get a comma separated string using java stream

Question

I have the following code using java Stream.

I am trying the get the function to build a string of value: "a,b" in this case. however, the output (separatedByComma in this case) is always "b".

Could somebody shed some light please?

@Test
public void testJoin() {
    List<MOccS> occList = new ArrayList<> (  );
    MOccS mOccS = new MOccS ();
    mOccS.setOccSCd ( "1" );
    mOccS.setOccSNm ( "a" );
    occList.add ( mOccS );

    MOccS mOccS2 = new MOccS ();
    mOccS2.setOccSCd ( "2" );
    mOccS2.setOccSNm ( "b" );
    occList.add ( mOccS2 );


    List<String> strings = new ArrayList<> (  );
    strings.add ( "1" );
    strings.add ( "2" );

    String separatedByComma = "";
    for(String word: strings) {
        separatedByComma = occList.stream ()
                                  .filter ( occ -> word.equalsIgnoreCase ( occ.getOccSCd () ) )         
                                  .map ( occ -> occ.getOccSNm () )
                                  .collect ( Collectors.joining ( "," ) );
     }

     System.out.println (separatedByComma);
}


class MOccS{
    String occSCd;
    String occSNm;
    ...
    getter/setter
    ...
}

Answer 1

Each iteration of your for loop overwrites the value of separatedByComma. The first iteration assigns the String "a" to it, and the second replaces it with "b".

You should Stream over the elements of the strings List in order to join the Strings that match each of them into a single output String:

String separatedByComma = 
    strings.stream()
           .flatMap(word -> occList.stream()
                                   .filter(occ -> word.equalsIgnoreCase (occ.getOccSCd()))
                                   .map (occ -> occ.getOccSNm()))
           .collect(Collectors.joining (","));

Output:

a,b