Join list of 2 different object to a list of a third object using Java 8 streams

Question

I came a cross a problem I didn;t manage to solve...

I have 3 types of objects

MyObject1 - with fields id and name
MyObject2 - with fields id and error
MyJoinObject - with fields id, name and error

I have 2 lists:

List of MyObject1
List of MyObject2

I want to create a third list of MyJoinObject, which is a join of the 2 lists. There will be MyJoinObject objects as MyObject1 objects, but they will also contain the error if exists (joined by id). I want to do that with Java 8 streams.

It would be really helpful if you'd give a minimal reproducible example including what you've tried so far. What happens if there are multiple errors for a single MyObject1? — Jon Skeet
– Jon Skeet, Commented Apr 14, 2016 at 6:07
Each list of objects contains a unique objects by the id attribute, so object can have one error or none. — SharonBL
– SharonBL, Commented Apr 14, 2016 at 6:18
Hmm. In LINQ (.NET) this would be trivial. Unfortunately I'm not sure of the equivalent for a group join in Java streams. I would still strongly encourage you to update your question with an minimal reproducible example which will make it easier for people to help you. Include sample input data (hard-coded) and expected output data... along with anything you've already tried. — Jon Skeet
– Jon Skeet, Commented Apr 14, 2016 at 6:23

SharonBL · Accepted Answer · 2016-04-14 08:45:17Z

2

You can do something like that :

List<MyJoinObject> result = 
    list1.stream().map( o1 -> {
        Optional<MyObject2> error = list2.steam()
                                         .filter( o2 -> o2.getId() == o1.getId() )
                                         .findAny();
        if ( error.isPresent() ) 
            return new MyJoinObject( o1.getId(), o1.getName(), error.get().getError() );      

        return new MyJoinObject( o1.getId(), o1.getName() );

    } ).collect( Collectors.toList() );

You can also construct a hasmap of errors mapped by id before doing that by doing :

final Map<Integer, MyObject2> errorsById = 
     list2.stream( )
          .collect( Collectors.toMap( MyObject2::getId, Function.identity( ) ) );

If you do that, you can use this map by calling methods containsKey( ) or get( ) to retreive the error

edited Apr 14, 2016 at 8:45

SharonBL

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answered Apr 14, 2016 at 7:18

Prim

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Comments

Raphael Roth · Accepted Answer · 2016-04-14 07:21:53Z

2

Something like this could work (although I didn't verify):

public static void main(String[] args) {
    List<MyObject1> object1list = new ArrayList<>(); // fill with data
    List<MyObject2> object2list = new ArrayList<>();// fill with data

    List<MyJoinObject> joinobjectlist = new ArrayList<>();

    object1list.stream().forEach(
            o1 -> object2list.stream().filter(
                    o2-> o1.getId()==o2.getId()
                    ).forEach(o2->joinobjectlist.add(
                            new JoinObject(o2.getId(), o1.getName(), o2.getError()))
                            )
            );

}

answered Apr 14, 2016 at 7:21

Raphael Roth

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Comments

zhouxin · Accepted Answer · 2016-04-14 07:42:24Z

For your information:

public static void main(String[] args) {

    List<MyObject1> myObject1s = new ArrayList<>();
    List<MyObject2> myObject2s = new ArrayList<>();

    // convert myObject2s to a map, it's convenient for the stream
    Map<Integer, MyObject2> map = myObject2s.stream().collect(Collectors.toMap(MyObject2::getId, Function.identity()));

    List<MyJoinObject> myJoinObjects = myObject1s.stream()
                                                 .map(myObject1 -> new MyJoinObject(myObject1, map.get(myObject1.getId()).getError()))
                                                 .collect(Collectors.toList());

}

Of course, there should be a new construction for MyJoinObject, like this:

public MyJoinObject(MyObject1 myObject1, String error){
    this.id = myObject1.getId();
    this.name = myObject1.getName();
    this.error = error;
}

That's all. :P

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