Java negative int to hex and back fails

问题:

public class Main3 {
    public static void main(String[] args) {
        Integer min = Integer.MIN_VALUE;
        String minHex = Integer.toHexString(Integer.MIN_VALUE);

        System.out.println(min + " " + minHex);
        System.out.println(Integer.parseInt(minHex, 16));
    }
}

Gives

-2147483648 80000000
Exception in thread "main" java.lang.NumberFormatException: For input string: "80000000"
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
    at java.lang.Integer.parseInt(Integer.java:459)
    at Main3.main(Main3.java:7)

Whats up?

回答1:

It's documented that Integer.toHexString returns a string representation of the integer as an unsigned value - while Integer.parseInt takes a signed int. If you use Integer.toString(value, 16) instead you'll get what you want.

回答2:

This is something that's always annoyed me. If you initialize an int with a hex literal, you can use the full range of positive values up to 0xFFFFFF; anything larger than 0x7FFFFF will really be a negative value. This is very handy for bit masking and other operations where you only care about the locations of the bits, not their meanings.

But if you use Integer.parseInt() to convert a string to an integer, anything larger than "0x7FFFFFFF" is treated as an error. There's probably a good reason why they did it that way, but it's still frustrating.

The simplest workaround is to use Long.parseLong() instead, then cast the result to int.

int n = (int)Long.parseLong(s, 16);

Of course, you should only do that if you're sure the number is going to be in the range Integer.MIN_VALUE..Integer.MAX_VALUE.

回答3:

According the the documentation, toHexString returns "a string representation of the integer argument as an unsigned integer in base 16. "

So the correct reverse operation is probably Integer.parseUnsignedInt that was introduced as part of Java 8:

public class Main3 {
    public static void main(String[] args) {
        Integer min = Integer.MIN_VALUE;
        String minHex = Integer.toHexString(Integer.MIN_VALUE);

        System.out.println(min + " " + minHex);
        System.out.println(Integer.parseUnsignedInt(minHex, 16));
    }

回答4:

Try this:

public class Main3 {
    public static void main(String[] args) {
        Integer min = Integer.MIN_VALUE;
        String minHex = Integer.toHexString(Integer.MIN_VALUE);

        System.out.println(min + " " + minHex);
        System.out.println(Integer.parseInt( "-" + minHex, 16));
    }

}

to get this:

-2147483648 80000000
-2147483648

回答5:

You need to include a negative sign.

I don't have access to test this right now but I'd bet if you tried this value instead:

Integer min = Integer.MIN_VALUE + 1;

It wouldn't bomb, but would give you a positive number (not negative) when you ran ParseInt(min,16).

A string of bits doesn't really have enough info to determine sign in this context so you need to provide it. (consider the case where you use min = "F". Is that +/-F? If you converted it to bits and saw 1111, and you knew it was a byte, you might conclude that it's negative, but that's a lot of ifs.

回答6:

This seem to work for me :

public class Main3 {
public static void main(String[] args) {
    Integer min = Integer.MIN_VALUE;
    String minHex = Integer.toHexString(Integer.MIN_VALUE);

    System.out.println(min + " " + minHex);
    System.out.println((int)Long.parseLong(minHex, 16));
}
}

The integer is parsed as a "signed long" that handle such big positive number and then the sign is found back by casting it to "int".