Java 8 streams - How do I manipulate value in a group by result?

Question

I have a list of objects with two String fields like

Name1, Active
Name2, InActive
Name3, InActive
Name4, Active

And I would like to have a

Map<Boolean, List<String>>

where Active=true and InActive=false.

I tried

active = dualFields.stream().collect(
       Collectors.groupingBy( 
           x -> StringUtils.equals(x.getPropertyValue(), "Active")));

But I received

Map<Boolean, List<DualField>>

score 9 · Accepted Answer · answered Oct 02 '17 at 10:11

First of all, you can use partitioningBy, which is a special case of groupingBy (where you are grouping by a Predicate into two groups). You can map the objects to the required Strings with mapping.

I'm assuming you want the List<String> to contain the values of the second property (i.e. not getPropertyValue(), which contains "Active" or "InActive"):

Map<Boolean,List<String>> map =
    dualFields.stream()
              .collect(Collectors.partitioningBy(x -> StringUtils.equals(x.getPropertyValue(), "Active"),
                                                 Collectors.mapping(DualField::getSecondPropertyValue,
                                                                    Collectors.toList()));

The code becomes more readable by replacing `StringUtils.equals(x.getPropertyValue(), "Active")` with `x.getPropertyValue().equals("Active")` or, if handling `null` is desired, `"Active".equals(x.getPropertyValue())`… There’s also the standard API method `Objects.equals` which handles `null`, so in either case, there is no need for a 3rd party library to provide redundant methods like `StringUtils.equals`… — Holger, Oct 04 '17 at 09:31

score 4 · Answer 2 · answered Oct 02 '17 at 10:05

If you want your List<String> to be composed of propertyValue, than:

active = dualFields.stream().collect(
        Collectors.groupingBy( 
            x -> StringUtils.equals(x.getPropertyValue(), "Active"),
            Collectors.mapping(DualField::getPropertyValue, Collectors.toList());
            ));

Java 8 streams - How do I manipulate value in a group by result?

2 Answers2