content: // Protocol Buffers - Google's data interchange format
  // Copyright 2008 Google Inc.  All rights reserved.
  // https://developers.google.com/protocol-buffers/
  //
  // Redistribution and use in source and binary forms, with or without
  // modification, are permitted provided that the following conditions are
  // met:
  //
  //     * Redistributions of source code must retain the above copyright
  // notice, this list of conditions and the following disclaimer.
  //     * Redistributions in binary form must reproduce the above
  // copyright notice, this list of conditions and the following disclaimer
  // in the documentation and/or other materials provided with the
  // distribution.
  //     * Neither the name of Google Inc. nor the names of its
  // contributors may be used to endorse or promote products derived from
  // this software without specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  package com.google.protobuf;
  
  import com.google.protobuf.Internal.ProtobufList;
  import java.util.AbstractList;
  import java.util.Collection;
  import java.util.List;
  import java.util.RandomAccess;
  
  /**
   * An abstract implementation of {@link ProtobufList} which manages mutability semantics. All mutate
   * methods must check if the list is mutable before proceeding. Subclasses must invoke {@link
   * #ensureIsMutable()} manually when overriding those methods.
   *
   * <p>This implementation assumes all subclasses are array based, supporting random access.
   */
  abstract class AbstractProtobufList<E> extends AbstractList<E> implements ProtobufList<E> {
  
    protected static final int DEFAULT_CAPACITY = 10;
  
    /** Whether or not this list is modifiable. */
    private boolean isMutable;
  
    /** Constructs a mutable list by default. */
    AbstractProtobufList() {
      isMutable = true;
    }
  
    @Override
    public boolean equals(Object o) {
      if (o == this) {
        return true;
      }
      if (!(o instanceof List)) {
        return false;
      }
      // Handle lists that do not support RandomAccess as efficiently as possible by using an iterator
      // based approach in our super class. Otherwise our index based approach will avoid those
      // allocations.
      if (!(o instanceof RandomAccess)) {
        return super.equals(o);
      }
  
      List<?> other = (List<?>) o;
      final int size = size();
      if (size != other.size()) {
        return false;
      }
      for (int i = 0; i < size; i++) {
        if (!get(i).equals(other.get(i))) {
          return false;
        }
      }
      return true;
    }
  
    @Override
    public int hashCode() {
      final int size = size();
      int hashCode = 1;
      for (int i = 0; i < size; i++) {
        hashCode = (31 * hashCode) + get(i).hashCode();
      }
      return hashCode;
    }
  
    @Override
    public boolean add(E e) {
      ensureIsMutable();
      return super.add(e);
    }
  
    @Override
    public void add(int index, E element) {
      ensureIsMutable();
      super.add(index, element);
    }
  
    @Override
    public boolean addAll(Collection<? extends E> c) {
      ensureIsMutable();
      return super.addAll(c);
    }
  
    @Override
    public boolean addAll(int index, Collection<? extends E> c) {
      ensureIsMutable();
      return super.addAll(index, c);
    }
  
    @Override
    public void clear() {
      ensureIsMutable();
      super.clear();
    }
  
    @Override
    public boolean isModifiable() {
      return isMutable;
    }
  
    @Override
    public final void makeImmutable() {
      isMutable = false;
    }
  
    @Override
    public E remove(int index) {
      ensureIsMutable();
      return super.remove(index);
    }
  
    @Override
    public boolean remove(Object o) {
      ensureIsMutable();
      return super.remove(o);
    }
  
    @Override
    public boolean removeAll(Collection<?> c) {
      ensureIsMutable();
      return super.removeAll(c);
    }
  
    @Override
    public boolean retainAll(Collection<?> c) {
      ensureIsMutable();
      return super.retainAll(c);
    }
  
    @Override
    public E set(int index, E element) {
      ensureIsMutable();
      return super.set(index, element);
    }
  
    /**
     * Throws an {@link UnsupportedOperationException} if the list is immutable. Subclasses are
     * responsible for invoking this method on mutate operations.
     */
    protected void ensureIsMutable() {
      if (!isMutable) {
        throw new UnsupportedOperationException();
      }
    }
  }
  