JavaInteger compareTo()-为什么使用比较和减法？

我发现 compareTo方法的 java.lang.Integer实现如下:

public int compareTo(Integer anotherInteger) {
int thisVal = this.value;
int anotherVal = anotherInteger.value;
return (thisVal<anotherVal ? -1 : (thisVal==anotherVal ? 0 : 1));
}

问题是为什么要用比较而不是减法:

return thisVal - anotherVal;
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最佳答案

This is due to integer overflow. When thisVal is very large and anotherVal is negative then subtracting the latter from the former yields a result that is bigger than thisVal which may overflow to the negative range.

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Perhaps it's to avoid overflow / underflow.

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The subtraction "trick" to compare two numerical value is broken!!!

        int a = -2000000000;
int b =  2000000000;
System.out.println(a - b);
// prints "294967296"

Here, a < b, yet a - b is positive.

DO NOT use this idiom. It doesn't work.

Moreover, even if it does work, it will NOT provide any significant improvement in performance, and may in fact cost readability.

See also

  • Java Puzzlers Puzzle 65: A Strange Saga of Suspicious Sort

    This puzzle has several lessons. The most specific is: Do not use a subtraction-based comparator unless you are sure that the difference between values will never be greater than Integer.MAX_VALUE. More generally, beware of int overflow. Another lesson is that you should avoid "clever" code. Strive to write clear, correct code, and do not optimize it unless it proves necessary.

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Simply speaking, the int type is not big enough to store the difference between two arbitrary int values. For example, the difference between 1.5 billion and -1.5 billion is 3.0 billion, but int cannot hold values greater than 2.1 billion.

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In addition to the overflow thing, you should note that the version with substraction does not give the same results.

  • The first compareTo version returns one of three possible values: -1, 0, or 1.
  • If you replace the last line with substraction, the result can be any integer value.

If you know there will be no overflow, you could use something like this:

public int compareTo(Integer anotherInteger) {
return sign(this.value - anotherInteger.valuel);
}

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