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      1 /*
      2  * Copyright (C) 2014 Google Inc.
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  * http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 package dagger.producers;
     17 
     18 import com.google.common.base.Objects;
     19 import dagger.internal.Beta;
     20 import java.util.concurrent.ExecutionException;
     21 import javax.annotation.Nullable;
     22 
     23 import static com.google.common.base.Preconditions.checkNotNull;
     24 
     25 /**
     26  * An interface that represents the result of a {@linkplain Producer production} of type {@code T},
     27  * or an exception that was thrown during that production. For any type {@code T} that can be
     28  * injected, you can also inject {@code Produced<T>}, which enables handling of any exceptions that
     29  * were thrown during the production of {@code T}.
     30  *
     31  * <p>For example: <pre><code>
     32  *   {@literal @}Produces Html getResponse(
     33  *       UserInfo criticalInfo, {@literal Produced<ExtraInfo>} noncriticalInfo) {
     34  *     try {
     35  *       return new Html(criticalInfo, noncriticalInfo.get());
     36  *     } catch (ExecutionException e) {
     37  *       logger.warning(e, "Noncritical info");
     38  *       return new Html(criticalInfo);
     39  *     }
     40  *   }
     41  * </code></pre>
     42  *
     43  * @author Jesse Beder
     44  */
     45 @Beta
     46 public abstract class Produced<T> {
     47   /**
     48    * Returns the result of a production.
     49    *
     50    * @throws ExecutionException if the production threw an exception
     51    */
     52   public abstract T get() throws ExecutionException;
     53 
     54   /**
     55    * Two {@code Produced} objects compare equal if both are successful with equal values, or both
     56    * are failed with equal exceptions.
     57    */
     58   @Override
     59   public abstract boolean equals(Object o);
     60 
     61   /** Returns an appropriate hash code to match {@link #equals). */
     62   @Override
     63   public abstract int hashCode();
     64 
     65   /** Returns a successful {@code Produced}, whose {@link #get} will return the given value. */
     66   public static <T> Produced<T> successful(@Nullable T value) {
     67     return new Successful<T>(value);
     68   }
     69 
     70   /**
     71    * Returns a failed {@code Produced}, whose {@link #get} will throw an
     72    * {@code ExecutionException} with the given cause.
     73    */
     74   public static <T> Produced<T> failed(Throwable throwable) {
     75     return new Failed<T>(checkNotNull(throwable));
     76   }
     77 
     78   private static final class Successful<T> extends Produced<T> {
     79     @Nullable private final T value;
     80 
     81     private Successful(@Nullable T value) {
     82       this.value = value;
     83     }
     84 
     85     @Override public T get() {
     86       return value;
     87     }
     88 
     89     @Override public boolean equals(Object o) {
     90       if (o == this) {
     91         return true;
     92       } else if (o instanceof Successful) {
     93         Successful<?> that = (Successful<?>) o;
     94         return Objects.equal(this.value, that.value);
     95       } else {
     96         return false;
     97       }
     98     }
     99 
    100     @Override public int hashCode() {
    101       return value == null ? 0 : value.hashCode();
    102     }
    103   }
    104 
    105   private static final class Failed<T> extends Produced<T> {
    106     private final Throwable throwable;
    107 
    108     private Failed(Throwable throwable) {
    109       this.throwable = checkNotNull(throwable);
    110     }
    111 
    112     @Override public T get() throws ExecutionException {
    113       throw new ExecutionException(throwable);
    114     }
    115 
    116     @Override public boolean equals(Object o) {
    117       if (o == this) {
    118         return true;
    119       } else if (o instanceof Failed) {
    120         Failed<?> that = (Failed<?>) o;
    121         return this.throwable.equals(that.throwable);
    122       } else {
    123         return false;
    124       }
    125     }
    126 
    127     @Override public int hashCode() {
    128       return throwable.hashCode();
    129     }
    130   }
    131 
    132   private Produced() {}
    133 }
    134