

        int x = 130;
        byte b = (byte) x;      // byte is only -128 to 127
        System.out.println(x + " -> " + b);
    }
}

The cast of a decimal to an int removes the decimal part (it does not round). Use Math.round() when you want rounding.

Casting in calculations

When you divide two int values, the answer is also an int. Cast one of them to get the decimal:

public class Divide {
    public static void main(String[] args) {
        int a = 7, b = 2;
        System.out.println(a / b);              // 3
        System.out.println((double) a / b);     // 3.5
        System.out.println(Math.round(7 / 2.0));// 4
    }
}

Numbers and strings

These are not casts. You use methods to change between a number and a String:

public class Convert {
    public static void main(String[] args) {
        int age = Integer.parseInt("25");
        double price = Double.parseDouble("99.50");
        String text = String.valueOf(42);
        String text2 = Integer.toString(7);

        System.out.println(age + 1);
        System.out.println(price + 0.5);
        System.out.println(text + text2);
    }
}

If the text is not a number, parseInt throws a NumberFormatException. We handle it in Java Exceptions.

public class Parse {
    public static void main(String[] args) {
        int n = Integer.parseInt("abc");
    }
}

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