A LRU HashMap that allows elements to be locked so that they don't expire.

git-svn-id: https://samskivert.googlecode.com/svn/trunk@939 6335cc39-0255-0410-8fd6-9bcaacd3b74c
2002-11-15 01:50:46 +00:00
parent e4e2eddeae
commit a4b242bf8f
1 changed files with 69 additions and 0 deletions
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+//
+// $Id: LockableLRUHashMap.java,v 1.1 2002/11/15 01:50:46 ray Exp $
+
+package com.samskivert.util;
+
+import java.util.HashSet;
+import java.util.Map;
+
+/**
+ * A LRU HashMap that allows specified keys to be locked such that they
+ * cannot be removed.
+ */
+public class LockableLRUHashMap extends LRUHashMap
+{
+    /**
+     * Construct a LockableLRUHashMap with the specified maximum number
+     * of <em>unlocked</em> elements.
+     */
+    public LockableLRUHashMap (int baseMaxSize)
+    {
+        super(baseMaxSize);
+    }
+
+    /**
+     * Lock the specified key from being removed.
+     * This has the side effect of increasing the maximum size by 1.
+     *
+     * @return false if the key was already locked and the max size was
+     * not altered.
+     */
+    public boolean lock (Object key)
+    {
+        if (_locks.add(key)) {
+            _maxSize++;
+            return true;
+        }
+        return false;
+    }
+
+    /**
+     * Unlock the specified key so that it can be removed.
+     * This has the side effect of decreasing the maximum size by 1.
+     * If the map is bigger than the new maximum size, the object with that
+     * key will be immediately removed.
+     *
+     * @return false if the key was not even locked in the first place.
+     */
+    public boolean unlock (Object key) 
+    {
+        if (_locks.remove(key)) {
+            _maxSize--;
+            if (size() > _maxSize) {
+                remove(key);
+            }
+            return true;
+        }
+        return false;
+    }
+
+    // documentation inherited
+    protected boolean removeEldestEntry (Map.Entry eldest)
+    {
+        return (! _locks.contains(eldest.getKey())) &&
+            super.removeEldestEntry(eldest);
+    }
+
+    /** The set of keys that are locked. */
+    protected HashSet _locks = new HashSet();
+}