how to convert list to map by java8?

Question

hello I'm new to the java world I have a question about how to convert a list to a map by use tree metrics.

public class AdditionalMetrics implements Serializable {
    private static final long serialVersionUID = 3813944465194104658L;

    private Long id;
    private String masterId;
    private String nodeKey;
    private String nodeValue;
}

I want to convert List<AdditionalMetrics> to Map<masterId, Map<nodeKey, nodeValue> and I search on the Internet it just shows

additionalMetrics.stream()
    .collect(Collectors.toMap(AdditionalMetrics::getMasterId,
                              AdditionalMetrics::getNodeValue)

I would avoid using nested maps. Instead I would create a `MetricsIdValueKey` with the attributes `masterId` and `nodeKey` and a `equals` and `hashCode` implementaion and then use it for a `Map` — magicmn, Feb 23 '21 at 10:33

score 1 · Accepted Answer · answered Feb 23 '21 at 09:54

You can't use toMap() because you have many nodeKey/nodeValue pairs to map to a single masterId value (unless you use a merge function).

This is easier to do grouping by masterId:

Map<String, Map<String, String>> result = additionalMetrics.stream()
        .collect(Collectors.groupingBy(
                AdditionalMetrics::getMasterId,
                Collectors.toMap(AdditionalMetrics::getNodeKey, 
                        AdditionalMetrics::getNodeValue)));

how to convert list to map by java8?

1 Answers1