Is map.size() always the same as map.entrySet().size()

Question

Say I have a HashMap:

var h = new HashMap();

my question is, is:

h.size() == h.entrySet().size()

always? (Assuming no concurrency issues).

Answer 1

Assuming no concurrency issues modifying the HashMap , then your expression

h.size() == h.entrySet().size()

will be true . From the source code for HashMap , the method size() :

public int size() {
    return size;
}

where size is an instance variable declared by HashMap .

When you call entrySet() , it returns an instance of a nested class called EntrySet , and its size() method is:

public final int size()                 { return size; }

Note that EntrySet doesn't declare its own size variable; it's returning size from HashMap .

Also note that keySet() and values() similarly return their own nested classes that are defined practically identically.

So, no matter where you call size() , it should return identically, assuming no interference from other threads or other structural modifications between calls.

Answer 2

The source code suggests it is...

final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
    public final int size() {
        return size;
    }
    // rest of code
}

...and my decompiler suggests it as well.

public final int size() {
    return HashMap.this.size;
}

Note that you will (somewhat ironically) get burned by ConcurrentHashMap ; this assumption may not hold due to the fact that any modifications to this class would put the state of the size in flux. Even though this is a thread-safe collection, it's not safe to rely on size() to do any kind of logical control like you would with a HashMap , for instance.

Answer 3

Yes, it's always true because a hashmap is just a list of entries. The difference is in the represented form, but the size is the same. (This answer is based on not modifying the HashMap class)