Java 8 Stream API :How to handle null value in Map [duplicate]

Question

I have an arraylist of Map type . I want to filter out null values from the list and find max and min values. The program is working fine except for null values.

ArrayList<Map<String, Float>> elements = new ArrayList<Map<String, Float>>();
Map<String, Float> element = new HashMap<String, Float>();
element.put("a", (float) 1.22);
element.put("b", (float) 1.33);
element.put("c", (float) 1.52);
element.put("d", (float) 1.452);
element.put("e", (float) 1.452);

Map<String, Float> element2 = new HashMap<String, Float>();
element2.put("a", (float) 1.2332);
element2.put("b", (float) 1.242);
element2.put("c", (float) 1.552);
element2.put("d", (float) 15.33);
element2.put("e",null);

elements.add(element);
elements.add(element1);
elements.add(element2);

System.out.println("Initial Mappings are: " + elements);
Map<String, Float> min = elements.stream() 
            .flatMap(m -> m.entrySet().stream())
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, Math::min));

System.out.println(min);

null map or a null element in a map? Your example does not contain any nulls — Thiyagu
– Thiyagu, Commented Dec 24, 2019 at 4:04
You must try and ensure that your Map doesn't allow null writes such as element2.put("e",null); in reality. This was corrected in JDK itself with Map.ofimplementation in Java-9 as well. — Naman
– Naman, Commented Dec 24, 2019 at 4:29

Naman · Accepted Answer · 2019-12-24 04:31:58Z

5

Add a filter to the chain.

Map<String, Float> min = elements.stream()
            .flatMap(m -> m.entrySet().stream())
            .filter(entry -> entry.getValue() != null)
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, Math::min));

edited Dec 24, 2019 at 4:31

Naman

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answered Dec 24, 2019 at 4:05

cbryant02

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Comments

Ryuzaki L · Accepted Answer · 2019-12-24 04:18:22Z

3

You can use filter to filter null values and use Collectors.summarizingDouble to get min,max and average

Map<String, DoubleSummaryStatistics> res = elements.stream() 
        .flatMap(m -> m.entrySet().stream())
        .filter(entry->Objects.nonNull(entry.getValue()) // check for null value
        .collect(Collectors.groupingBy(Map.Entry::getKey, Collectors.summarizingDouble(entry->entry.getValue())));

res.forEach((k,v)->System.out.println("Key : "+k+", Min : "+v.getMin()+", Max : "+v.getMax()));

edited Dec 24, 2019 at 4:18

answered Dec 24, 2019 at 4:09

Ryuzaki L

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1 Comment

Naman Over a year ago

The question doesn't really focus around using **SummaryStatistics, but a good suggestion, though the actual answer already exists.

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