使用流对 Map 进行排序抛出“无法调用”java.lang.Comparable.compareTo(Object) 流“

Question

I have a HashMap with an Integer key and value. I am trying to sort this hashMap by values and store the keys of this sorted map in a list.

I want to use stream to sort HashMap. Below is my code snippet

Map<Integer, Integer> hm
            = new HashMap<Integer, Integer>();

int arr[]=new int[]{1,2,2,3,3,3,4,4,5};
n-arr.length;

    for (int i = 0; i < n; i++) {
        if(hm.get(arr[i])!=null)
            hm.put(arr[i], hm.get(arr[i]) + 1);
        else
            hm.put(arr[i],1);

List<Integer> temp= hm.entrySet()
                .stream()
                .sorted(Map.Entry.comparingByValue())
                .map(entry -> entry.getKey())
                .collect(Collectors.toList());

How can I fix it?

The error is thrown:

java.lang.NullPointerException: Cannot invoke "java.lang.Comparable.compareTo(Object)" because the return value of "java.util.Map$Entry.getValue()" is null
  at line 541, java.base/java.util.Map$Entry.lambda$comparingByValue$1065357e$1
  at line 355, java.base/java.util.TimSort.countRunAndMakeAscending
  at line 220, java.base/java.util.TimSort.sort
  at line 1307, java.base/java.util.Arrays.sort
  at line 353, java.base/java.util.stream.SortedOps$SizedRefSortingSink.end
  at line 510, java.base/java.util.stream.AbstractPipeline.copyInto
  at line 499, java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto
  at line 921, java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential
  at line 234, java.base/java.util.stream.AbstractPipeline.evaluate
  at line 682, java.base/java.util.stream.ReferencePipeline.collect
  at line 16, Solution.topKFrequent
  at line 54, __DriverSolution__.__helper__
  at line 87, __Driver__.main

Answer 1

Your error says you have a null value in the map so that is what is causing the error. So either ensure there are no nulls when the map is constructed or filter them out as shown below.

List<Integer> temp= hm.entrySet()
                .stream()
                .filter(e -> e.getValue() != null) // <-- added
                .sorted(Entry.comparingByValue())
                .map(Entry::getKey)
                .collect(Collectors.toList());