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
    ...
}

Haig

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

Mail

Thank you very much, your solution produces exactly what I needed. – Haig

Answer 2

In your loop for(String word: strings) you overwrite your separatedByComma variable.

Paulpaula

I think you are right, could you let me know how to fix it? sorry I am new to java stream... – Haig

If you want to filter occList and allow only those which getOccSCd is in list of string I can give you code sample. – Paulpaula

thanks so much for your quick answer as well. It was very helpful! – Haig

Answer 3

You can also use String.join(",", strings)

Glucose