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.Preconditions.checkNotNull;
018
019import com.google.common.annotations.GwtCompatible;
020import com.google.errorprone.annotations.CanIgnoreReturnValue;
021import com.google.errorprone.annotations.ForOverride;
022import com.google.errorprone.annotations.concurrent.LazyInit;
023import com.google.j2objc.annotations.RetainedWith;
024import java.io.Serializable;
025import java.util.Iterator;
026import org.checkerframework.checker.nullness.qual.Nullable;
027
028/**
029 * A function from {@code A} to {@code B} with an associated <i>reverse</i> function from {@code B}
030 * to {@code A}; used for converting back and forth between <i>different representations of the same
031 * information</i>.
032 *
033 * <h3>Invertibility</h3>
034 *
035 * <p>The reverse operation <b>may</b> be a strict <i>inverse</i> (meaning that {@code
036 * converter.reverse().convert(converter.convert(a)).equals(a)} is always true). However, it is very
037 * common (perhaps <i>more</i> common) for round-trip conversion to be <i>lossy</i>. Consider an
038 * example round-trip using {@link com.google.common.primitives.Doubles#stringConverter}:
039 *
040 * <ol>
041 *   <li>{@code stringConverter().convert("1.00")} returns the {@code Double} value {@code 1.0}
042 *   <li>{@code stringConverter().reverse().convert(1.0)} returns the string {@code "1.0"} --
043 *       <i>not</i> the same string ({@code "1.00"}) we started with
044 * </ol>
045 *
046 * <p>Note that it should still be the case that the round-tripped and original objects are
047 * <i>similar</i>.
048 *
049 * <h3>Nullability</h3>
050 *
051 * <p>A converter always converts {@code null} to {@code null} and non-null references to non-null
052 * references. It would not make sense to consider {@code null} and a non-null reference to be
053 * "different representations of the same information", since one is distinguishable from
054 * <i>missing</i> information and the other is not. The {@link #convert} method handles this null
055 * behavior for all converters; implementations of {@link #doForward} and {@link #doBackward} are
056 * guaranteed to never be passed {@code null}, and must never return {@code null}.
057 *
058 *
059 * <h3>Common ways to use</h3>
060 *
061 * <p>Getting a converter:
062 *
063 * <ul>
064 *   <li>Use a provided converter implementation, such as {@link Enums#stringConverter}, {@link
065 *       com.google.common.primitives.Ints#stringConverter Ints.stringConverter} or the {@linkplain
066 *       #reverse reverse} views of these.
067 *   <li>Convert between specific preset values using {@link
068 *       com.google.common.collect.Maps#asConverter Maps.asConverter}. For example, use this to
069 *       create a "fake" converter for a unit test. It is unnecessary (and confusing) to <i>mock</i>
070 *       the {@code Converter} type using a mocking framework.
071 *   <li>Extend this class and implement its {@link #doForward} and {@link #doBackward} methods.
072 *   <li><b>Java 8 users:</b> you may prefer to pass two lambda expressions or method references to
073 *       the {@link #from from} factory method.
074 * </ul>
075 *
076 * <p>Using a converter:
077 *
078 * <ul>
079 *   <li>Convert one instance in the "forward" direction using {@code converter.convert(a)}.
080 *   <li>Convert multiple instances "forward" using {@code converter.convertAll(as)}.
081 *   <li>Convert in the "backward" direction using {@code converter.reverse().convert(b)} or {@code
082 *       converter.reverse().convertAll(bs)}.
083 *   <li>Use {@code converter} or {@code converter.reverse()} anywhere a {@link
084 *       java.util.function.Function} is accepted (for example {@link java.util.stream.Stream#map
085 *       Stream.map}).
086 *   <li><b>Do not</b> call {@link #doForward} or {@link #doBackward} directly; these exist only to
087 *       be overridden.
088 * </ul>
089 *
090 * <h3>Example</h3>
091 *
092 * <pre>
093 *   return new Converter&lt;Integer, String&gt;() {
094 *     protected String doForward(Integer i) {
095 *       return Integer.toHexString(i);
096 *     }
097 *
098 *     protected Integer doBackward(String s) {
099 *       return parseUnsignedInt(s, 16);
100 *     }
101 *   };</pre>
102 *
103 * <p>An alternative using Java 8:
104 *
105 * <pre>{@code
106 * return Converter.from(
107 *     Integer::toHexString,
108 *     s -> parseUnsignedInt(s, 16));
109 * }</pre>
110 *
111 * @author Mike Ward
112 * @author Kurt Alfred Kluever
113 * @author Gregory Kick
114 * @since 16.0
115 */
116@GwtCompatible
117public abstract class Converter<A, B> implements Function<A, B> {
118  private final boolean handleNullAutomatically;
119
120  // We lazily cache the reverse view to avoid allocating on every call to reverse().
