Java - Collection.max 比较器的内联定义
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Java - Inline definition of comparator for Collection.max
提问by Nullpoet
I am looking for a Java equivalent for python snippet
我正在寻找 Python 代码段的 Java 等价物
max_valued_key = max(str_to_double_map.keys(), key=lambda x: str_to_double_map[x])
I want to something standard like Collections.max
Is there a way to do this with inline definition of Comparatorsince I don't want to write one more class for every other comparator.
我想要像 Collections.max 这样的标准
有没有办法用Comparator 的内联定义来做到这一点，因为我不想为每个其他比较器再写一个类。
I tried following code unsuccessfully
我尝试以下代码失败
depScores = foo();
String dep = Collections.max(depScores.keySet(), new Comparator<String>() {
@Override
public int compare(String o1, String o2) {
return depScores.get(o1).compareTo(depScores.get(o2));
}
});
depScores variable is not readable from comparator.
Looks like in java inner class cannot access non-final variable from outside!
depScores 变量无法从比较器读取。
看起来在java内部类中无法从外部访问非最终变量！
Thanks in advance!
提前致谢！
采纳答案by Mike Strobel
Just declare depScoresas a final variable. If for some reason you can't, create a second (final) variable that points to it.
只需声明depScores为最终变量。如果由于某种原因你不能，创建指向它的第二个（最终）变量。
Local classes can capture variables only if they are final.
局部类只有在它们是 final 时才能捕获变量。
As a (very) late addendum, it is trivial to create a custom Comparatorfrom a lambda in Java 8:
作为（非常）晚的附录，Comparator在 Java 8 中从 lambda创建自定义是微不足道的：
String dep = Collections.max(
depScores.keySet(),
Comparator.comparing(k -> depScores.get(k))
);
You can get even more terse by replacing the lambda k -> depScores.get(k)with the method reference depScores::get.
通过将 lambda 替换k -> depScores.get(k)为方法引用，您可以获得更简洁的信息depScores::get。
The rules for capturing local variables like depScoreare a little more flexible for lambdas than inner classes: captured variables need only be effectively final. In other words, they must be assigned exactly once, though they needn't be explicitly marked final.
depScore对于 lambdas 来说，捕获局部变量的规则比内部类更灵活一些：捕获的变量只需要有效的 final。换句话说，它们必须只分配一次，尽管它们不需要显式标记final。
回答by Holger
What you want is (will be) possible with Java?8:
您想要的是（将）可以使用 Java？8：
Map<String,Double> map…
String maxKey=Collections.max(map.keySet(), (x,y)->Double.compare(map.get(x),map.get(y)));
or even shorter
甚至更短
String maxKey = Collections.max(map.keySet(), Comparator.comparingDouble(map::get));
For previous Java version you have to use:
对于以前的 Java 版本，您必须使用：
String maxKey=Collections.max(map.keySet(), new Comparator<String>(){
public int compare(String x, String y) {
return Double.compare(map.get(x),map.get(y));
}
});
Problems with mapnot being finalcan be circumvented by assigning it to a final variable right before the invocation:
有问题的map不是final可以通过将其分配给调用之前右边最后一个变量被规避：
final Map<String,Double> fmap=map;
String maxKey=Collections.max(map.keySet(), new Comparator<String>(){
public int compare(String x, String y) {
return Double.compare(fmap.get(x),fmap.get(y));
}
});
But I think even more straightforward and more efficient will be the following helper method as it does not require any hash lookups:
但我认为更直接、更有效的是以下辅助方法，因为它不需要任何哈希查找：
static <K,V extends Comparable<V>> K keyForHighestValue(Map<K,V> map) {
V maxValue=Collections.max(map.values());
for(Map.Entry<K,V> e:map.entrySet()) {
if(e.getValue()==maxValue) return e.getKey();
}
throw new ConcurrentModificationException();
}
