Eclipse Collections by Example: Detect

Learn how to find the first element of a collection that matches a condition in Java using Eclipse Collections.

Tomato <-> To.ma.to.

Detect / DetectWith

Use detect to find the first element of a collection that matches a given Predicate. Use detectWith to find the first element of a collection that matches a given Predicate2 with an extra parameter. The method detectWith works well with method references. If no element is found that matches then null is returned.

MutableList<String> rainbow = Lists.mutable.with(
        "Red", "Orange", "Yellow", "Green",
        "Blue", "Indigo", "Violet");
String red = rainbow.detect(each -> each.startsWith("R"));
Assert.assertEquals("Red", red);

String red2 = rainbow.detectWith(String::startsWith, "R");
Assert.assertEquals("Red", red2);

Assert.assertNull(rainbow.detect(each -> each.startsWith("T")));

Assert.assertNull(rainbow.detectWith(String::startsWith, "T"));

DetectIfNone / DetectWithIfNone

Use detectIfNone to find the first element of a collection that matches a given Predicate. If there is no match returns the result of evaluating the given Function0. The method detectWithIfNone is similar to detectWith, and also works well with method references.

String violetIfNone =
        rainbow.detectIfNone(
                each -> each.startsWith("T"),
                () -> "Violet");
Assert.assertEquals("Violet", violetIfNone);

String violetIfNone2 =
        rainbow.detectWithIfNone(
                String::startsWith,
                "T",
                () -> "Violet");
Assert.assertEquals("Violet", violetIfNone2);

DetectOptional / DetectWithOptional

The method detectOptional is the equivalent of filter().findFirst() on a Java Stream. This method will return an Optional which can be queried to determine if any element has matched the given Predicate. The method detectWithOptional is similar to detectWith, and will work equally well with method references.

Optional<String> violetOptional =
        rainbow.detectOptional(each -> each.startsWith("T"));
Assert.assertEquals("Violet", violetOptional.orElse("Violet"));

Optional<String> violetOptional2 =
        rainbow.detectWithOptional(String::startsWith, "T");
Assert.assertEquals("Violet", violetOptional2.orElse("Violet"));

APIs covered in the examples

1. detect / detectWith — finds the first element of a collection that matches a given Predicate or Predicate2. If there is no match, then null is returned.
2. detectIfNone / detectWithIfNone — finds the first element of a collection that matches a given Predicate or Predicate2. If there is no match, then the result of evaluating the given Function0 is returned.
3. detectOptional / detectWithOptional — finds the first element of a collection that matches a given Predicate or Predicate2. The result of the call will be an Optional, which can be queried to determine if there was a successful match.

Check out this presentation to learn more about the origins, design and evolution of the Eclipse Collections API.

I am a Project Lead and Committer for the Eclipse Collections OSS project at the Eclipse Foundation. Eclipse Collections is open for contributions. If you like the library, you can let us know by starring it on GitHub.