Will Java 8 create a new List after using Stream "filter" and "collect"?

Question

I have code using Java8:

List<Integer> list = new ArrayList<Integer>();
list.add(3);
list.add(5);
list.add(4);
list.add(2);
list.add(5);
list = list.stream().filter(i -> i >= 3).collect(Collectors.toList());

Original list is [3, 5, 4, 2, 5]. After "filter" and "collect" operation, the list changes to [3, 5, 4, 5].

Are all the operations perform on original list and does not create a new list? Or after "filter" and "collect" operation, return a new created list and ignore original list?

Answer 1

According to the Javadoc , passing the Collector returned by Collectors.toList() into the collect method will create a new list.

public static <T> Collector<T,?,List<T>> toList()

Returns a Collector that accumulates the input elements into a new List . There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned; if more control over the returned List is required, use toCollection(Supplier) .

The original list remains unaffected.

Answer 2

If you actually want to modify the original list, consider using removeIf :

list.removeIf(i -> i < 2);

Answer 3

Stream operations are either intermediate or terminal. Intermediate operations return a stream so you can chain multiple intermediate operations. Terminal operations return void or something else.

Most of stream operations are non-interfering, it means that they don't modify the data source of the stream. But by calling the collect method you are creating a new list and you're assigning it to list

Answer 4

Try this:

List<Integer> list = new ArrayList<Integer>();
list.add(3);
list.add(5);
list.add(4);
list.add(2);
list.add(5);
List<Integer> list2 = list.stream().filter(i -> i >= 3).collect(Collectors.toList());
System.out.println("list:  "+list);
System.out.println("list2: "+list2);

Answer 5

Referring to Guava Lists.transform() function will help you understand why a new list needs to generate. Before JDK 7, Lists.transform() function is strongly recommended to implement similar feature.