Java 8: Merging two lists containing objects by key

Question

I have two lists:

List<Server> servers1 = new ArrayList<>(); Server s1 = new Server("MyServer"); s1.setAttribute1("Attribute1"); servers1.add(s1); List<Server> servers2 = new ArrayList<>(); Server s2 = new Server("MyServer"); s2.setAttribute2("Attribute2"); servers2.add(s2); 

servers1 contains servers with name and attribute1 (but not attribute2 ).
servers2 contains servers with name and attribute2 (but not attribute1 ).

 public class Server { private String name; private String attribute1; private String attribute2; public Server(String name) { this.name = name; this.attribute1 = ""; this.attribute2 = ""; } //Getters & Setters } 

Does anyone know how I can combine these two lists into one list containing each Server only once (via name ), but with both attributes?

There are servers that exist on only one or the other list. The final list should contain all servers.

  List<Server> servers1 = new ArrayList<>(); Server s1 = new Server("MyServer"); s1.setAttribute1("Attribute1"); Server s2 = new Server("MyServer2"); s2.setAttribute1("Attribute1.2"); servers1.add(s1); servers1.add(s2); List<Server> servers2 = new ArrayList<>(); Server s3 = new Server("MyServer"); s3.setAttribute2("Attribute2"); Server s4 = new Server("MyServer3"); s4.setAttribute2("Attribute2.2"); servers2.add(s3); servers2.add(s4); 

should get:

[Server [name = MyServer, attribute1 = Attribute1, attribute2 = Attribute2],

Server [name = MyServer2, attribute1 = Attribute1.2, attribute2 =]]

Server [name = MyServer3, attribute1 =, attribute2 = Attribute2.2]]

// SOLUTION (thanks for the help!)

 Map<String, Server> serverMap1 = Stream.concat(servers1.stream(), servers2.stream()) .collect(Collectors.toMap(Server::getName, Function.identity(), (server1, server2) -> { server1.setAttribute2(server2.getAttribute2()); return server1; })); 

Answer 1

Convert each list into a map and merge it (I use Lombok to avoid writing boilerplate code):

 @Data @NoArgsConstructor @AllArgsConstructor class Server { private String name; private String attribute1; private String attribute2; } public class ServerMain { public static void main(String[] args) { List<Server> servers1 = Arrays.asList( new Server("name1", "attr1.1", null), new Server("name2", "attr1.2", null)); List<Server> servers2 = Arrays.asList( new Server("name1", null, "attr2.1"), new Server("name2", null, "attr2.2")); Map<String, Server> serverMap1 = servers1.stream().collect(Collectors.toMap(Server::getName, Function.identity())); Map<String, Server> serverMap2 = servers2.stream().collect(Collectors.toMap(Server::getName, Function.identity())); serverMap1.keySet().forEach(key -> serverMap1.merge(key, serverMap2.get(key), (server1, server2) -> { server1.setAttribute2(server2.getAttribute2()); return server1; })); System.out.println(serverMap1); } } 

Answer 2

For each server on servers1, find the corresponding server2 on servers2 and set the missing attribute of the first to fix the else attribute, if there is no corresponding server by name, just return null. Also note that this implementation will only return a list of server names contained in servers1.

 @AllArgsConstructor @Data private class Server { private String name; private String attribute1; private String attribute2; } List<Server> completServersInfo = servers1.stream() .map((server1 -> { Server matchingServer = servers2.stream() .filter(server2 -> server2.name.equals(server1.name)) .findFirst() .orElse(null); server1.setAttribute2(matchingServer.attribute2); return server1; })) .collect(Collectors.toList()); 

Answer 3

Maybe you can use a map to do this? The card key will be the server name, and the card value will be the Server object.

 Map<String, Server> map = new HashMap<>(); for (Server s : servers1) { map.put(s.getName(), s); } for (Server s : servers2) { String key = s.getName(); if (map.containsKey(key)) { map.get(key).setAttribute2(s.getAttribute2()); } else { map.put(key, s); } } List<Server> servers = new ArrayList<>(map.values()); // or other impl.