001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.collect.Collections2.containsAllImpl;
020import static com.google.common.collect.Collections2.toStringImpl;
021
022import com.google.common.annotations.GwtCompatible;
023import com.google.errorprone.annotations.CanIgnoreReturnValue;
024import java.util.Collection;
025import java.util.Iterator;
026import java.util.Objects;
027import org.jspecify.annotations.Nullable;
028
029/**
030 * A collection which forwards all its method calls to another collection. Subclasses should
031 * override one or more methods to modify the behavior of the backing collection as desired per the
032 * <a href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
033 *
034 * <p><b>Warning:</b> The methods of {@code ForwardingCollection} forward <b>indiscriminately</b> to
035 * the methods of the delegate. For example, overriding {@link #add} alone <b>will not</b> change
036 * the behavior of {@link #addAll}, which can lead to unexpected behavior. In this case, you should
037 * override {@code addAll} as well, either providing your own implementation, or delegating to the
038 * provided {@code standardAddAll} method.
039 *
040 * <p><b>{@code default} method warning:</b> This class does <i>not</i> forward calls to {@code
041 * default} methods. Instead, it inherits their default implementations. When those implementations
042 * invoke methods, they invoke methods on the {@code ForwardingCollection}.
043 *
044 * <p>The {@code standard} methods are not guaranteed to be thread-safe, even when all of the
045 * methods that they depend on are thread-safe.
046 *
047 * @author Kevin Bourrillion
048 * @author Louis Wasserman
049 * @since 2.0
050 */
051@GwtCompatible
052public abstract class ForwardingCollection<E extends @Nullable Object> extends ForwardingObject
053    implements Collection<E> {
054  // TODO(lowasser): identify places where thread safety is actually lost
055
056  /** Constructor for use by subclasses. */
057  protected ForwardingCollection() {}
058
059  @Override
060  protected abstract Collection<E> delegate();
061
062  @Override
063  public Iterator<E> iterator() {
064    return delegate().iterator();
065  }
066
067  @Override
068  public int size() {
069    return delegate().size();
070  }
071
072  @CanIgnoreReturnValue
073  @Override
074  public boolean removeAll(Collection<?> collection) {
075    return delegate().removeAll(collection);
076  }
077
078  @Override
079  public boolean isEmpty() {
080    return delegate().isEmpty();
081  }
082
083  @Override
084  public boolean contains(@Nullable Object object) {
085    return delegate().contains(object);
086  }
087
088  @CanIgnoreReturnValue
089  @Override
090  public boolean add(@ParametricNullness E element) {
091    return delegate().add(element);
092  }
093
094  @CanIgnoreReturnValue
095  @Override
096  public boolean remove(@Nullable Object object) {
097    return delegate().remove(object);
098  }
099
100  @Override
101  public boolean containsAll(Collection<?> collection) {
102    return delegate().containsAll(collection);
103  }
104
105  @CanIgnoreReturnValue
106  @Override
107  public boolean addAll(Collection<? extends E> collection) {
108    return delegate().addAll(collection);
109  }
110
111  @CanIgnoreReturnValue
112  @Override
113  public boolean retainAll(Collection<?> collection) {
114    return delegate().retainAll(collection);
115  }
116
117  @Override
118  public void clear() {
119    delegate().clear();
120  }
121
122  @Override
123  public @Nullable Object[] toArray() {
124    return delegate().toArray();
125  }
126
127  @CanIgnoreReturnValue
128  @Override
129  @SuppressWarnings("nullness") // b/192354773 in our checker affects toArray declarations
130  public <T extends @Nullable Object> T[] toArray(T[] array) {
131    return delegate().toArray(array);
132  }
133
134  /**
135   * A sensible definition of {@link #contains} in terms of {@link #iterator}. If you override
136   * {@link #iterator}, you may wish to override {@link #contains} to forward to this
137   * implementation.
138   *
139   * @since 7.0
140   */
141  protected boolean standardContains(@Nullable Object object) {
142    return Iterators.contains(iterator(), object);
143  }
144
145  /**
146   * A sensible definition of {@link #containsAll} in terms of {@link #contains} . If you override
147   * {@link #contains}, you may wish to override {@link #containsAll} to forward to this
148   * implementation.
