如何使用 Java Streams 比较两个列表并将共同添加到字段

Question

I have two lists List<Product> products and List<AssetMap> mapAssetIds . Both the Product and AssetMap classes have two fields productId and assetId .

public class Product {
    public String productId;
    public String assetId;
}

public class AssetMap {
    public String productId;
    public String assetId;
}

List<Product> products = new ArrayList<>();
products.add(new Product("pid1", "aid1"));
products.add(new Product("pid2", "aid2"));
List<AssetMap> assetMaps = new ArrayList<>();
assetMaps.add(new AssetMap("pid560", "aid1"));
assetMaps.add(new AssetMap("pid3", "aid3"));

The output should be:

["pid560", "aid2"]

I want to check if the assetId s in products are present in assetId s in mapAssetIds .

If they are, then take the productId from mapAssetIds else take the assetId from Product and put them in a List<String> .

How do I do this using Java Stream?

Answer 1

List<Product> products = new ArrayList<>();
    products.add(new Product("aaa"));
    products.add(new Product("bbb"));
    List<AssetMap> assetMaps = new ArrayList<>();
    assetMaps.add(new AssetMap("bbb"));
    assetMaps.add(new AssetMap("ccc"));

    List<String> commons = products.stream().map(Product::getProductId).collect(Collectors.toList());
    commons.retainAll(assetMaps.stream().map(AssetMap::getAssetId).collect(Collectors.toList()));
    for (String common : commons) {
        System.out.println(common);
    }

output "bbb"

Answer 2

You can do like this:

first create a map from assetMaps so that getAssetId as key and getProductId as value .

Map<String, String> map = assetMaps.stream()
      .collect(Collectors.toMap(AssetMap::getAssetId, AssetMap::getProductId));

then stream over products items and find value based on previous result and in case absent value use default value.

List<String> result = products.stream()
            .map(Product::getAssetId)
            .map(assetId -> map.getOrDefault(assetId, assetId))
            .collect(Collectors.toList());

Answer 3

  List<String> result = new ArrayList<>();
        
        List<String> assestIdsInMap = assetMaps.stream().map(assest -> assest.assestId).collect(Collectors.toList());
        products.stream().forEach(i -> {
            if (assestIdsInMap.contains(i.assestId)) {
                result.add(assetMaps.stream().filter(assetMap -> assetMap.assestId.equals(i.assestId)).findAny().get().productId);
            } else {
                result.add(i.assestId);
            }
        });