What is the collection that HashMap.values() method in Java returns?

Question

Java SE 6.0 API says values() method in java.util.HashMap returns a Collection type. How does JVM decide which collection to return at run time. Is it jvm specific or are there any general guidelines that Java follows. I browsed the source-code of HashMap but couldn't get a clue. Any help is highly appreciated, or if the question is lame, kindly point me why.Thanks.

Answer 1

You can see in the sources :

public Collection<V> values() {
    if (values == null) {
        values = new AbstractCollection<V>() {
          ...

They actually give a custom implementation of AbstractCollection .

An important thing to know about this collection is that it is NOT Serializable : never try to send it as-is between client-server.

Please note that this extract comes from the Sun JDK sources. It means that it is specific to the vendor implementation .

Answer 2

It's not JVM who decides which collection to return at runtime but the actual implementation of Map interface. In case of HashMap this is HashMap.Values inner class, see HashMap src