Best way to replace an element of a Collection with multiple elements in Java

Question

I have a Collection of String values. If one of the values is an '*', I'd like to replace that value with 3 others, let's say "X", "Y", and "Z".

In other words, I'd like [ "A", "B", "*", "C"] to turn into ["A","B","X","Y","Z","C"]. Order does not matter, so it is simply get rid of one and add the others. These are the ways I can think of for doing it, using that example:

Collection<String> additionalValues = Arrays.asList("X","Y","Z"); // or set or whatever
if (attributes.contains("*")) {
    attributes.remove("*");
    attributes.addAll(additionalValues);
}

or

attributes.stream()
          .flatMap(val -> "*".equals(val) ? additionalValues.stream() : Stream.of(val))
          .collect(Collectors.toList());

What's the most efficient way of doing this? Again, order doesn't matter, and ideally I'd like to remove duplicates (so maybe distinct() on stream, or HashSet?).

that would always add the attributes, even if there wasn't a '*'. I only want it to add the values if the * exists — Sean, Jul 13 '18 at 21:49
Best practice is to focus on readability and maintainability. If you have reason to believe there's a performance concern, do some [benchmarking](https://stackoverflow.com/q/504103/1553851). — shmosel, Jul 13 '18 at 21:49
@shmosel you have right that readability and maintainability are high priority, but since java9 and jigsaw project we can put java on smaller devices than PC, where performance is a priority not readability — Thomas Banderas, Jul 13 '18 at 21:57
@ThomasBanderas Well sure, that's true since way before Java 9. But that would be a reason to believe performance is a concern. — shmosel, Jul 13 '18 at 21:58

score 2 · Answer 1 · answered Jul 13 '18 at 22:05

I'd do it very similarly to your first way:

if (attributes.remove("*")) {
    attributes.addAll(additionalValues);
}

You don't need a separate remove and contains call for a correctly-implemented collection:

[Collection.remove(Object)] Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if this collection contains one or more such elements. Returns true if this collection contained the specified element (or equivalently, if this collection changed as a result of the call).

wow, totally forgot .remove() returned a boolean. Good call. — Sean, Jul 13 '18 at 22:11

Oleg Cherednik · Accepted Answer · 2018-07-13T21:59:51.537

I think that second one is better. Arrays.asList("X","Y","Z") retrieve ArrayList i.e. an array. It is not good for replace values in it.

In general case, if you want to modify a colletion (e.g. replace * with X,Y and Z), the do create new collection in some way.

Do look at LinkedList if you want to modify collection itself.

Using Streams:

public static List<String> replace(Collection<String> attributes, String value, Collection<String> additionalValues) {
    return attributes.stream()
                     .map(val -> value.equals(val) ? additionalValues.stream() : Stream.of(val))
                     .flatMap(Function.identity())
                     .collect(Collectors.toList());
}

Not using Streams

public static List<String> replace(Collection<String> attributes, String value, Collection<String> additionalValues) {
    List<String> res = new LinkedList<>();

    for (String attribute : attributes) {
        if (value.equals(attribute))
            res.addAll(additionalValues);
        else
            res.add(attribute);
    }

    return res;
}

Demo:

List<String> attributes = Arrays.asList("A", "B", "*", "C");
List<String> res = replace(attributes, "*", Arrays.asList("X", "Y", "Z"));  // ["A", "B", "X", "Y", "Z", "C"]

additionalValues can be any kind of collection, it's just how I happened to define the list up there. In the actual code, it's a collection coming through the method, so it has already been defined elsewhere — Sean, Jul 13 '18 at 21:48
Ah I see what you're saying for creating a new list now. Ok, thanks! — Sean, Jul 13 '18 at 22:11

score 0 · Answer 3 · answered Jul 13 '18 at 22:13

For best performance, and to insert replacement values in the correct position, use indexOf(o), remove(index), and addAll(index, c):

Demo

List<String> attributes = new ArrayList<>(Arrays.asList("A", "B", "*", "C"));
Collection<String> additionalValues = Arrays.asList("X","Y","Z");

int idx = attributes.indexOf("*");
if (idx != -1) {
    attributes.remove(idx);
    attributes.addAll(idx, additionalValues);
}

System.out.println(attributes);

Output

[A, B, X, Y, Z, C]

If order doesn't matter, use the return value of remove(o):

if (attributes.remove("*"))
    attributes.addAll(additionalValues);

Output

[A, B, C, X, Y, Z]

Best way to replace an element of a Collection with multiple elements in Java

3 Answers3