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package edu.stanford.nlp.util;

import java.lang.ref.WeakReference;
import java.util.Map;
import java.util.Set;

/**
 * For interning (canonicalizing) things.
 * 

* It maps any object to a unique interned version which .equals the * presented object. If presented with a new object which has no * previous interned version, the presented object becomes the * interned version. You can tell if your object has been chosen as * the new unique representative by checking whether o == intern(o). * The interners use WeakHashMap, meaning that if the only pointers * to an interned item are the interners' backing maps, that item can * still be garbage collected. Since the gc thread can silently * remove things from the backing map, there's no public way to get * the backing map, but feel free to add one at your own risk. *

* Note that in general it is just as good or better to use the * static Interner.globalIntern() method rather than making an * instance of Interner and using the instance-level intern(). *

* Author: Dan Klein * Date: 9/28/03 * * @author Dan Klein */ public class Interner { protected static Interner interner = Generics.newInterner(); /** * For getting the instance that global methods use. */ public static Interner getGlobal() { return interner; } /** * For supplying a new instance for the global methods to use. * * @return the previous global interner. */ public static Interner setGlobal(Interner interner) { Interner oldInterner = Interner.interner; Interner.interner = interner; return oldInterner; } /** * Returns a unique object o' that .equals the argument o. If o * itself is returned, this is the first request for an object * .equals to o. */ @SuppressWarnings("unchecked") public static T globalIntern(T o) { return (T) getGlobal().intern(o); } protected Map> map = Generics.newWeakHashMap(); public void clear() { map = Generics.newWeakHashMap(); } /** * Returns a unique object o' that .equals the argument o. If o * itself is returned, this is the first request for an object * .equals to o. */ public synchronized T intern(T o) { WeakReference ref = map.get(o); if (ref == null) { ref = Generics.newWeakReference(o); map.put(o, ref); } // else { // System.err.println("Found dup for " + o); // } return ref.get(); } /** * Returns a Set such that each element in the returned set * is a unique object e' that .equals the corresponding element e in the * original set. */ public Set internAll(Set s) { Set result = Generics.newHashSet(); for (T o : s) { result.add(intern(o)); } return result; } public int size() { return map.size(); } /** * Test method: interns its arguments and says whether they == themselves. */ public static void main(String[] args) { for (int i = 0; i < args.length; i++) { String str = args[i]; System.out.println(Interner.globalIntern(str) == str); } } }