121  @LazyInit @RetainedWith private transient @Nullable Converter<B, A> reverse;
122
123  /** Constructor for use by subclasses. */
124  protected Converter() {
125    this(true);
126  }
127
128  /** Constructor used only by {@code LegacyConverter} to suspend automatic null-handling. */
129  Converter(boolean handleNullAutomatically) {
130    this.handleNullAutomatically = handleNullAutomatically;
131  }
132
133  // SPI methods (what subclasses must implement)
134
135  /**
136   * Returns a representation of {@code a} as an instance of type {@code B}. If {@code a} cannot be
137   * converted, an unchecked exception (such as {@link IllegalArgumentException}) should be thrown.
138   *
139   * @param a the instance to convert; will never be null
140   * @return the converted instance; <b>must not</b> be null
141   */
142  @ForOverride
143  protected abstract B doForward(A a);
144
145  /**
146   * Returns a representation of {@code b} as an instance of type {@code A}. If {@code b} cannot be
147   * converted, an unchecked exception (such as {@link IllegalArgumentException}) should be thrown.
148   *
149   * @param b the instance to convert; will never be null
150   * @return the converted instance; <b>must not</b> be null
151   * @throws UnsupportedOperationException if backward conversion is not implemented; this should be
152   *     very rare. Note that if backward conversion is not only unimplemented but
153   *     unimplement<i>able</i> (for example, consider a {@code Converter<Chicken, ChickenNugget>}),
154   *     then this is not logically a {@code Converter} at all, and should just implement {@link
155   *     Function}.
156   */
157  @ForOverride
158  protected abstract A doBackward(B b);
159
160  // API (consumer-side) methods
161
162  /**
163   * Returns a representation of {@code a} as an instance of type {@code B}.
164   *
165   * @return the converted value; is null <i>if and only if</i> {@code a} is null
166   */
167  @CanIgnoreReturnValue
168  public final @Nullable B convert(@Nullable A a) {
169    return correctedDoForward(a);
170  }
171
172  @Nullable
173  B correctedDoForward(@Nullable A a) {
174    if (handleNullAutomatically) {
175      // TODO(kevinb): we shouldn't be checking for a null result at runtime. Assert?
176      return a == null ? null : checkNotNull(doForward(a));
177    } else {
178      return doForward(a);
179    }
180  }
181
182  @Nullable
183  A correctedDoBackward(@Nullable B b) {
184    if (handleNullAutomatically) {
185      // TODO(kevinb): we shouldn't be checking for a null result at runtime. Assert?
186      return b == null ? null : checkNotNull(doBackward(b));
187    } else {
188      return doBackward(b);
189    }
190  }
191
192  /**
193   * Returns an iterable that applies {@code convert} to each element of {@code fromIterable}. The
194   * conversion is done lazily.
195   *
196   * <p>The returned iterable's iterator supports {@code remove()} if the input iterator does. After
197   * a successful {@code remove()} call, {@code fromIterable} no longer contains the corresponding
198   * element.
199   */
200  @CanIgnoreReturnValue
201  public Iterable<B> convertAll(final Iterable<? extends A> fromIterable) {
202    checkNotNull(fromIterable, "fromIterable");
203    return new Iterable<B>() {
204      @Override
205      public Iterator<B> iterator() {
206        return new Iterator<B>() {
207          private final Iterator<? extends A> fromIterator = fromIterable.iterator();
208
209          @Override
210          public boolean hasNext() {
211            return fromIterator.hasNext();
212          }
213
214          @Override
215          public B next() {
216            return convert(fromIterator.next());
217          }
218
219          @Override
220          public void remove() {
221            fromIterator.remove();
222          }
223        };
224      }
225    };
226  }
227
228  /**
229   * Returns the reversed view of this converter, which converts {@code this.convert(a)} back to a
230   * value roughly equivalent to {@code a}.
231   *
232   * <p>The returned converter is serializable if {@code this} converter is.
233   *
234   * <p><b>Note:</b> you should not override this method. It is non-final for legacy reasons.
