Multi tool use

Combine two list based on result data to create a new list in java 8

I want to combine two lists into a single list based on certain criteria.
Sample is the list 1 contains

final Org ro1= new Org(1, 1001, "Name 01");
final Org ro2 = new Org (2, 1001, "Name 02");
final Org ro3 = new Org (3, 1002, "Name 03");
final Org ro4 = new Org (4, 1003, "Name 04");
final Org ro5 = new Org (5, 1004, "Name 05");
List<Org> listOrg = new ArrayList<>();
// Add all the object to the listOrg


The list 2 contains

final Candidate can1 = new Candidate(1001, "Candidate01", "100");
final Candidate can2 = new Candidate(1002, "Candidate02", "150");
final Candidate can3 = new Candidate(1003, "Candidate03", "200");
List<Candidate > listCandidate = new ArrayList<>();
// Add all the Candidate object to the listCandidate


My final list will look like

List<Result > listResult = new ArrayList<>();
// Each individual object of the listResult is followed-
final Result rs1= new Result (1, 1001, "Name 01", "Candidate01", "100");
final Result rs2 = new Result (2, 1001, "Name 02", "Candidate01", "100");
final Result rs3 = new Result (3, 1002, "Name 03", "Candidate02", "150");
final Result rs4 = new Result (4, 1003, "Name 04", "Candidate03", "200");
final Result rs5 = new Result (5, 1004, "Name 05", null, null);


I want to achieve the same using Java 8 stream features. Can anyone please help me in this?

My class details

public class Candidate {
private int canId;
private String candidateName;
private String score;
//Getter setter with constructors.
}

public class Org{
private int id;
private int canId;
private String name;
//Getter setter with constructors
}

public class Result {
private int id;
private int canId;
private String name;
private String candidateName;
private String score;
//Getter setter with constructors.
}


The Org class have a canId, which serves as the mapping point for the CandidateClass.

new Result

Org

Candidate

for

2 Answers
2

You can do it like so,

Map<Integer, Candidate> candidateById = listCandidate.stream()
.collect(Collectors.toMap(Candidate::getId, Function.identity()));
List<Result> resultList = listOrg.stream()
.map(o -> {
Candidate candidate = candidateById.getOrDefault(o.getCandidateId(), new Candidate(-1, null, null));
return new Result(o.getId(), o.getCandidateId(), o.getName(), candidate.getName(), candidate.getNum());
})
.collect(Collectors.toList());


First create a Map of Candidate objects against their Id value. Then iterate over the List of Org objects, get the associated Candidate instance from the map given the Id value, and merge them together to form a Result. Finally collect all the Results into a List.

Map

Candidate

List

Org

Candidate

Result

Results

List

Update

As per the comment below, this can further be improved as,

List<Result> resultList = listOrg.stream()
.map(o -> new Result(o, candidateById.get(o.getCandidateId())))
.collect(Collectors.toList());


The Result constructor now looks like this,

Result

public Result(Org org, Candidate candidate) {
super();
this.orgId = org.getId();
this.candidateId = org.getCandidateId();
this.orgName = org.getName();
this.candidateName = Optional.ofNullable(candidate).map(c -> c.getName()).orElse(null);
this.candidateNum = Optional.ofNullable(candidate).map(c -> c.getNum()).orElse(null);
}


null

null

Result

Org

Candidate

null

You iterate over your Org and for each one you look for the corresponding Candidate if it exists, and then build a Result from both Org and Candidate, a constructor Result(Org o, Candidate c) would be helpful as you can see below :

Org

Candidate

Result

Org

Candidate

Result(Org o, Candidate c)

To avoid to iterate the listCandidate each time, you can prepare them in a map to get them by canId

listCandidate

canId

1. Detailled solution

Map<Integer, Candidate> candidates = listCandidate.stream()
.collect(Collectors.toMap(Candidate::getCanId, Function.identity()));
List<Result > listResult = new ArrayList<>();
listOrg.forEach(org -> {
Candidate can = candidates.getOrDefault(ro.getCanId(), null);
Result r = new Result(org, can);
listResult.add(r);
});


2. Short solution using Streams

Streams

Map<Integer, Candidate> candidates = listCandidate.stream()
.collect(Collectors.toMap(Candidate::getCanId, Function.identity()));

List<Result > listResult = listOrg.stream()
.map(org -> new Result(org, candidates.getOrDefault(ro.getCanId(), null)))
.collect(Collectors.toList());


3. Result constructor to add

Result

public Result(Org o, Candidate c){
this.id = org.getId();
this.canId = org.getCanId();
this.name = org.getName();
this.candidateName = c==null ? null : c.getCandidateName(); // return null if c is null
this.score = c==null ? null : c.getScore()); // return null if c is null
}


listOrg.size()

listCandidate.size()

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