001/*
002 * Copyright (C) 2008 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.base;
016
017import static com.google.common.base.NullnessCasts.uncheckedCastNullableTToT;
018import static com.google.common.base.Preconditions.checkNotNull;
019
020import com.google.common.annotations.GwtCompatible;
021import com.google.errorprone.annotations.CheckReturnValue;
022import com.google.errorprone.annotations.ForOverride;
023import com.google.errorprone.annotations.InlineMe;
024import com.google.errorprone.annotations.concurrent.LazyInit;
025import com.google.j2objc.annotations.RetainedWith;
026import java.io.Serializable;
027import java.util.Iterator;
028import javax.annotation.CheckForNull;
029
030/**
031 * A function from {@code A} to {@code B} with an associated <i>reverse</i> function from {@code B}
032 * to {@code A}; used for converting back and forth between <i>different representations of the same
033 * information</i>.
034 *
035 * <h3>Invertibility</h3>
036 *
037 * <p>The reverse operation <b>may</b> be a strict <i>inverse</i> (meaning that {@code
038 * converter.reverse().convert(converter.convert(a)).equals(a)} is always true). However, it is very
039 * common (perhaps <i>more</i> common) for round-trip conversion to be <i>lossy</i>. Consider an
040 * example round-trip using {@link com.google.common.primitives.Doubles#stringConverter}:
041 *
042 * <ol>
043 *   <li>{@code stringConverter().convert("1.00")} returns the {@code Double} value {@code 1.0}
044 *   <li>{@code stringConverter().reverse().convert(1.0)} returns the string {@code "1.0"} --
045 *       <i>not</i> the same string ({@code "1.00"}) we started with
046 * </ol>
047 *
048 * <p>Note that it should still be the case that the round-tripped and original objects are
049 * <i>similar</i>.
050 *
051 * <h3>Nullability</h3>
052 *
053 * <p>A converter always converts {@code null} to {@code null} and non-null references to non-null
054 * references. It would not make sense to consider {@code null} and a non-null reference to be
055 * "different representations of the same information", since one is distinguishable from
056 * <i>missing</i> information and the other is not. The {@link #convert} method handles this null
057 * behavior for all converters; implementations of {@link #doForward} and {@link #doBackward} are
058 * guaranteed to never be passed {@code null}, and must never return {@code null}.
059 *
060 * <h3>Common ways to use</h3>
061 *
062 * <p>Getting a converter:
063 *
064 * <ul>
065 *   <li>Use a provided converter implementation, such as {@link Enums#stringConverter}, {@link
066 *       com.google.common.primitives.Ints#stringConverter Ints.stringConverter} or the {@linkplain
067 *       #reverse reverse} views of these.
068 *   <li>Convert between specific preset values using {@link
069 *       com.google.common.collect.Maps#asConverter Maps.asConverter}. For example, use this to
070 *       create a "fake" converter for a unit test. It is unnecessary (and confusing) to <i>mock</i>
071 *       the {@code Converter} type using a mocking framework.
072 *   <li>Extend this class and implement its {@link #doForward} and {@link #doBackward} methods.
073 *   <li><b>Java 8+ users:</b> you may prefer to pass two lambda expressions or method references to
074 *       the {@link #from from} factory method.
075 * </ul>
076 *
077 * <p>Using a converter:
078 *
079 * <ul>
080 *   <li>Convert one instance in the "forward" direction using {@code converter.convert(a)}.
081 *   <li>Convert multiple instances "forward" using {@code converter.convertAll(as)}.
082 *   <li>Convert in the "backward" direction using {@code converter.reverse().convert(b)} or {@code
083 *       converter.reverse().convertAll(bs)}.
084 *   <li>Use {@code converter} or {@code converter.reverse()} anywhere a {@link
085 *       java.util.function.Function} is accepted (for example {@link java.util.stream.Stream#map
086 *       Stream.map}).
087 *   <li><b>Do not</b> call {@link #doForward} or {@link #doBackward} directly; these exist only to
088 *       be overridden.
089 * </ul>
090 *
091 * <h3>Example</h3>
092 *
093 * <pre>
094 *   return new Converter&lt;Integer, String&gt;() {
095 *     protected String doForward(Integer i) {
096 *       return Integer.toHexString(i);
097 *     }
098 *
099 *     protected Integer doBackward(String s) {
100 *       return parseUnsignedInt(s, 16);
101 *     }
102 *   };</pre>
103 *
104 * <p>An alternative using Java 8:
105 *
106 * <pre>{@code
107 * return Converter.from(
108 *     Integer::toHexString,
109 *     s -> parseUnsignedInt(s, 16));
110 * }</pre>
111 *
112 * @author Mike Ward
113 * @author Kurt Alfred Kluever
114 * @author Gregory Kick
115 * @since 16.0
116 */
117@GwtCompatible
118/*
119 * 1. The type parameter is <T> rather than <T extends @Nullable> so that we can use T in the
120 * doForward and doBackward methods to indicate that the parameter cannot be null. (We also take
121 * advantage of that for convertAll, as discussed on that method.)
