Type Casting and Type Conversion

Pouring water from a small glass into a big jug is safe; nothing spills. Pouring a full jug into a small glass is risky; some water ends up on the floor. Converting numbers between types works the same way. Going from a smaller type to a bigger one is safe, so Java does it for you. Going from bigger to smaller can lose data, so Java makes you say so explicitly with a cast.

Widening and Narrowing

Widening: Automatic

public class Widening {
    public static void main(String[] args) {
        int fare = 450;
        long bigFare = fare;
        double exactFare = fare;
        System.out.println(bigFare + " and " + exactFare);
    }
}

450 and 450.0

Lines 4 and 5 copy an int into a long and a double. Every int value fits in both, so no cast is needed.

Narrowing: You Must Cast

Going the other way without a cast is a compile error:

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

Writing the target type in brackets, a cast, tells the compiler "I know data may be lost":

public class Narrowing {
    public static void main(String[] args) {
        double price = 99.99;
        int rupees = (int) price;
        System.out.println("Rupees: " + rupees);
 
        int big = 300;
        byte small = (byte) big;
        System.out.println("300 as a byte: " + small);
 
        long rounded = Math.round(price);
        System.out.println("Rounded: " + rounded);
    }
}

Rupees: 99
300 as a byte: 44
Rounded: 100

How it works

1. Line 4 casts 99.99 to int. A cast cuts off the decimal part; it does not round. The result is 99.
2. Line 8 squeezes 300 into a byte, whose range is -128 to 127. Only the lowest 8 bits are kept: 300 − 256 = 44. The value is wrong, and Java gives no warning at runtime.
3. Line 11 uses Math.round, which rounds to the nearest whole number instead: 100.

Try it yourself: cast -99.99 to int. Is the result -99 or -100?

char and int

A char is a number underneath, so it widens to int automatically, and an int can be cast to char:

public class Letters {
    public static void main(String[] args) {
        int code = 'A';
        char next = (char) (code + 2);
        System.out.println(code + " " + next);
    }
}

65 C

Conversion That Isn't Casting

Turning text into a number, or a number into text, isn't a cast; you call a method:

(int) "450" doesn't compile: casts only work between numeric types (and between related object types, which Level 2 covers).

Common Mistakes

Expecting a cast to round. (int) 9.99 is 9. Use Math.round to round.

Casting after dividing. (double) (7 / 2) is 3.0, because 7 / 2 is already 3. Cast first: (double) 7 / 2 is 3.5.

Using a cast to "fix" a compile error. If a long doesn't fit in an int, casting hides the bug. Change the variable's type instead.

Interview Corner

Practice

1. Easy: Write the result of each without running it: (int) 7.9, (int) -7.9, (long) 2.5, (char) 66.
2. Harder: A price is 249.75. Print it as whole rupees by cutting off the paise, and again by rounding. Then print the paise part alone as an int (75).

Key Takeaways