Question

I’m interested how I can very quickly change the Boolean values into this hashmap:

HashMap<String, Boolean> selectedIds = new HashMap<>(); 

I want very quickly to replace the Boolean values all to be true. How I can do this?

Answer 1

The fastest way is this:

for (Map.Entry<String, Boolean> entry : selectedIds.entrySet()) {
    entry.setValue(true);
}

This code avoids any lookups whatsoever, because it iterates though the entire map’s entries and sets their values directly.

Note that whenever HashMap.put() is called, a key look up occurs in the internal Hashtable. While the code is highly optimized, it nevertheless requires work to calculate and compare hashcodes, then employ an algorithm to ultimately find the entry (if it exists). This is all "work", and consumes CPU cycles.

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Java 8 update:

Java 8 introduced a new method replaceAll() for just such a purpose, making the code required even simpler:

selectedIds.replaceAll((k, v) -> true);