122 *
123 * 2. The supertype of this class could be `Function<@Nullable A, @Nullable B>`, since
124 * Converter.apply (like Converter.convert) is capable of accepting null inputs. However, a
125 * supertype of `Function<A, B>` turns out to be massively more useful to callers in practice: They
126 * want their output to be non-null in operations like `stream.map(myConverter)`, and we can
127 * guarantee that as long as we also require the input type to be non-null[*] (which is a
128 * requirement that existing callers already fulfill).
129 *
130 * Disclaimer: Part of the reason that callers are so well adapted to `Function<A, B>` may be that
131 * that is how the signature looked even prior to this comment! So naturally any change can break
132 * existing users, but it can't *fix* existing users because any users who needed
133 * `Function<@Nullable A, @Nullable B>` already had to find a workaround. Still, there is a *ton* of
134 * fallout from trying to switch. I would be shocked if the switch would offer benefits to anywhere
135 * near enough users to justify the costs.
136 *
137 * Fortunately, if anyone does want to use a Converter as a `Function<@Nullable A, @Nullable B>`,
138 * it's easy to get one: `converter::convert`.
139 *
140 * [*] In annotating this class, we're ignoring LegacyConverter.
141 */
142public abstract class Converter<A, B> implements Function<A, B> {
143  private final boolean handleNullAutomatically;
144
145  // We lazily cache the reverse view to avoid allocating on every call to reverse().
146  @LazyInit @RetainedWith @CheckForNull private transient Converter<B, A> reverse;
147
148  /** Constructor for use by subclasses. */
149  protected Converter() {
150    this(true);
151  }
152
153  /** Constructor used only by {@code LegacyConverter} to suspend automatic null-handling. */
154  Converter(boolean handleNullAutomatically) {
155    this.handleNullAutomatically = handleNullAutomatically;
156  }
157
158  // SPI methods (what subclasses must implement)
159
160  /**
161   * Returns a representation of {@code a} as an instance of type {@code B}. If {@code a} cannot be
162   * converted, an unchecked exception (such as {@link IllegalArgumentException}) should be thrown.
163   *
164   * @param a the instance to convert; will never be null
165   * @return the converted instance; <b>must not</b> be null
166   */
167  @ForOverride
168  protected abstract B doForward(A a);
169
170  /**
171   * Returns a representation of {@code b} as an instance of type {@code A}. If {@code b} cannot be
172   * converted, an unchecked exception (such as {@link IllegalArgumentException}) should be thrown.
173   *
174   * @param b the instance to convert; will never be null
175   * @return the converted instance; <b>must not</b> be null
176   * @throws UnsupportedOperationException if backward conversion is not implemented; this should be
177   *     very rare. Note that if backward conversion is not only unimplemented but
178   *     unimplement<i>able</i> (for example, consider a {@code Converter<Chicken, ChickenNugget>}),
179   *     then this is not logically a {@code Converter} at all, and should just implement {@link
180   *     Function}.
181   */
182  @ForOverride
183  protected abstract A doBackward(B b);
184
185  // API (consumer-side) methods
186
187  /**
188   * Returns a representation of {@code a} as an instance of type {@code B}.
189   *
190   * @return the converted value; is null <i>if and only if</i> {@code a} is null
191   */
192  @CheckForNull
193  public final B convert(@CheckForNull A a) {
194    return correctedDoForward(a);
195  }
196
197  @CheckForNull
198  B correctedDoForward(@CheckForNull A a) {
199    if (handleNullAutomatically) {
200      // TODO(kevinb): we shouldn't be checking for a null result at runtime. Assert?
201      return a == null ? null : checkNotNull(doForward(a));
202    } else {
203      return unsafeDoForward(a);
204    }
205  }
206
207  @CheckForNull
208  A correctedDoBackward(@CheckForNull B b) {
209    if (handleNullAutomatically) {
210      // TODO(kevinb): we shouldn't be checking for a null result at runtime. Assert?
211      return b == null ? null : checkNotNull(doBackward(b));
212    } else {
213      return unsafeDoBackward(b);
214    }
215  }
216
217  /*
218   * LegacyConverter violates the contract of Converter by allowing its doForward and doBackward
219   * methods to accept null. We could avoid having unchecked casts in Converter.java itself if we
220   * could perform a cast to LegacyConverter, but we can't because it's an internal-only class.
