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Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. Guava has only one code dependency - javax.annotation, per the JSR-305 spec.

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/*
 * Copyright (C) 2013 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.collect.CollectPreconditions.checkEntryNotNull;

import com.google.common.annotations.GwtIncompatible;

import javax.annotation.Nullable;

/**
 * Implementation of {@code Map.Entry} for {@link ImmutableMap} that adds extra methods to traverse
 * hash buckets for the key and the value. This allows reuse in {@link RegularImmutableMap} and
 * {@link RegularImmutableBiMap}, which don't have to recopy the entries created by their
 * {@code Builder} implementations.
 *
 * @author Louis Wasserman
 */
@GwtIncompatible("unnecessary")
abstract class ImmutableMapEntry extends ImmutableEntry {
  /**
   * Creates an {@code ImmutableMapEntry} array to hold parameterized entries. The
   * result must never be upcast back to ImmutableMapEntry[] (or Object[], etc.), or
   * allowed to escape the class.
   */
  @SuppressWarnings("unchecked") // Safe as long as the javadocs are followed
  static  ImmutableMapEntry[] createEntryArray(int size) {
    return new ImmutableMapEntry[size];
  }
  
  ImmutableMapEntry(K key, V value) {
    super(key, value);
    checkEntryNotNull(key, value);
  }

  ImmutableMapEntry(ImmutableMapEntry contents) {
    super(contents.getKey(), contents.getValue());
    // null check would be redundant
  }

  @Nullable
  abstract ImmutableMapEntry getNextInKeyBucket();

  @Nullable
  abstract ImmutableMapEntry getNextInValueBucket();

  static final class TerminalEntry extends ImmutableMapEntry {
    TerminalEntry(ImmutableMapEntry contents) {
      super(contents);
    }

    TerminalEntry(K key, V value) {
      super(key, value);
    }

    @Override
    @Nullable
    ImmutableMapEntry getNextInKeyBucket() {
      return null;
    }

    @Override
    @Nullable
    ImmutableMapEntry getNextInValueBucket() {
      return null;
    }
  }
}