How to Get a List from Some Class Properties with Java 8 Stream

How can I get a List from some class properties with Java 8 Stream?

You can use map :

List<String> names = 
    personList.stream()
              .map(Person::getName)
              .collect(Collectors.toList());

EDIT :

In order to combine the Lists of friend names, you need to use flatMap :

List<String> friendNames = 
    personList.stream()
              .flatMap(e->e.getFriends().stream())
              .collect(Collectors.toList());

Get aggregated list of properties from list of Objects(Java 8)

You may do it using the flatMap operator. Here's how it looks.

List<Section> sections = divisions.stream()
    .flatMap(d -> d.getSections().stream())
    .collect(Collectors.toList());

How to collect objects to List based on properties of the class and then Map to HashMap in java 8 or 11?

Class Source needs to properly override hashCode and equals methods to be used as a key in Map.

However, another POJO should be implemented to contain the desired output:

@AllArgsConstructor
class Grouped {
    @JsonProperty("Source")
    Source source;

    @JsonProperty("Target")
    List<Target> target;
}

Then the list of responses may be converted as follows:

List<Response> responses; // set up the input list

List<Grouped> grouped = responses.stream()
    .collect(Collectors.groupingBy(
        Response::getSource,
        Collectors.mapping(Response::getTarget, Collectors.toList())
    )) // Map<Source, List<Target>> is built
    .entrySet()
    .stream() // Stream<Map.Entry<Source, List<Target>>>
    .map(e -> new Grouped(e.getKey(), e.getValue()))
    .collect(Collectors.toList());

Java: Obtain class member list, for a list within multilevel nested list

Convert sub-list to a stream and use flatMap to convert stream of streams of elements to stream of elements.

Example:

package x.mvmn.demo;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Demo {

    public static class CustomerSales {
        public List<Product> productList;
    }

    public static class Product {
        public List<ProductSubItem> productSubItemList;

        public List<ProductSubItem> getProductSubItemList() {
            return productSubItemList;
        }
    }

    public static class ProductSubItem {
        public String itemName;

        public ProductSubItem(String itemName) {
            this.itemName = itemName;
        }

        public String getItemName() {
            return itemName;
        }
    }

    public static void main(String args[]) throws Exception {
        // Setup mock data
        CustomerSales customerSales = new CustomerSales();
        Product p1 = new Product();
        p1.productSubItemList = Arrays.asList(new ProductSubItem("p1 item one"), new ProductSubItem("p1 item two"));
        Product p2 = new Product();
        p2.productSubItemList = Arrays.asList(new ProductSubItem("p2 item one"), new ProductSubItem("p2 item two"));
        customerSales.productList = Arrays.asList(p1, p2);

        // Get list of item names
        System.out.println(customerSales.productList.stream().map(Product::getProductSubItemList).flatMap(List::stream)
                .map(ProductSubItem::getItemName).collect(Collectors.toList()));
        // Alternative syntax
        System.out.println(customerSales.productList.stream().flatMap(product -> product.productSubItemList.stream())
                .map(subItem -> subItem.itemName).collect(Collectors.toList()));
    }
}

Output:

[p1 item one, p1 item two, p2 item one, p2 item two]
[p1 item one, p1 item two, p2 item one, p2 item two]

Collecting lists from an object list using Java 8 Stream API

Use flatMap:

List<Integer> concat = examples.stream()
    .flatMap(e -> e.getIds().stream())
    .collect(Collectors.toList());