[英]Comparator for sorting by frequency without Creating a Comparator implementation class
Just curious to know on whether we can sort a List according to the frequency of repeated numbers using Java 8 without writing a Custom Comparator class. 只是想知道我们是否可以在不编写Custom Comparator类的情况下使用Java 8根据重复数字的频率对List进行排序。
I need to sort the given integers based on its frequency and then by the natural numerical order. 我需要根据给定的整数频率和自然数值顺序对其进行排序。
I'm getting error at Comparator.naturalOrder(); 我在Comparator.naturalOrder()时出错。
Here is the code which I tried: 这是我尝试的代码:
Integer[] given = new Integer[]{0,0,1,22,11,22,22,11,44,555,55,66,77,88,99};
List<Integer> intList = Arrays.asList(given);
Map<Integer, Long> frequencyMap = intList.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
List<Integer> newList = intList.stream().sorted(Comparator.comparing(frequencyMap::get).thenComparing(Comparator.naturalOrder())).collect(Collectors.toList());
System.out.println(newList.toString());
The expected output is 预期的输出是
[1, 44, 55, 66, 77, 88, 99, 555, 0, 0, 11, 11, 22, 22, 22]
PS: Used arrays in first line in order to avoid list.add() in multiple lines and for clear understanding. PS:在第一行中使用数组,以避免在多行中使用list.add()并获得清晰的理解。
Unfortunately, Java's type inference can't recognize the type of the compared object when chaining Comparator.comparing(frequencyMap::get)
with thenComparing(Comparator.naturalOrder())
. 不幸的是,当用
thenComparing(Comparator.naturalOrder())
链接Comparator.comparing(frequencyMap::get)
时,Java的类型推断无法识别比较对象的类型。 Since the method signature of Map.get
is get(Object)
, the compiler infers Comparator<Object>
as result type of Comparator.comparing(frequencyMap::get)
. 由于
Map.get
的方法签名为get(Object)
,因此编译器将Comparator<Object>
推断为Comparator.comparing(frequencyMap::get)
结果类型。
You can fix this by inserting an explicit type. 您可以通过插入显式类型来解决此问题。 But note that you are not using the result of
collect(Collectors.toList())
but just printing the original, unaffected List
. 但是请注意,您不是在使用
collect(Collectors.toList())
的结果,而只是打印未受影响的原始List
。 On the other hand, you don't need the List
when the array is given: 另一方面,给出数组时,您不需要
List
:
Integer[] given = {0,0,1,22,11,22,22,11,44,555,55,66,77,88,99};
Map<Integer, Long> frequencyMap = Arrays.stream(given)
.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
Arrays.sort(given,
Comparator.<Integer>comparingLong(frequencyMap::get)
.thenComparing(Comparator.naturalOrder()));
System.out.println(Arrays.toString(given));
For printing without changing the array you can also use the following alternative 对于不更改阵列的打印,您也可以使用以下替代方法
Arrays.stream(given)
.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
.entrySet().stream()
.sorted(Map.Entry.<Integer, Long>comparingByValue()
.thenComparing(Map.Entry.comparingByKey()))
.flatMap(e -> LongStream.range(0, e.getValue()).mapToObj(l -> e.getKey()))
.forEach(System.out::println);
This sorts the groups instead of the individual values and prints identical values as often as they've been counted. 这将对组进行排序,而不是对单个值进行排序,并按计数的频率打印相同的值。
you need to add a type witness, minor compiler weakness: 您需要添加一个类型见证者,较小的编译器弱点:
intList.stream()
.sorted(Comparator.comparing((Integer x) -> frequencyMap.get(x))
.thenComparing(Comparator.naturalOrder()))
.forEachOrdered(System.out::println);
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