Why Collector keyword is ignored?

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When you convert a Stream Object into a List Object, Why numbers .filter(number -> number > 0) .collect(Collector.toList()); does not work?

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Guadalupe Gagnon

Level 37 , Tampa, United States

15 August 2022, 13:54

Or are you asking why you have to use toList() instead of Collectors.toList()? These bits of code are exactly the same but you need a static import such as:

import static java.util.stream.Collectors.toList;

To simply use toList() without typing Collectors. What do the task conditions say you have to do?

Whisper

Level 31 , Ireland

15 August 2022, 13:58

I have never noticed this piece of code..... my bad.... but the condition says to call a static toList() method of the Collector class.

Thomas

Level 41 , Bayreuth, Germany

16 August 2022, 06:44

Collectors.toList() Some hints:

// if you do not have an Integer stream but an IntStream... map it to Integer
IntStream.range(0,10)
        .boxed()// convert IntStream to Stream<Integer>, same as mapToObj(i -> i)
        // mapToObject is far more flexible though
        .collect(Collectors.toList());

// if you have an Integer stream... just collect. Newer Java (16?) versions have a toList() method besides the
// collect(Collectors.toxxx()) method, the difference
List<Integer> testList = Stream.of(1, 2, 3). // creates an Integer stream Stream<Integer>
        collect(Collectors.toList());
for (int num : testList)
    System.out.println(num);
// backed by an ImmutableCollection
System.out.println(testList.getClass());
// can't modify testList
// testList.add(4);
List<Integer> testList2 = Stream.of(1, 2, 3)
        .collect(Collectors.toList());
// backed by an ArrayList
System.out.println(testList2.getClass());
testList2.add(4);

// if Stream.of does not work...
List<Integer> testList3 = Arrays.stream(new int[] {1,2,3})// that's an IntStream again.. needs boxing
        .boxed()
        .collect(Collectors.toList());
// or create a list, set and stream from there
List<Integer> testList4 = new ArrayList<>();
Collections.addAll(testList4, 1, 2, 3);
testList4.stream() // Stream<Integer>
        .collect(Collectors.toList());

Guadalupe Gagnon

Level 37 , Tampa, United States

15 August 2022, 13:45

Did you convert the Stream to a List of numbers such as List<Integer>? If not then you have a List<Object> and you will not be able to compare an Object with 0.