Java tutorial
/******************************************************************************* * Copyright (c) 2004, 2013 IBM Corporation and others. * * This program and the accompanying materials * are made available under the terms of the Eclipse Public License 2.0 * which accompanies this distribution, and is available at * https://www.eclipse.org/legal/epl-2.0/ * * SPDX-License-Identifier: EPL-2.0 * * Contributors: * IBM Corporation - initial API and implementation *******************************************************************************/ package org.eclipse.jdt.internal.core.util; import java.lang.ref.ReferenceQueue; import java.lang.ref.WeakReference; /** * A hashset whose values can be garbage collected. */ @SuppressWarnings({ "unchecked", "rawtypes" }) public class WeakHashSet { public static class HashableWeakReference extends WeakReference { public int hashCode; public HashableWeakReference(Object referent, ReferenceQueue queue) { super(referent, queue); this.hashCode = referent.hashCode(); } @Override public boolean equals(Object obj) { if (!(obj instanceof HashableWeakReference)) return false; Object referent = get(); Object other = ((HashableWeakReference) obj).get(); if (referent == null) return other == null; return referent.equals(other); } @Override public int hashCode() { return this.hashCode; } @Override public String toString() { Object referent = get(); if (referent == null) return "[hashCode=" + this.hashCode + "] <referent was garbage collected>"; //$NON-NLS-1$ //$NON-NLS-2$ return "[hashCode=" + this.hashCode + "] " + referent.toString(); //$NON-NLS-1$ //$NON-NLS-2$ } } HashableWeakReference[] values; public int elementSize; // number of elements in the table int threshold; ReferenceQueue referenceQueue = new ReferenceQueue(); public WeakHashSet() { this(5); } public WeakHashSet(int size) { this.elementSize = 0; this.threshold = size; // size represents the expected number of elements int extraRoom = (int) (size * 1.75f); if (this.threshold == extraRoom) extraRoom++; this.values = new HashableWeakReference[extraRoom]; } /* * Adds the given object to this set. * If an object that is equals to the given object already exists, do nothing. * Returns the existing object or the new object if not found. */ public Object add(Object obj) { cleanupGarbageCollectedValues(); int valuesLength = this.values.length, index = (obj.hashCode() & 0x7FFFFFFF) % valuesLength; HashableWeakReference currentValue; while ((currentValue = this.values[index]) != null) { Object referent; if (obj.equals(referent = currentValue.get())) { return referent; } if (++index == valuesLength) { index = 0; } } this.values[index] = new HashableWeakReference(obj, this.referenceQueue); // assumes the threshold is never equal to the size of the table if (++this.elementSize > this.threshold) rehash(); return obj; } private void addValue(HashableWeakReference value) { Object obj = value.get(); if (obj == null) return; int valuesLength = this.values.length; int index = (value.hashCode & 0x7FFFFFFF) % valuesLength; HashableWeakReference currentValue; while ((currentValue = this.values[index]) != null) { if (obj.equals(currentValue.get())) { return; } if (++index == valuesLength) { index = 0; } } this.values[index] = value; // assumes the threshold is never equal to the size of the table if (++this.elementSize > this.threshold) rehash(); } private void cleanupGarbageCollectedValues() { HashableWeakReference toBeRemoved; while ((toBeRemoved = (HashableWeakReference) this.referenceQueue.poll()) != null) { int hashCode = toBeRemoved.hashCode; int valuesLength = this.values.length; int index = (hashCode & 0x7FFFFFFF) % valuesLength; HashableWeakReference currentValue; while ((currentValue = this.values[index]) != null) { if (currentValue == toBeRemoved) { // replace the value at index with the last value with the same hash int sameHash = index; int current; while ((currentValue = this.values[current = (sameHash + 1) % valuesLength]) != null && currentValue.hashCode == hashCode) sameHash = current; this.values[index] = this.values[sameHash]; this.values[sameHash] = null; this.elementSize--; break; } if (++index == valuesLength) { index = 0; } } } } public boolean contains(Object obj) { return get(obj) != null; } /* * Return the object that is in this set and that is equals to the given object. * Return null if not found. */ public Object get(Object obj) { cleanupGarbageCollectedValues(); int valuesLength = this.values.length; int index = (obj.hashCode() & 0x7FFFFFFF) % valuesLength; HashableWeakReference currentValue; while ((currentValue = this.values[index]) != null) { Object referent; if (obj.equals(referent = currentValue.get())) { return referent; } if (++index == valuesLength) { index = 0; } } return null; } private void rehash() { WeakHashSet newHashSet = new WeakHashSet(this.elementSize * 2); // double the number of expected elements newHashSet.referenceQueue = this.referenceQueue; HashableWeakReference currentValue; for (int i = 0, length = this.values.length; i < length; i++) if ((currentValue = this.values[i]) != null) newHashSet.addValue(currentValue); this.values = newHashSet.values; this.threshold = newHashSet.threshold; this.elementSize = newHashSet.elementSize; } /* * Removes the object that is in this set and that is equals to the given object. * Return the object that was in the set, or null if not found. */ public Object remove(Object obj) { cleanupGarbageCollectedValues(); int valuesLength = this.values.length; int index = (obj.hashCode() & 0x7FFFFFFF) % valuesLength; HashableWeakReference currentValue; while ((currentValue = this.values[index]) != null) { Object referent; if (obj.equals(referent = currentValue.get())) { this.elementSize--; this.values[index] = null; rehash(); return referent; } if (++index == valuesLength) { index = 0; } } return null; } public int size() { return this.elementSize; } @Override public String toString() { StringBuffer buffer = new StringBuffer("{"); //$NON-NLS-1$ for (int i = 0, length = this.values.length; i < length; i++) { HashableWeakReference value = this.values[i]; if (value != null) { Object ref = value.get(); if (ref != null) { buffer.append(ref.toString()); buffer.append(", "); //$NON-NLS-1$ } } } buffer.append("}"); //$NON-NLS-1$ return buffer.toString(); } }