Converting Java List to another using java streams

Question

I have a class Test

public class Test{
  String codes;
  String field 1;
  ....
  String field n;
}

I have a list of test objects

List<Test> objects, code can be one or more with a comma separated
testObj1("A", "field1".."fieldn")
testObj2("B,C", ...)
testObj3("D,E,F", ....)
testObj4("G", ...)

Trying to convert this list1 to new list2 with each code A, B, C... to its own object by retaining the remaining fields.

List<Test>
testObj1("A", ....)
testObj2("B", ....)
testObj3("C", ....)

list1.stream().collect(Collectors.toList())

I achieved this using loops (Sudo code) but looking for better logic

for(loop thru list1){
  String[] codesArr = testObj1.codes.split(",");
  for (String code : codesArr) {
    //Create new Obj 
    Test obj = new Test(code, testObj1.copyotherfields);
    //Add obj to list2
  }
}

In the future, please be explicit about what you are asking for. I cannot know by your question whether there were performance issues, memory issues, correctness issues... — Joe C
– Joe C, Commented Jan 18, 2019 at 18:30

Naman · Accepted Answer · 2019-01-18 10:22:50Z

3

You can use Stream.map with flatMap as :

List<Test> finalList = list1.stream()
        .flatMap(e -> Arrays.stream(e.getCodes().split(","))
                .map(c -> new Test(c, e.getField1(), e.getFieldn())))
        .collect(Collectors.toList());

This assumes that your Test class would have a constructor similar to the following implementation:

class Test {
    String codes;
    String field1;
    String fieldn;

    // would vary with the number of 'field's
    Test(String codes, String field1, String fieldn) {
        this.codes = codes;
        this.field1 = field1;
        this.fieldn = fieldn;
    }
    // getters and setters
}

edited Jan 18, 2019 at 10:22

answered Jan 18, 2019 at 0:59

Naman

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Comments

GBlodgett · Accepted Answer · 2019-01-18 00:51:09Z

0

You can simplify this to:

List<Test> copy = list.stream()
                      .map(e -> Arrays.stream(e.codes.split(""))            
                                      .map(c -> new Test(c, e.otherField))
                     .collect(Collectors.toList()))
                     .findAny().orElse(...);

Which will stream through the given list, then stream through the Array yielded from split() and map to a new Test object and collect it to a List. It retrieves it through findAny(), which returns an Optional<List<Test>>, so I would recommend using orElse to retrieve a default value.

edited Jan 18, 2019 at 0:51

answered Jan 17, 2019 at 21:46

GBlodgett

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Comments

Blake Ordway · Accepted Answer · 2019-01-17 21:59:24Z

-1

You can use a map function and then flatMap it to be like so:

List<String> testList = Arrays.asList("one,two,three,four", "five", "six", "seven", 
"eight, nine", "ten");

 List<String> reMappedList = testList.stream()
 .map(s -> {
     String[] array = s.split(",");
     return Arrays.asList(array);
 })
 .flatMap(List::stream)
 .collect(Collectors.toList());

 System.out.println(reMappedList);

answered Jan 17, 2019 at 21:59

Blake Ordway

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1 Comment

GBlodgett Over a year ago

I believe OP wanted a List<Test> not a List<String>

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