removeEldestEntry

removeEldestEntry is a protected hook method in the LinkedHashMap class that allows a class to impose a policy for removing the eldest entry when a new one is inserted. The method signature is protected boolean removeEldestEntry(Map.Entry<K,V> eldest). By default, it returns false, so no removal occurs. After put or putAll inserts a new entry, LinkedHashMap calls this method and, if the result is true, removes the eldest entry from the map.

Usage and pattern: removeEldestEntry is commonly used to implement bounded caches. You subclass LinkedHashMap and override removeEldestEntry to return true when size() exceeds a configured maximum. Because LinkedHashMap can be constructed to follow insertion order or access order, you can implement an LRU cache by creating it with accessOrder set to true and keeping the size under a maximum.

Example (plain text):

class LruCache<K,V> extends LinkedHashMap<K,V> {

private final int maxEntries;

public LruCache(int maxEntries) {

super(maxEntries, 0.75f, true);

this.maxEntries = maxEntries;

}

protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {

return size() > maxEntries;

}

}

Notes: removeEldestEntry is a subclassing hook, not part of the Map interface, and is not applicable to all map implementations. It is not inherently thread-safe; external synchronization is required if used from multiple threads. This mechanism is central to the common “LRU cache” pattern in Java collections. See LinkedHashMap and related documentation for details on constructor options and iteration order.