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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) 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 java.util.Map;

import javax.annotation.Nullable;

/**
 * A map, each entry of which maps a {@link TypeToken} to an instance of that type.
 * In addition to implementing {@code Map}, the additional type-safe operations
 * {@link #putInstance} and {@link #getInstance} are available.
 *
 * Generally, implementations don't support {@link #put} and {@link #putAll}
 * because there is no way to check an object at runtime to be an instance of a
 * {@link TypeToken}. Instead, caller should use the type safe {@link #putInstance}.
 * 
 * 
Also, if caller suppresses unchecked warnings and passes in an {@code Iterable}
 * for type {@code Iterable}, the map won't be able to detect and throw type error.
 *
 * 
Like any other {@code Map}, this map may contain entries
 * for primitive types, and a primitive type and its corresponding wrapper type
 * may map to different values.
 *
 * @param  the common supertype that all entries must share; often this is
 *     simply {@link Object}
 *
 * @author Ben Yu
 * @since 13.0
 */
@Beta
public interface TypeToInstanceMap extends Map, B>  {

  /**
   * Returns the value the specified class is mapped to, or {@code null} if no
   * entry for this class is present. This will only return a value that was
   * bound to this specific class, not a value that may have been bound to a
   * subtype.
   * 
   * 
{@code getInstance(Foo.class)} is equivalent to
   * {@code getInstance(TypeToken.of(Foo.class))}.
   */
  @Nullable
   T getInstance(Class type);

  /**
   * Maps the specified class to the specified value. Does not associate
   * this value with any of the class's supertypes.
   * 
   * {@code putInstance(Foo.class, foo)} is equivalent to
   * {@code putInstance(TypeToken.of(Foo.class), foo)}.
   *
   * @return the value previously associated with this class (possibly {@code null}),
   *         or {@code null} if there was no previous entry.
   */
  @Nullable
   T putInstance(Class type, @Nullable T value);

  /**
   * Returns the value the specified type is mapped to, or {@code null} if no
   * entry for this type is present. This will only return a value that was
   * bound to this specific type, not a value that may have been bound to a subtype.
   */
  @Nullable
   T getInstance(TypeToken type);

  /**
   * Maps the specified type to the specified value. Does not associate
   * this value with any of the type's supertypes.
   *
   * @return the value previously associated with this type (possibly {@code null}),
   *         or {@code null} if there was no previous entry.
   */
  @Nullable
   T putInstance(TypeToken type, @Nullable T value);
}