Best way to convert a signed integer to an unsigned long?

问题

For certain hash functions in Java it would be nice to see the value as an unsigned integer (e.g. for comparison to other implementations) but Java supports only signed types. We can convert a signed int to an "unsigned" long as such:

public static final int BITS_PER_BYTE = 8;
public static long getUnsignedInt(int x) {
  ByteBuffer buf = ByteBuffer.allocate(Long.SIZE / BITS_PER_BYTE);
  buf.putInt(Integer.SIZE / BITS_PER_BYTE, x);
  return buf.getLong(0);
}
getUnsignedInt(-1); // => 4294967295

However, this solution seems like overkill for what we're really doing. Is there a more efficient way to achieve the same thing?

回答1:

Something like this?

int x = -1;
long y = x & 0x00000000ffffffffL;

Or am I missing something?

public static long getUnsignedInt(int x) {
    return x & 0x00000000ffffffffL;
}

回答2:

The standard way in Java 8 is Integer.toUnsignedLong(someInt), which is equivalent to @Mysticial's answer.

回答3:

Guava provides UnsignedInts.toLong(int)...as well as a variety of other utilities on unsigned integers.

回答4:

You can use a function like

public static long getUnsignedInt(int x) {
    return x & (-1L >>> 32);
}

however in most cases you don't need to do this. You can use workarounds instead. e.g.

public static boolean unsignedEquals(int a, int b) {
    return a == b;
}

For more examples of workarounds for using unsigned values. Unsigned utility class

回答5:

other solution.

public static long getUnsignedInt(int x) {
    if(x > 0) return x;
    long res = (long)(Math.pow(2, 32)) + x;
    return res;
}

回答6:

Just my 2 cents here, but I think it's a good practice to use:

public static long getUnsignedInt(int x) { return x & (~0L); // ~ has precedence over & so no real need for brackets }

instead of:

return x & 0xFFFFFFFFL;

In this situation there's not your concern how many 'F's the mask has. It shall always work!

回答7:

long abs(int num){
    return num < 0 ? num * -1 : num;
}

来源：https://stackoverflow.com/questions/9578639/best-way-to-convert-a-signed-integer-to-an-unsigned-long