guava/src/com/google/common/reflect/ImmutableTypeToInstanceMap.java

/*
 * Copyright (C) 2012 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.reflect;

import com.google.common.annotations.Beta;
import com.google.common.collect.ForwardingMap;
import com.google.common.collect.ImmutableMap;

import java.util.Map;

/**
 * A type-to-instance map backed by an {@link ImmutableMap}. See also {@link
 * MutableTypeToInstanceMap}.
 *
 * @author Ben Yu
 * @since 13.0
 */
@Beta
public final class ImmutableTypeToInstanceMap<B> extends ForwardingMap<TypeToken<? extends B>, B>
    implements TypeToInstanceMap<B> {

  /** Returns an empty type to instance map. */
  public static <B> ImmutableTypeToInstanceMap<B> of() {
    return new ImmutableTypeToInstanceMap<B>(ImmutableMap.<TypeToken<? extends B>, B>of());
  }

  /** Returns a new builder. */
  public static <B> Builder<B> builder() {
    return new Builder<B>();
  }

  /**
   * A builder for creating immutable type-to-instance maps. Example:
   * <pre>   {@code
   *
   *   static final ImmutableTypeToInstanceMap<Handler<?>> HANDLERS =
   *       ImmutableTypeToInstanceMap.<Handler<?>>builder()
   *           .put(new TypeToken<Handler<Foo>>() {}, new FooHandler())
   *           .put(new TypeToken<Handler<Bar>>() {}, new SubBarHandler())
   *           .build();}</pre>
   *
   * <p>After invoking {@link #build()} it is still possible to add more entries
   * and build again. Thus each map generated by this builder will be a superset
   * of any map generated before it.
   *
   * @since 13.0
   */
  @Beta
  public static final class Builder<B> {
    private final ImmutableMap.Builder<TypeToken<? extends B>, B> mapBuilder
        = ImmutableMap.builder();

    private Builder() {}

    /**
     * Associates {@code key} with {@code value} in the built map. Duplicate
     * keys are not allowed, and will cause {@link #build} to fail.
     */
    public <T extends B> Builder<B> put(Class<T> key, T value) {
      mapBuilder.put(TypeToken.of(key), value);
      return this;
    }

    /**
     * Associates {@code key} with {@code value} in the built map. Duplicate
     * keys are not allowed, and will cause {@link #build} to fail.
     */
    public <T extends B> Builder<B> put(TypeToken<T> key, T value) {
      mapBuilder.put(key.rejectTypeVariables(), value);
      return this;
    }

    /**
     * Returns a new immutable type-to-instance map containing the entries
     * provided to this builder.
     *
     * @throws IllegalArgumentException if duplicate keys were added
     */
    public ImmutableTypeToInstanceMap<B> build() {
      return new ImmutableTypeToInstanceMap<B>(mapBuilder.build());
    }
  }

  private final ImmutableMap<TypeToken<? extends B>, B> delegate;

  private ImmutableTypeToInstanceMap(ImmutableMap<TypeToken<? extends B>, B> delegate) {
    this.delegate = delegate;
  }

  @Override public <T extends B> T getInstance(TypeToken<T> type) {
    return trustedGet(type.rejectTypeVariables());
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always
   */
  @Override public <T extends B> T putInstance(TypeToken<T> type, T value) {
    throw new UnsupportedOperationException();
  }

  @Override public <T extends B> T getInstance(Class<T> type) {
    return trustedGet(TypeToken.of(type));
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always
   */
  @Override public <T extends B> T putInstance(Class<T> type, T value) {
    throw new UnsupportedOperationException();
  }

  @Override protected Map<TypeToken<? extends B>, B> delegate() {
    return delegate;
  }

  @SuppressWarnings("unchecked") // value could not get in if not a T
  private <T extends B> T trustedGet(TypeToken<T> type) {
    return (T) delegate.get(type);
  }
}