Convert ArrayList to Set with Java streams

Question

I want to convert an ArrayList<String> to Set<ScopeItem> with Java streams.

ScopeItem is a enum;
items is an ArrayList<String>;

Set<ScopeItem> scopeItems = items.stream()
                    .map(scopeString -> ScopeItem.valueOf(scopeString))
                    .filter(Objects::nonNull)
                    .collect(Collectors.toSet());

On a string that isn't in the enum this throws the following:

java.lang.IllegalArgumentException: No enum const...

Ideally, I would like to skip past any Strings that don't match.

I think maybe a using flatmap? Any ideas how to do it?

Answer 1

You could add the following method to your ScopeItem:

public static ScopeItem valueOfOrNull(String name) {
    try {
        return valueOf(name);
    } catch (IllegalArgumentException e) {
        // no such element
        return null;
    }
}

and use that to map your enum values:

.map(scopeString -> ScopeItem.valueOfOrNull(scopeString))

Subsequent .filter() on non-null values (which you already have) will filter-out those nulls that correspond to non-matching strings.

Answer 2

You can put a try-catch inside your map to return null instead of throwing an exception:

Set<ScopeItem> scopeItems = items.stream()
    .map(scopeString ->
        {
           try
           {
              return ScopeItem.valueOf(scopeString);
           }
           catch (IllegalArgumentException e)
           {
              return null;
           }
        })
    .filter(Objects::nonNull)
    .collect(Collectors.toSet());

You could also use filter beforehand to check whether the array of values contains the string you're looking for:

Set<ScopeItem> scopeItems = items.stream()
    .filter(scopeString -> Arrays.stream(ScopeItem.values())
                               .anyMatch(scopeItem -> scopeItem.name().equals(scopeString)))
    .map(ScopeItem::valueOf)
    .collect(Collectors.toSet());

Answer 3

Unlike others, I won't recommend using exceptions, as I feel they should be used for exceptional situations, and not for something that will likely to occur. A simple solution, is to have a static set of acceptable strings, and simply check, if a string you want to use valueOf with is in said set.

package test;

import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;

public class Test {

    public static enum ScopeItem {
        ScopeA,
        ScopeB;

        private static Set<String> castableStrings;

        static {
            castableStrings = new HashSet<>();
            for (ScopeItem i : ScopeItem.values()) {
                castableStrings.add(i.name());
            }
        }

        public static boolean acceptable(String s) {
            return castableStrings.contains(s);
        }
    }

    public static void main(String[] args) {

       List<String> items = Arrays.asList("ScopeA", "RandomString", "ScopeB");
       Set<ScopeItem> scopeItems = items.stream()
               .filter(ScopeItem::acceptable)
               .map(ScopeItem::valueOf)
               .collect(Collectors.toSet());

       System.out.println(scopeItems.size());
    }

}

Answer 4

There's a more elegant approach here. You don't have to add any new fields to your enum; you can simply run a stream against it as well and determine if there's any matches in your collection.

The below code assumes an enum declaration of:

enum F {
    A, B, C, D, E
}

and looks as thus:

List<String> bad = Arrays.asList("A", "a", "B", "b", "C", "c");

final Set<F> collect = bad.stream()
                                .filter(e -> Arrays.stream(F.values())
                                                     .map(F::name)
                                                     .anyMatch(m -> Objects.equals(e, m)))
                                .map(F::valueOf)
                                .collect(Collectors.toSet());

Two parts to pay attention to here:

• We do the internal filter on the values of our enum, and map that to a String through F::name.
• We determine if there's any match on the elements of our base collection with the elements of our enum in a null-safe way (Objects.equals does The Right Thing™ with nulls here)

Answer 5

You could use Apache Common's commons-lang3 EnumUtils.getEnum() instead of valueOf(). This returns null if there is no matching enum entry (which you can then filter exactly as you have in your code).

Answer 6

The easiest and clearest method would be to filter your Enum's values() method on items.contains():

Set<ScopeItem> enumVals = Arrays.stream(ScopeItem.values())
        .filter(e -> items.contains(e.name()))
        .collect(Collectors.toSet());

No added functionality just to get the stream to do what you want, and it is obvious what this does at a glance.