Maintain a transitive mapping in Java

Question

Scenario

class A{
  String aId;
;}

class B{
  String bId;
;}

class C{
  String bId;
;}

class D{
    B b;
    C c;
;}

I hava a List<A>. I traverse this list using a for loop, perform some DB operations and create a List<B>. I need a mapping from A to B, so I create a Map<A,B> also.

From this List<B>, I make a bulk API call and fetch a List<D>. I require a mapping from B to C also, so I traverse over this List<D> and create a Map<B,C> .

My main requirement is a mapping from A to C. I can't remove the bulk API call but want to remove the extra overhead of maintaining two maps. Is there some way of maintaining a transitive mapping, a mapping of A to C directly in JAVA?

Answer 1

You can generate it as follows:

for loop

import java.util.HashMap;
import java.util.Map;

public class Main
{

    public static void main(String[] args)
    {
        Map<String,String> m1= new HashMap<>();
        Map<String,String> m2 = new HashMap<>();
        m1.put("aaa", "bbb");
        m2.put("bbb", "ccc");
        Map<String,String> combined = new HashMap<>();
        for(String key : m1.keySet())
        {
            // TODO: add error handling or make sure get never returns null 
            combined.put(key, m2.get(m1.get(key)));
        }
        System.out.println(combined.get("aaa"));
    }
}

streams

import java.util.HashMap;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;;

public class Main
{

    public static void main(String[] args)
    {
        Map<String,String> m1= new HashMap<>();
        Map<String,String> m2 = new HashMap<>();
        m1.put("aaa", "bbb");
        m2.put("bbb", "ccc");
        Map<String,String> combined = m1.keySet().stream().collect(Collectors.toMap( Function.identity(), x->m2.get(m1.get(x))));
        System.out.println(combined);
    }

}

Edit: As requested, added a version using streams, even though I prefer the first version.