149   *
150   * @since 7.0
151   */
152  protected boolean standardContainsAll(Collection<?> collection) {
153    return containsAllImpl(this, collection);
154  }
155
156  /**
157   * A sensible definition of {@link #addAll} in terms of {@link #add}. If you override {@link
158   * #add}, you may wish to override {@link #addAll} to forward to this implementation.
159   *
160   * @since 7.0
161   */
162  protected boolean standardAddAll(Collection<? extends E> collection) {
163    return Iterators.addAll(this, collection.iterator());
164  }
165
166  /**
167   * A sensible definition of {@link #remove} in terms of {@link #iterator}, using the iterator's
168   * {@code remove} method. If you override {@link #iterator}, you may wish to override {@link
169   * #remove} to forward to this implementation.
170   *
171   * @since 7.0
172   */
173  protected boolean standardRemove(@Nullable Object object) {
174    Iterator<E> iterator = iterator();
175    while (iterator.hasNext()) {
176      if (Objects.equals(iterator.next(), object)) {
177        iterator.remove();
178        return true;
179      }
180    }
181    return false;
182  }
183
184  /**
185   * A sensible definition of {@link #removeAll} in terms of {@link #iterator}, using the iterator's
186   * {@code remove} method. If you override {@link #iterator}, you may wish to override {@link
187   * #removeAll} to forward to this implementation.
188   *
189   * @since 7.0
190   */
191  protected boolean standardRemoveAll(Collection<?> collection) {
192    return Iterators.removeAll(iterator(), collection);
193  }
194
195  /**
196   * A sensible definition of {@link #retainAll} in terms of {@link #iterator}, using the iterator's
197   * {@code remove} method. If you override {@link #iterator}, you may wish to override {@link
198   * #retainAll} to forward to this implementation.
199   *
200   * @since 7.0
201   */
202  protected boolean standardRetainAll(Collection<?> collection) {
203    return Iterators.retainAll(iterator(), collection);
204  }
205
206  /**
207   * A sensible definition of {@link #clear} in terms of {@link #iterator}, using the iterator's
208   * {@code remove} method. If you override {@link #iterator}, you may wish to override {@link
209   * #clear} to forward to this implementation.
210   *
211   * @since 7.0
212   */
213  protected void standardClear() {
214    Iterators.clear(iterator());
215  }
216
217  /**
218   * A sensible definition of {@link #isEmpty} as {@code !iterator().hasNext}. If you override
219   * {@link #isEmpty}, you may wish to override {@link #isEmpty} to forward to this implementation.
220   * Alternately, it may be more efficient to implement {@code isEmpty} as {@code size() == 0}.
221   *
222   * @since 7.0
223   */
224  protected boolean standardIsEmpty() {
225    return !iterator().hasNext();
226  }
227
228  /**
229   * A sensible definition of {@link #toString} in terms of {@link #iterator}. If you override
230   * {@link #iterator}, you may wish to override {@link #toString} to forward to this
231   * implementation.
232   *
233   * @since 7.0
234   */
235  protected String standardToString() {
236    return toStringImpl(this);
237  }
238
239  /**
240   * A sensible definition of {@link #toArray()} in terms of {@link #toArray(Object[])}. If you
241   * override {@link #toArray(Object[])}, you may wish to override {@link #toArray} to forward to
242   * this implementation.
243   *
244   * @since 7.0
245   */
246  protected @Nullable Object[] standardToArray() {
247    @Nullable Object[] newArray = new @Nullable Object[size()];
248    return toArray(newArray);
249  }
250
251  /**
252   * A sensible definition of {@link #toArray(Object[])} in terms of {@link #size} and {@link
253   * #iterator}. If you override either of these methods, you may wish to override {@link #toArray}
254   * to forward to this implementation.
255   *
256   * @since 7.0
257   */
258  protected <T extends @Nullable Object> T[] standardToArray(T[] array) {
259    return ObjectArrays.toArrayImpl(this, array);
260  }
261}