Why is map stream sorted after filtering not working?-CodePudding

I have Map<String,Integer> map. I want to filter and sort the map by key and then get 5 percent of their number , I have such a function:

public List<String> getValuableSubStr(){
        List<String> result = new ArrayList<>();
        long size = map.entrySet().stream().filter(e -> e.getKey().length() ==3).count();
        map.entrySet().stream()
                      .filter(e -> e.getKey().length() ==3)
                      .sorted(Map.Entry.<String,Integer>comparingByKey().reversed())
                      .limit((size*5)/100)
                      .peek(e -> result.add(e.getKey()));
        return result;

    }

But after calling the function , I get an empty list , although the map is not empty and forEach are printed normally .What did I do wrong?

CodePudding user response：

peek is not a terminal operation, so it doesn't cause the stream to be evaluated.

Use forEach instead of peek.

Or, better, collect directly into a list.

return map.entrySet().stream()
    .filter(e -> e.getKey().length() ==3)
    .sorted(Map.Entry.<String,Integer>comparingByKey().reversed())
    .limit((size*5)/100)
    .map(Map.Entry::getKey)
    .collect(Collectors.toList());

Actually, since you're dealing only with keys:

return map.keySet().stream()
    .filter(e -> e.length() ==3)
    .sorted(Comparator.reverseOrder())
    .limit((size*5)/100)
    .collect(Collectors.toList());

CodePudding user response：

peek is more useful for debugging. See here:

it's an intermediate operation and we didn't apply a terminal operation

from https://www.baeldung.com/java-streams-peek-api

I would do

public List<String> getValuableSubStr(){
    List<String> result = new ArrayList<>();
    long size = map.entrySet().stream().filter(e -> e.getKey().length() ==3).count();
    return map.entrySet().stream()
                  .filter(e -> e.getKey().length() ==3)
                  .sorted(Map.Entry.<String,Integer>comparingByKey().reversed())
                  .limit((size*5)/100)
                  .map(a -> a.getKey())
                  .collect(Collectors.toList());

}