How to convert a plain Java for-loop to a Java 8 stream?

Question

How can I convert this code to Java 8 stream?

String getFirst(String key) {
    for (Param p : params) {
        if (key.equals(p.getKey())) {
            if (!p.getValues().isEmpty()) {
                return p.getValues().get(0);
            }
        }
    }
    return "";
}

Some IDEs will do this conversion for you. I suggest you see if the IDE will do this for you. — Peter Lawrey
– Peter Lawrey, Commented Apr 5, 2015 at 1:05

Misha · Accepted Answer · 2015-04-24 17:56:15Z

8

return params.stream()
  .filter(p -> key.equals(p.getKey())
  .filter(p -> ! p.getValues().isEmpty())
  .map(p -> p.getValues().get(0))
  .findFirst()
  .orElse("");

If p.getValues() is a List, you could shorten it as:

return params.stream()
  .filter(p -> key.equals(p.getKey())
  .flatMap(p -> p.getValues().stream())
  .findFirst()
  .orElse("");

If it's not important to get the first matching value and you are ok with just getting any match, replace findFirst() with findAny(). It will more clearly mark your intent and, if somebody makes the stream parallel later on, findAny() may perform better.

edited Apr 24, 2015 at 17:56

answered Apr 5, 2015 at 0:43

Misha

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1 Comment

Vadzim Over a year ago

Two filters could be merged into single predicate for efficiency.

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