221   *
222   * TODO(cpovirk): So make it part of the open-source build, albeit package-private there?
223   *
224   * So we use uncheckedCastNullableTToT here. This is a weird usage of that method: The method is
225   * documented as being for use with type parameters that have parametric nullness. But Converter's
226   * type parameters do not. Still, we use it here so that we can suppress a warning at a smaller
227   * level than the whole method but without performing a runtime null check. That way, we can still
228   * pass null inputs to LegacyConverter, and it can violate the contract of Converter.
229   *
230   * TODO(cpovirk): Could this be simplified if we modified implementations of LegacyConverter to
231   * override methods (probably called "unsafeDoForward" and "unsafeDoBackward") with the same
232   * signatures as the methods below, rather than overriding the same doForward and doBackward
233   * methods as implementations of normal converters do?
234   *
235   * But no matter what we do, it's worth remembering that the resulting code is going to be unsound
236   * in the presence of LegacyConverter, at least in the case of users who view the converter as a
237   * Function<A, B> or who call convertAll (and for any checkers that apply @PolyNull-like semantics
238   * to Converter.convert). So maybe we don't want to think too hard about how to prevent our
239   * checkers from issuing errors related to LegacyConverter, since it turns out that
240   * LegacyConverter does violate the assumptions we make elsewhere.
241   */
242
243  @CheckForNull
244  private B unsafeDoForward(@CheckForNull A a) {
245    return doForward(uncheckedCastNullableTToT(a));
246  }
247
248  @CheckForNull
249  private A unsafeDoBackward(@CheckForNull B b) {
250    return doBackward(uncheckedCastNullableTToT(b));
251  }
252
253  /**
254   * Returns an iterable that applies {@code convert} to each element of {@code fromIterable}. The
255   * conversion is done lazily.
256   *
257   * <p>The returned iterable's iterator supports {@code remove()} if the input iterator does. After
258   * a successful {@code remove()} call, {@code fromIterable} no longer contains the corresponding
259   * element.
260   */
261  /*
262   * Just as Converter could implement `Function<@Nullable A, @Nullable B>` instead of `Function<A,
263   * B>`, convertAll could accept and return iterables with nullable element types. In both cases,
264   * we've chosen to instead use a signature that benefits existing users -- and is still safe.
265   *
266   * For convertAll, I haven't looked as closely at *how* much existing users benefit, so we should
267   * keep an eye out for problems that new users encounter. Note also that convertAll could support
268   * both use cases by using @PolyNull. (By contrast, we can't use @PolyNull for our superinterface
269   * (`implements Function<@PolyNull A, @PolyNull B>`), at least as far as I know.)
270   */
271  public Iterable<B> convertAll(Iterable<? extends A> fromIterable) {
272    checkNotNull(fromIterable, "fromIterable");
273    return new Iterable<B>() {
274      @Override
275      public Iterator<B> iterator() {
276        return new Iterator<B>() {
277          private final Iterator<? extends A> fromIterator = fromIterable.iterator();
278
279          @Override
280          public boolean hasNext() {
281            return fromIterator.hasNext();
282          }
283
284          @Override
285          public B next() {
286            return convert(fromIterator.next());
287          }
288
289          @Override
290          public void remove() {
291            fromIterator.remove();
292          }
293        };
294      }
295    };
296  }
297
298  /**
299   * Returns the reversed view of this converter, which converts {@code this.convert(a)} back to a
300   * value roughly equivalent to {@code a}.
301   *
302   * <p>The returned converter is serializable if {@code this} converter is.
303   *
304   * <p><b>Note:</b> you should not override this method. It is non-final for legacy reasons.
305   */
306  @CheckReturnValue
307  public Converter<B, A> reverse() {
308    Converter<B, A> result = reverse;
309    return (result == null) ? reverse = new ReverseConverter<>(this) : result;
310  }
311
312  private static final class ReverseConverter<A, B> extends Converter<B, A>
313      implements Serializable {
314    final Converter<A, B> original;
315
316    ReverseConverter(Converter<A, B> original) {
317      this.original = original;
318    }
319
320    /*
321     * These gymnastics are a little confusing. Basically this class has neither legacy nor
322     * non-legacy behavior; it just needs to let the behavior of the backing converter shine
323     * through. So, we override the correctedDo* methods, after which the do* methods should never
324     * be reached.
