[FIXED] Filtered view of collection in Java

Issue

is there a filtered view of a collection (a subset of it) in Java, so that adding new item to it can also affect the source collection?

e.g.

List<String> source = new ArrayList<>();
source.add("a");
source.add("b");
List<String> view = source.stream().filter(i -> "b".equals(i)).collect(...);
view.add("c"); // source now contains also "c"

Solution

JDK provides only index-based view of the list with the method List::subList:

The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa.

Thus, the changes to the view can be applied to the source list too if the "filter" is constructed using List::indexOf and/or List::lastIndexOf.

This should be enough for a limited case of having a single range matching the condition:

List<String> source = new ArrayList<>();
source.add("a");
source.add("b");
source.add("b");

List<String> view = source.subList(source.indexOf("b"), source.lastIndexOf("b") + 1);

System.out.println("view: " + view);
view.add("c");

System.out.println("added view: " + view);
System.out.println("fixed src: " + source);

Output:

view: [b, b]
added view: [b, b, c]
fixed src: [a, b, b, c]

---

As for the filtered view of a list which stores all the source's elements matching some condition, such view would have to maintain a collection of sublists/subranges in general case and implement List interface appropriately.

The Guava Collections2.filter mentioned by @Wilderness Ranger above may be an example of the filtered view, however, its documentation states that its methods add/addAll would throw IllegalArgumentException if the predicate is not satisfied, so in your example an attempt to add "c" would fail for the predicate "b"::equals.

Answered By - Nowhere Man
Answer Checked By - David Marino (JavaFixing Volunteer)