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Java example source code file (MapMaker.java)

This example Java source code file (MapMaker.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

computationexception, computingmap, concurrenthashmap, concurrentmap, function, illegalargumentexception, linkedhashmap, mapmaker, nullpointerexception, override, threading, threads, throwable, util

The MapMaker.java Java example source code

/*
 * Copyright (C) 2009 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 com.google.common.base.Function;

import java.util.LinkedHashMap;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

/**
 * MapMaker emulation. Since Javascript is single-threaded and have no references, this reduces to
 * the creation of expiring and computing maps.
 *
 * @author Charles Fry
 */
public final class MapMaker {

  // TODO(fry,kak): ConcurrentHashMap never throws a CME when mutating the map during iteration, but
  // this implementation (based on a LHM) does. This will all be replaced soon anyways, so leaving
  // it as is for now.
  private static class ComputingMap<K, V> extends LinkedHashMap
      implements ConcurrentMap<K, V> {
    private final Function<? super K, ? extends V> computer;

    ComputingMap(int initialCapacity) {
      this(null, initialCapacity);
    }

    ComputingMap(Function<? super K, ? extends V> computer, int initialCapacity) {
      super(initialCapacity, /* ignored loadFactor */ 0.75f, true);
      this.computer = computer;
    }

    @Override
    public V putIfAbsent(K key, V value) {
      if (!containsKey(key)) {
        return put(key, value);
      } else {
        return get(key);
      }
    }

    @Override
    public boolean remove(Object key, Object value) {
      if (containsKey(key) && get(key).equals(value)) {
        remove(key);
        return true;
      }
      return false;
    }

    @Override
    public boolean replace(K key, V oldValue, V newValue) {
      if (containsKey(key) && get(key).equals(oldValue)) {
        put(key, newValue);
        return true;
      }
      return false;
    }

    @Override
    public V replace(K key, V value) {
      return containsKey(key) ? put(key, value) : null;
    }

    @Override
    public V get(Object k) {
      // from CustomConcurrentHashMap
      V result = super.get(k);
      if (result == null && computer != null) {
        /*
         * This cast isn't safe, but we can rely on the fact that K is almost always passed to
         * Map.get(), and tools like IDEs and Findbugs can catch situations where this isn't the
         * case.
         *
         * The alternative is to add an overloaded method, but the chances of a user calling get()
         * instead of the new API and the risks inherent in adding a new API outweigh this little
         * hole.
         */
        @SuppressWarnings("unchecked")
        K key = (K) k;
        result = compute(key);
      }
      return result;
    }

    private V compute(K key) {
      // from MapMaker
      V value;
      try {
        value = computer.apply(key);
      } catch (Throwable t) {
        throw new ComputationException(t);
      }

      if (value == null) {
        String message = computer + " returned null for key " + key + ".";
        throw new NullPointerException(message);
      }
      put(key, value);
      return value;
    }
  }

  private int initialCapacity = 16;
  private boolean useCustomMap;

  public MapMaker() {}

  public MapMaker initialCapacity(int initialCapacity) {
    if (initialCapacity < 0) {
      throw new IllegalArgumentException();
    }
    this.initialCapacity = initialCapacity;
    return this;
  }

  public MapMaker concurrencyLevel(int concurrencyLevel) {
    if (concurrencyLevel < 1) {
      throw new IllegalArgumentException("GWT only supports a concurrency level of 1");
    }
    // GWT technically only supports concurrencyLevel == 1, but we silently
    // ignore other positive values.
    return this;
  }

  public <K, V> ConcurrentMap makeMap() {
    return useCustomMap
        ? new ComputingMap<K, V>(null, initialCapacity)
        : new ConcurrentHashMap<K, V>(initialCapacity);
  }

  public <K, V> ConcurrentMap makeComputingMap(Function computer) {
    return new ComputingMap<K, V>(computer, initialCapacity);
  }
}

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