Skip to content

java.util.Map values() method performance

I have a map like this with several million entries:

private final Map<String, SomeItem> tops = new HashMap<>();

I need to get list of values, which could be done by calling java.util.Map values() method.

Is Collection of values created every time I call values() method or is it pre-computed from performance perspective?

As my Map has several millions elements, I do not want to create new list object every time values() is called.



Below is the copied implementation of Map.values() in java.util.HashMap:

public Collection<V> values() {
    Collection<V> vs = values;
    if (vs == null) {
        vs = new Values();
        values = vs;
    return vs;

This clearly shows that the value collection isn’t created unless necessary. So, there should not be additional overhead caused by calls to values()