Java 8 Stream从Map中提取数据

Question

I have a Map<String,ExtractedData> extractedDatas and I want to extract some data as return result. I'm quite new with the Stream API and I don't understand what I have to do. I tried with

public Map<String,ExtractedData> getExtractedData(String name)
{
    return extractedDatas.entrySet().stream()
            .filter(entry -> entry.getKey().startsWith(name))
            .filter(entry -> entry.getValue().getFieldValue() != null && entry.getValue().getFieldValue() != "")
            .collect(Collectors.toMap(...);
}

What do I have to put in the Collectors.toMap ?

Answer 1

You simply have to pass the functions that map an element of your Stream to both the key and the value of the output Map .

In your case it's simply the key and the value of the Map.Entry elements of the Stream.

public Map<String,ExtractedData> getExtractedData(String name)
{
    return extractedDatas.entrySet().stream()
            .filter(entry -> entry.getKey().startsWith(name))
            .filter(entry -> entry.getValue().getFieldValue() != null && entry.getValue().getFieldValue() != "")
            .collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue));
}

Answer 2

you could do it a bit different if you are OK altering the initial Map :

extractedDatas
     .entrySet()
     .removeIf(entry -> 
                 !(entry.getKey().startsWith(name) || 
                   entry.getValue().getFieldValue() != null && entry.getValue().getFieldValue() != "")
                  )
              );