325     */
326
327    @Override
328    protected A doForward(B b) {
329      throw new AssertionError();
330    }
331
332    @Override
333    protected B doBackward(A a) {
334      throw new AssertionError();
335    }
336
337    @Override
338    @CheckForNull
339    A correctedDoForward(@CheckForNull B b) {
340      return original.correctedDoBackward(b);
341    }
342
343    @Override
344    @CheckForNull
345    B correctedDoBackward(@CheckForNull A a) {
346      return original.correctedDoForward(a);
347    }
348
349    @Override
350    public Converter<A, B> reverse() {
351      return original;
352    }
353
354    @Override
355    public boolean equals(@CheckForNull Object object) {
356      if (object instanceof ReverseConverter) {
357        ReverseConverter<?, ?> that = (ReverseConverter<?, ?>) object;
358        return this.original.equals(that.original);
359      }
360      return false;
361    }
362
363    @Override
364    public int hashCode() {
365      return ~original.hashCode();
366    }
367
368    @Override
369    public String toString() {
370      return original + ".reverse()";
371    }
372
373    private static final long serialVersionUID = 0L;
374  }
375
376  /**
377   * Returns a converter whose {@code convert} method applies {@code secondConverter} to the result
378   * of this converter. Its {@code reverse} method applies the converters in reverse order.
379   *
380   * <p>The returned converter is serializable if {@code this} converter and {@code secondConverter}
381   * are.
382   */
383  public final <C> Converter<A, C> andThen(Converter<B, C> secondConverter) {
384    return doAndThen(secondConverter);
385  }
386
387  /** Package-private non-final implementation of andThen() so only we can override it. */
388  <C> Converter<A, C> doAndThen(Converter<B, C> secondConverter) {
389    return new ConverterComposition<>(this, checkNotNull(secondConverter));
390  }
391
392  private static final class ConverterComposition<A, B, C> extends Converter<A, C>
393      implements Serializable {
394    final Converter<A, B> first;
395    final Converter<B, C> second;
396
397    ConverterComposition(Converter<A, B> first, Converter<B, C> second) {
398      this.first = first;
399      this.second = second;
400    }
401
402    /*
403     * These gymnastics are a little confusing. Basically this class has neither legacy nor
404     * non-legacy behavior; it just needs to let the behaviors of the backing converters shine
405     * through (which might even differ from each other!). So, we override the correctedDo* methods,
406     * after which the do* methods should never be reached.
407     */
408
409    @Override
410    protected C doForward(A a) {
411      throw new AssertionError();
412    }
413
414    @Override
415    protected A doBackward(C c) {
416      throw new AssertionError();
417    }
418
419    @Override
420    @CheckForNull
421    C correctedDoForward(@CheckForNull A a) {
422      return second.correctedDoForward(first.correctedDoForward(a));
423    }
424
425    @Override
426    @CheckForNull
427    A correctedDoBackward(@CheckForNull C c) {
428      return first.correctedDoBackward(second.correctedDoBackward(c));
429    }
430
431    @Override
432    public boolean equals(@CheckForNull Object object) {
433      if (object instanceof ConverterComposition) {
434        ConverterComposition<?, ?, ?> that = (ConverterComposition<?, ?, ?>) object;
435        return this.first.equals(that.first) && this.second.equals(that.second);
436      }
437      return false;
438    }
439
440    @Override
441    public int hashCode() {
442      return 31 * first.hashCode() + second.hashCode();
443    }
444
445    @Override
446    public String toString() {
447      return first + ".andThen(" + second + ")";
448    }
449
450    private static final long serialVersionUID = 0L;
451  }
452
453  /**
454   * @deprecated Provided to satisfy the {@code Function} interface; use {@link #convert} instead.
455   */
456  @Deprecated
457  @Override
458  @InlineMe(replacement = "this.convert(a)")
459  public final B apply(A a) {
460    /*
461     * Given that we declare this method as accepting and returning non-nullable values (because we
462     * implement Function<A, B>, as discussed in a class-level comment), it would make some sense to
463     * perform runtime null checks on the input and output. (That would also make NullPointerTester
464     * happy!) However, since we didn't do that for many years, we're not about to start now.
465     * (Runtime checks could be particularly bad for users of LegacyConverter.)
466     *
467     * Luckily, our nullness checker is smart enough to realize that `convert` has @PolyNull-like
468     * behavior, so it knows that `convert(a)` returns a non-nullable value, and we don't need to
469     * perform even a cast, much less a runtime check.
470     *
471     * All that said, don't forget that everyone should call converter.convert() instead of
472     * converter.apply(), anyway. If clients use only converter.convert(), then their nullness
473     * checkers are unlikely to ever look at the annotations on this declaration.
