Build map from list by lambda-CodePudding

I have a Group model with an inner field List<Entity> entities

How is possible to change the below code to one line by lambda and stream

 Map<String, String> entityGroup = new HashMap<>();
 groups.forEach(g -> g.getEntities()
                .forEach(e -> entityGroup.put(e.getKey(), g.getKey()))
 );

Each entity in the inner list should be the key in the map and the value should be the Group itself

Thanks

CodePudding user response：

To do this using streams, you would need to make a stream of map-entries and then collect it into a Map.

I'm using Map.entry in my example, which is available in Java 9 and up. If you're on Java 8, you can use AbstractMap.SimpleEntry or some other kind of Pair-class.

Something like this:

Map<String, String> entityGroup = groups.stream()
    .flatMap(group -> group.getEntities().stream()
        .map(entity -> Map.entry(entity.getKey(), group.getKey())))
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

CodePudding user response：

Here's a runnable example which doesn't require you to explicitly create a new entry like the solution above:

import java.util.*;
import java.util.stream.*;

public class MyClass {
    public static void main(String args[]) {
        List<B> b1 = List.of(new B("a", "1"), new B("b", "2"));
        List<B> b2 = List.of(new B("x", "3"), new B("y", "4"));
        A a1 = new A(b1);
        A a2 = new A(b2);
        List<A> allAs = List.of(a1, a2);
        Map<String, String> map =
               allAs.stream().flatMap(a -> a.vals.stream())
               .collect(Collectors.toMap(B::getKey, B::getValue));
        System.out.println(map); // {a=1, b=2, x=3, y=4}
    }
}

class A {
    List<B> vals;
    public A(List<B> vals) { this.vals = vals; }
}

class B {
    String k, v;
    public B(String k, String v) { this.k = k; this.v = v; }
    public String getKey() { return this.k; }
    public String getValue() { return this.v; }
}