Type Casting

Sometimes a value of one type is needed as another: an int in a calculation with a double, or a double stored in an int. Java converts automatically when nothing can be lost, and requires an explicit cast when information might be.

Widening: automatic

JAVA

void main() {
    int items = 7;
    long bigItems = items;
    double precise = items;
    System.out.println(bigItems);
    System.out.println(precise);
}

Output

7
7.0

Converting to a larger type is safe, so Java does it automatically: byte → short → int → long → float → double.

Narrowing: explicit cast

JAVA

void main() {
    double price = 9.99;
    int whole = (int) price;
    System.out.println(whole);
}

Output

Going to a smaller type may lose information, so you must write the target type in parentheses. Casting a double to int truncates — it drops the fraction, it doesn't round. Without the cast it's a compile error:

JAVA

void main() {
    double price = 9.99;
    int whole = price;
}

Compiler error

Main.java:3: error: incompatible types: possible lossy conversion from double to int

Rounding instead of truncating

JAVA

void main() {
    double value = 9.99;
    System.out.println((int) value);
    System.out.println(Math.round(value));
    System.out.println((int) -9.99);
    System.out.println(Math.round(-9.5));
}

Output

Math.round rounds to the nearest whole number (halves round up, towards positive infinity) and returns a long.

Casting can overflow

JAVA

void main() {
    int big = 300;
    byte small = (byte) big;
    System.out.println(small);
}

Output

300 doesn't fit in a byte, so only the lowest 8 bits are kept. A cast tells the compiler "I know what I'm doing" — make sure you do.

Integer division

JAVA

void main() {
    int a = 7, b = 2;
    System.out.println(a / b);
    System.out.println((double) a / b);
    System.out.println(a / (double) b);
    System.out.println((double) (a / b));
}

Output

3
3.5
3.5
3.0

Dividing two ints gives an int. Cast one operand before dividing to get a decimal result; casting the result afterwards is too late.

Numbers and text

JAVA

void main() {
    String text = "42";
    int number = Integer.parseInt(text);
    double d = Double.parseDouble("3.75");
    String back = String.valueOf(number + 1);
    String also = "" + d;

    System.out.println(number * 2);
    System.out.println(d);
    System.out.println(back + also);
}

Output

84
3.75
433.75

Casting can't turn text into a number. Use Integer.parseInt, Double.parseDouble and friends; go the other way with String.valueOf or by concatenating with a string. Parsing invalid text throws a NumberFormatException:

JAVA

void main() {
    int n = Integer.parseInt("forty-two");
}

Output

Exception in thread "main" java.lang.NumberFormatException: For input string: "forty-two"
    at Main.main(Main.java:2)

Promotion in expressions

JAVA

void main() {
    byte x = 10, y = 20;
    int sum = x + y;
    char c = 'A';
    System.out.println(sum);
    System.out.println(c + 1);
    System.out.println((char) (c + 1));
}

Output

30
66
B

In arithmetic, byte, short and char are promoted to int, and if either operand is long, float or double the result takes the larger type.

Try it yourself

Calculate an average score as a decimal, then show it rounded and truncated.

Output

Average: 85.66666666666667
Rounded: 86
Truncated: 85

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