java Java中双波浪号（~~）是什么意思？
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What is the meaning of double tilde (~~) in Java?
提问by Halle Knast
When browsing the source code of Guava, I came across the following piece of code (part of the implementation of hashCodefor the inner class CartesianSet):
在浏览Guava的源码时，遇到了如下一段代码（hashCode为内部类的实现部分CartesianSet）：
int adjust = size() - 1;
for (int i = 0; i < axes.size(); i++) {
adjust *= 31;
adjust = ~~adjust;
// in GWT, we have to deal with integer overflow carefully
}
int hash = 1;
for (Set<E> axis : axes) {
hash = 31 * hash + (size() / axis.size() * axis.hashCode());
hash = ~~hash;
}
hash += adjust;
return ~~hash;
Both of adjustand hashare ints. From what I know about Java, ~means bitwise negation, so adjust = ~~adjustand hash = ~~hashshould leave the variables unchanged. Running the small test (with assertions enabled, of course),
两者的adjust和hash是int秒。根据我对 Java 的了解，~意味着按位否定，因此adjust = ~~adjust并且hash = ~~hash应该保持变量不变。运行小测试（当然，启用断言），
for (int i = Integer.MIN_VALUE; i < Integer.MAX_VALUE; i++) {
assert i == ~~i;
}
confirms this. Assuming that the Guava guys know what they are doing, there must be a reason for them to do this. The question is what?
证实了这一点。假设 Guava 家伙知道他们在做什么，那么他们这样做肯定是有原因的。问题是什么？
EDITAs pointed out in the comments, the test above doesn't include the case where iequals Integer.MAX_VALUE. Since i <= Integer.MAX_VALUEis always true, we will need to check that case outside the loop to prevent it from looping forever. However, the line
编辑正如评论中所指出的，上面的测试不包括iequals的情况Integer.MAX_VALUE。由于i <= Integer.MAX_VALUE始终为真，我们需要在循环外检查这种情况以防止它永远循环。然而，该行
assert Integer.MAX_VALUE == ~~Integer.MAX_VALUE;
yields the compiler warning "Comparing identical expressions", which pretty much nails it.
产生编译器警告“比较相同的表达式”，这几乎可以解决它。
回答by yshavit
In Java, it means nothing.
在 Java 中，它没有任何意义。
But that comment says that the line is specifically for GWT, which is a way to compile Java to JavaScript.
但该评论说该行专门用于 GWT，这是一种将 Java 编译为 JavaScript 的方法。
In JavaScript, integers are kind of like doubles-that-act-as-integers. They have a max value of 2^53, for instance. But bitwise operatorstreat numbers as if they're 32-bit, which is exactly what you want in this code. In other words, ~~hashsays "treat hashas a 32-bit number" in JavaScript. Specifically, it discards all but the bottom 32 bits (since the bitwise ~operators only looks at the bottom 32 bits), which is identical to how Java's overflow works.
在 JavaScript 中，整数有点像 doubles-that-act-integers。例如，它们的最大值为 2^53。但按位运算符将数字视为 32 位，这正是您在此代码中想要的。换句话说，在 JavaScript 中~~hash表示“hash视为 32 位数字”。具体来说，它会丢弃除底部 32 位之外的所有位（因为按位运算~符只查看底部 32 位），这与 Java 溢出的工作方式相同。
If you didn't have that, the hash code of the object would be different depending on whether it's evaluated in Java-land or in JavaScript land (via a GWT compilation).
如果没有，对象的哈希码会有所不同，具体取决于它是在 Java 领域还是在 JavaScript 领域（通过 GWT 编译）进行评估。
