Get amount of specific element in List

Question

I'm looking for a quick method to find the amount of elements of a List that are one specific element:

List<String> list = new ArrayList<String>();
list.add("apple");
list.add("banana");
list.add("apple");
list.add("kiwi");

// I'm looking for a method as List.amountOf(Object obj):

list.amountOf("apple");     // should return 2
list.amountOf("kiwi");      // should return 1
list.amountOf("pear");      // should return 0

Answer 1

You can use Collections.frequency :

int amountOfApple = Collections.frequency(list,"apple");

With Java 8 you will also be able to do this using streams :

long amountOfApple = list.stream().filter(s -> "apple".equals(s)).count();

Answer 2

If you use Eclipse Collections you can use either a MutableBag or MutableList, depending on whether order matters for the collection.

// If order doesn't matter 
MutableBag<String> bag = Bags.mutable.with("apple", "banana", "apple", "kiwi");

// O(1) for bag.occurrencesOf()
Assert.assertEquals(2, bag.occurrencesOf("apple"));
Assert.assertEquals(1, bag.occurrencesOf("kiwi"));
Assert.assertEquals(0, bag.occurrencesOf("pear"));

// If order does matter 
MutableList<String> list = Lists.mutable.with("apple", "banana", "apple", "kiwi");

// O(n) for collection.count()
// Java 5 - 7
Assert.assertEquals(2, list.count(Predicates.equal("apple")));
Assert.assertEquals(1, list.count(Predicates.equal("kiwi")));
Assert.assertEquals(0, list.count(Predicates.equal("pear")));

// using Java 8 Lambdas
Assert.assertEquals(2, list.count(fruit -> fruit.equals("apple")));
Assert.assertEquals(1, list.count(fruit -> fruit.equals("kiwi")));
Assert.assertEquals(0, list.count(fruit -> fruit.equals("pear")));

// using Java 8 Method References
Assert.assertEquals(2, list.count("apple"::equals));
Assert.assertEquals(1, list.count("kiwi"::equals));
Assert.assertEquals(0, list.count("pear"::equals));

// O(n) for collection.countWith()
// using Java 8 Method References
Assert.assertEquals(2, list.countWith(Object::equals, "apple"));
Assert.assertEquals(1, list.countWith(Object::equals, "kiwi"));
Assert.assertEquals(0, list.countWith(Object::equals, "pear"));

Note: I am a committer for Eclipse Collections