如何转换地图<String, List<String> &gt; 设置<String>使用流？

Question

I'm trying to convert Map> to a List/Set of Strings in single line but couldn't do it unless I use traditional forEach for map. Is anyone aware to do it in single line using streams.

import java.util.*;

public class StreamDemo {
    public static void main(String[] args) {
        HashMap<String, List<String>> map = new HashMap<>();
        ArrayList<String> s1 = new ArrayList<>();
        map.put("A", Arrays.asList("A1, A2"));
        map.put("B", Arrays.asList("B1, B2"));
        map.put("C", Arrays.asList("C1, C2"));

        Set<String> stringValues = new HashSet<>();

        // LOGIC TO GET ALL VALUES A1, A2, B1, B2, C1, C2 INTO stringValues

        System.out.println(stringValues);
        // Expected output
        // [A1, A2, B1, B2, C1, C2]

    }
}

Answer 1

you could use the following stream which basically streams over your values and collect their items inside of a HashSet but note that the HashSet doesn't guarantee sorting. For that you'd want to use TreeSet::new in the end

Set<String> stringValues = map.values()
   .stream()
   .flatMap(List::stream)
   .collect(Collectors.toCollection(HashSet::new));

Answer 2

不使用流的解决方案可以更简单：

map.values().forEach(l -> l.forEach(stringValues::add));

Answer 3

Try the below code:

HashMap<String, List<String>> map = new HashMap<>();
        ArrayList<String> s1 = new ArrayList<>();
        map.put("A", Arrays.asList("A1, A2"));
        map.put("B", Arrays.asList("B1, B2"));
        map.put("C", Arrays.asList("C1, C2"));
        HashSet<String> strings = new LinkedHashSet<>();
        map.values().stream().sequential().forEach(value -> {
            value.forEach(value1 -> {
                strings.add(value1);
            });
        });
        System.out.println(strings);

Answer 4

Since your expected output is [A1, A2, B1, B2, C1, C2] , use LinkedHashSet instead of a HashSet .

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;

public class Main {
    public static void main(String[] args) throws InterruptedException {
        HashMap<String, List<String>> map = new HashMap<>();
        ArrayList<String> s1 = new ArrayList<>();
        map.put("A", Arrays.asList("A1, A2"));
        map.put("B", Arrays.asList("B1, B2"));
        map.put("C", Arrays.asList("C1, C2"));
        Set<String> stringValues = new LinkedHashSet<>();
        map.values().forEach(list -> list.forEach(stringValues::add));
        System.out.println(stringValues);
    }
}

Output:

[A1, A2, B1, B2, C1, C2]