474     *
475     * Historical note: At one point, we'd declared this method as accepting and returning nullable
476     * values. For details on that, see earlier revisions of this file.
477     */
478    return convert(a);
479  }
480
481  /**
482   * Indicates whether another object is equal to this converter.
483   *
484   * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}.
485   * However, an implementation may also choose to return {@code true} whenever {@code object} is a
486   * {@link Converter} that it considers <i>interchangeable</i> with this one. "Interchangeable"
487   * <i>typically</i> means that {@code Objects.equal(this.convert(a), that.convert(a))} is true for
488   * all {@code a} of type {@code A} (and similarly for {@code reverse}). Note that a {@code false}
489   * result from this method does not imply that the converters are known <i>not</i> to be
490   * interchangeable.
491   */
492  @Override
493  public boolean equals(@CheckForNull Object object) {
494    return super.equals(object);
495  }
496
497  // Static converters
498
499  /**
500   * Returns a converter based on separate forward and backward functions. This is useful if the
501   * function instances already exist, or so that you can supply lambda expressions. If those
502   * circumstances don't apply, you probably don't need to use this; subclass {@code Converter} and
503   * implement its {@link #doForward} and {@link #doBackward} methods directly.
504   *
505   * <p>These functions will never be passed {@code null} and must not under any circumstances
506   * return {@code null}. If a value cannot be converted, the function should throw an unchecked
507   * exception (typically, but not necessarily, {@link IllegalArgumentException}).
508   *
509   * <p>The returned converter is serializable if both provided functions are.
510   *
511   * @since 17.0
512   */
513  public static <A, B> Converter<A, B> from(
514      Function<? super A, ? extends B> forwardFunction,
515      Function<? super B, ? extends A> backwardFunction) {
516    return new FunctionBasedConverter<>(forwardFunction, backwardFunction);
517  }
518
519  private static final class FunctionBasedConverter<A, B> extends Converter<A, B>
520      implements Serializable {
521    private final Function<? super A, ? extends B> forwardFunction;
522    private final Function<? super B, ? extends A> backwardFunction;
523
524    private FunctionBasedConverter(
525        Function<? super A, ? extends B> forwardFunction,
526        Function<? super B, ? extends A> backwardFunction) {
527      this.forwardFunction = checkNotNull(forwardFunction);
528      this.backwardFunction = checkNotNull(backwardFunction);
529    }
530
531    @Override
532    protected B doForward(A a) {
533      return forwardFunction.apply(a);
534    }
535
536    @Override
537    protected A doBackward(B b) {
538      return backwardFunction.apply(b);
539    }
540
541    @Override
542    public boolean equals(@CheckForNull Object object) {
543      if (object instanceof FunctionBasedConverter) {
544        FunctionBasedConverter<?, ?> that = (FunctionBasedConverter<?, ?>) object;
545        return this.forwardFunction.equals(that.forwardFunction)
546            && this.backwardFunction.equals(that.backwardFunction);
547      }
548      return false;
549    }
550
551    @Override
552    public int hashCode() {
553      return forwardFunction.hashCode() * 31 + backwardFunction.hashCode();
554    }
555
556    @Override
557    public String toString() {
558      return "Converter.from(" + forwardFunction + ", " + backwardFunction + ")";
559    }
560  }
561
562  /** Returns a serializable converter that always converts or reverses an object to itself. */
563  @SuppressWarnings("unchecked") // implementation is "fully variant"
564  public static <T> Converter<T, T> identity() {
565    return (IdentityConverter<T>) IdentityConverter.INSTANCE;
566  }
567
568  /**
569   * A converter that always converts or reverses an object to itself. Note that T is now a
570   * "pass-through type".
571   */
572  private static final class IdentityConverter<T> extends Converter<T, T> implements Serializable {
573    static final Converter<?, ?> INSTANCE = new IdentityConverter<>();
574
575    @Override
576    protected T doForward(T t) {
577      return t;
578    }
579
580    @Override
581    protected T doBackward(T t) {
582      return t;
583    }
584
585    @Override
586    public IdentityConverter<T> reverse() {
587      return this;
588    }
589
590    @Override
591    <S> Converter<T, S> doAndThen(Converter<T, S> otherConverter) {
592      return checkNotNull(otherConverter, "otherConverter");
593    }
594
595    /*
596     * We *could* override convertAll() to return its input, but it's a rather pointless
597     * optimization and opened up a weird type-safety problem.
598     */
599
600    @Override
601    public String toString() {
602      return "Converter.identity()";
603    }
604
605    private Object readResolve() {
606      return INSTANCE;
607    }
608
609    private static final long serialVersionUID = 0L;
610  }
611}