Java Map Lambda异常

Question

I have a List of Maps with certain keys that map to String values.

Something like List<Map<String,String>> aMapList;

Objective : Stream over this List of maps and collect values of a single key in all Maps.

How I'm doing this ->

key = "somekey";
aMapList.stream().map(a -> a.get(key)).collect(Collectors.averagingInt());

The Problem: I get exceptions due to a.get(key) if there is no such key! because averaging this will give a null. How do I check or make lambda ignore any such maps and move on.

I do know that I can add a filter on a -> a.contains(key) and then proceed. Edit : I can also add more filters or simple check multiple conditions on one filter. Possible Solution:

    aMapList.stream().filter(a -> a.contains(key)).
         map(a -> a.get(key)).collect(Collectors.averagingInt());

Can this be made prettier? Instead of halting the operation, simply skip over them? Is there some more generic way to skip over exceptions or nulls. For eg. We can expand the lambda and put a try-catch block, but I still need to return something, what if I wish to do an equivalent of "continue".

Eg.

(a -> {return a.get(key) }).

Can be expanded to -->

(a -> {try{return a.get(key)} 
catch(Exception e){return null} }).

The above still returns a null, instead of just skipping over.

I'm selecting the best answer for giving two options, But I do not find any of them prettier. Chaining filters seems to be the solution to this.

Answer 1

How about wrapping the result with Optional :

List<Optional<String>> values = aMapList.stream()
            .map(a -> Optional.ofNullable(a.get(key)))
            .collect(Collectors.toList());

Later code will know to expect possible empty elements.

---

The solution you propose has a potential bug for maps that allow null values. For example:

Map<String, String> aMap = new HashMap<>();
aMap.put("somekey", null);

aMapList.add(aMap);

aMapList.straem()
    .filter(a -> a.contains("somekey")) // true returned for contains
    .map(a -> a.get("somekey")) // null returned for get
    .collect(Collectors.toList());

Answer 2

Based on the Map documentation , and on your comment under your question, you're not actually getting an exception from a.get(key) . Rather, that expression produces a null value, and you're having problems later when you run into these null values. So simply filtering out these null values right away should work just fine:

aMapList.stream()
    .map(a -> a.get(key))
    .filter(v -> v != null)
    .collect(Collectors.toList());

This is prettier, simpler, and performs better than the workaround in your question.

I should mention that I usually prefer the Optional<> type when dealing with null values, but this filtering approach works better in this case since you specifically said you wanted to ignore elements where the key doesn't exist in a map list.

Answer 3

The simplest I could come up with was:

aMapList.stream()
        .filter(map -> map.containsKey(key))
        .map(map -> map.get(key))
        .collect(Collectors.toList());

By formatting the lambda in this fashion, it is easier to see the distinct steps that the code processes.

Answer 4

尽管我认为这并不是一种更漂亮的方法，但是您可以这样做：

aMapList.stream().map(a -> a.containsKey(key) ? a.get(key) : null).collect(Collectors.toList());