Java - removeIf example

Question

HashMap<Integer, ArrayList<Integer>> cityMap = new HashMap<>();
...    
for (ArrayList<Integer> list : cityMap.values()) {
    int size = list.size();
    if (size > 0) {
        list.removeIf(i -> true);
    }
}

I don't quite understand what the removeIf does in this case. Especially the part (i -> true). Thank you for any explanation.

Answer 1

The Javadoc of removeIf() states:

Removes all of the elements of this collection that satisfy the given predicate.

The predicate in your example is always true because you map each integer i in your list to trueby the expression: i -> true.

I added a simpler example which removes all even integers and keeps all odd integers by the predicate i % 2 == 0:

Ugly setup:

List<List<Integer>> lists = new ArrayList<List<Integer>>() {{
    add(new ArrayList<>(Arrays.asList(1,2,3,4)));
    add(new ArrayList<>(Arrays.asList(2,4,6,8)));
    add(new ArrayList<>(Arrays.asList(1,3,5,7)));
}};

Keep only odd numbers:

for (List<Integer> list : lists) {
    list.removeIf(i -> i % 2 == 0);
    System.out.println(list);
}

Output:

[1, 3]
[]
[1, 3, 5, 7]