public class WeakHashMap<K, V>

java.util.WeakHashMap: a map that does not keep its keys alive. An entry disappears once nothing else refers to its key, which is what lets desktop code hang data off a component – a layout manager’s cached constraints, say – without ever removing it.

Keys are held through the device’s WeakReference and compared with equals, as the JDK’s are. Entries whose key is gone are dropped by the next change to the map, by size(), and by reads too, in a sweep that runs once per roughly as many reads as the map has keys – so a cache that is only ever read still lets go of its values, at an amortized constant cost per read. There is no reference queue on a device, so nothing is dropped in between, and size() counts only the entries still alive.

entrySet(), keySet() and values() are snapshots taken when they are called: removing through them, or through their iterators, does not change the map. The JDK’s are live views; code that edits a weak map through a view is rare enough that the simpler contract is kept.

Like every shared JDK class here it is not synchronized, and neither is the JDK’s.