In our introduction to unsigned arithmetic in Java, we began by comparing to a language such as C/C++. We saw that in C/C++, when you declare a variable as unsigned, the compiler will know to perform unsigned arithmetic on that variable. Arithmetic operations are operations such as addition and division, where we're not simply treating the number as a "raw string of bits", but where the bits have some "numerical" meaning. Perhaps less obviously, arithmetic operations include comparison. In our overview, we mentioned that to perform unsigned bitwise operations in Java— such as XOR, AND etc, where we do just treat the number as a series of bits— these are essentially the same whether or not we later treat the resulting integer as signed or unsigned. See the previous page's introduction to unsigned operations in C/C++ and Java for more details. On this page, we consider cases where implementing unsigned arithmetic correctly in Java is not so straightforward.
The reason that unsigned arithmetic is inherently tricky in Java is that in most cases, Java only "natively" caters for signed arithmetic: that is, it expects to interpret the top bit of an integer type (int, but also byte, short and long) as the sign bit. This is a departure from other languages, even including modern examples such as Swift, where the programmer generally has the option of specifying signed or unsigned values and arithmetic, even if signed is often the default.
So, what are the trickier cases that we must consider, and how do we deal with them in Java
If you are dealing purely with numbers that are 32 bits or smaller— i.e. the size of a Java int— then you can perform unsigned arithmetic reliably by using a long to perofmr the arithmetic. Only the top bit of a long is interpreted as the sign bit. So provided you keep to the bottom bits, you will effectively be performing unsigned arithmetic. We just need to be careful of two things:
Wherever the result of an operation could be larger than the bottom 32 bits, we "chop off" the top 32 bits of the long. We can chop them off by casting back to an int, or ANDing with 0xffffffffL if we want to keep the result as a long, e.g. for further arithmetic. For example, here is one way to perform an unsigned 32-bit division in Java:
public static int unsignedDiv(int i1, int i2) { long l1 = i1 & 0xffffffffL, l2 = i2 & 0xffffffffL; return (int) (l1 / l2); }
We can perform an unsigned 32-bit comparison by "casting" the numbers to a long (remembering to AND with the bottom 32 bits as mentioned), then perform the comparison on the longs:
public static boolean lessThanUnsigned(int n1, int n2) { return (n1 & 0xffffffffL) < (n2 & 0xffffffffL); }
Performing 64-bit unsigned arithmetic in Java is a bit more tricky, because there's no "next size up" primitive that you can use. We need to watch out for "special cases" and "manuall" interpret the sign bit as magnitude. In general:
The idea of unsigned addition/subtraction being identical to their signed counterparts may seem counterintuitive, but numbers are deliberately stored in a form (called "two's complement"), that allows them to be identical at the bit level.
We're of course talking about "less than" or "greater than": "equals" or "not equals" are of course identical whether the number is signed or unsigned. We'll take the example of unsigned less than. To understand how the comparison works, we start by considering the four possible combinations of the sign bit set or not set on the two numbers (henceforth x and y). In signed land, if the sign bit is set, this would mean less than zero. But we want to re-interpret that as is a big number— or more specifically, is greater or equal to 1<<31. So our "truth table" of sign bit set or not set for the two numbers looks as follows:
Top bit of x | ||
---|---|---|
Top bit of y | 0 | 1 |
0 | x < y (Signed comparison) | false |
1 | true | x < y (Signed comparison) |
The idea is that if the sign bits are the same, then we can perform an "ordinary" signed comparison— in effect the result will be the same as just comparing the bottom 63 bits. If the sign bits are different, then the number that has the sign bit set (is really "negative" as a signed number) should be treated as bigger, and vice versa. In other words, if the sign bits are different, we reverse the comparison. Logically this would look as follows:
public static boolean isLessThanUnsigned(long n1, long n2) { boolean comp = (n1 < n2); if ((n1 < 0) != (n2 < 0)) { comp = !comp; } return comp; }
However, with a bit of bitwise trickery, we can actually write this as a single expression. It's not very common in Java, but we can use the exclusive OR operator (^) with booleans. The result of x ^ y is:
This means we can write the above as:
public static boolean isLessThanUnsigned(long n1, long n2) { return (n1 < n2) ^ ((n1 < 0) != (n2 < 0)); }
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Editorial page content written by Neil Coffey. Copyright © Javamex UK 2021. All rights reserved.