235   */
236  @CanIgnoreReturnValue
237  public Converter<B, A> reverse() {
238    Converter<B, A> result = reverse;
239    return (result == null) ? reverse = new ReverseConverter<>(this) : result;
240  }
241
242  private static final class ReverseConverter<A, B> extends Converter<B, A>
243      implements Serializable {
244    final Converter<A, B> original;
245
246    ReverseConverter(Converter<A, B> original) {
247      this.original = original;
248    }
249
250    /*
251     * These gymnastics are a little confusing. Basically this class has neither legacy nor
252     * non-legacy behavior; it just needs to let the behavior of the backing converter shine
253     * through. So, we override the correctedDo* methods, after which the do* methods should never
254     * be reached.
255     */
256
257    @Override
258    protected A doForward(B b) {
259      throw new AssertionError();
260    }
261
262    @Override
263    protected B doBackward(A a) {
264      throw new AssertionError();
265    }
266
267    @Override
268    @Nullable
269    A correctedDoForward(@Nullable B b) {
270      return original.correctedDoBackward(b);
271    }
272
273    @Override
274    @Nullable
275    B correctedDoBackward(@Nullable A a) {
276      return original.correctedDoForward(a);
277    }
278
279    @Override
280    public Converter<A, B> reverse() {
281      return original;
282    }
283
284    @Override
285    public boolean equals(@Nullable Object object) {
286      if (object instanceof ReverseConverter) {
287        ReverseConverter<?, ?> that = (ReverseConverter<?, ?>) object;
288        return this.original.equals(that.original);
289      }
290      return false;
291    }
292
293    @Override
294    public int hashCode() {
295      return ~original.hashCode();
296    }
297
298    @Override
299    public String toString() {
300      return original + ".reverse()";
301    }
302
303    private static final long serialVersionUID = 0L;
304  }
305
306  /**
307   * Returns a converter whose {@code convert} method applies {@code secondConverter} to the result
308   * of this converter. Its {@code reverse} method applies the converters in reverse order.
309   *
310   * <p>The returned converter is serializable if {@code this} converter and {@code secondConverter}
311   * are.
312   */
313  public final <C> Converter<A, C> andThen(Converter<B, C> secondConverter) {
314    return doAndThen(secondConverter);
315  }
316
317  /** Package-private non-final implementation of andThen() so only we can override it. */
318  <C> Converter<A, C> doAndThen(Converter<B, C> secondConverter) {
319    return new ConverterComposition<>(this, checkNotNull(secondConverter));
320  }
321
322  private static final class ConverterComposition<A, B, C> extends Converter<A, C>
323      implements Serializable {
324    final Converter<A, B> first;
325    final Converter<B, C> second;
326
327    ConverterComposition(Converter<A, B> first, Converter<B, C> second) {
328      this.first = first;
329      this.second = second;
330    }
331
332    /*
333     * These gymnastics are a little confusing. Basically this class has neither legacy nor
334     * non-legacy behavior; it just needs to let the behaviors of the backing converters shine
335     * through (which might even differ from each other!). So, we override the correctedDo* methods,
336     * after which the do* methods should never be reached.
337     */
338
339    @Override
340    protected C doForward(A a) {
341      throw new AssertionError();
342    }
343
344    @Override
345    protected A doBackward(C c) {
346      throw new AssertionError();
347    }
348
349    @Override
350    @Nullable
351    C correctedDoForward(@Nullable A a) {
352      return second.correctedDoForward(first.correctedDoForward(a));
353    }
354
355    @Override
356    @Nullable
357    A correctedDoBackward(@Nullable C c) {
358      return first.correctedDoBackward(second.correctedDoBackward(c));
359    }
360
361    @Override
362    public boolean equals(@Nullable Object object) {
363      if (object instanceof ConverterComposition) {
364        ConverterComposition<?, ?, ?> that = (ConverterComposition<?, ?, ?>) object;
365        return this.first.equals(that.first) && this.second.equals(that.second);
366      }
367      return false;
368    }
369
370    @Override
371    public int hashCode() {
372      return 31 * first.hashCode() + second.hashCode();
373    }
374
375    @Override
376    public String toString() {
377      return first + ".andThen(" + second + ")";
378    }
379
380    private static final long serialVersionUID = 0L;
381  }
382
383  /**
384   * @deprecated Provided to satisfy the {@code Function} interface; use {@link #convert} instead.
385   */
386  @Deprecated
387  @Override
388  @CanIgnoreReturnValue
389  public final @Nullable B apply(@Nullable A a) {
390    return convert(a);
391  }
392
393  /**
394   * Indicates whether another object is equal to this converter.
395   *
396   * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}.
397   * However, an implementation may also choose to return {@code true} whenever {@code object} is a
398   * {@link Converter} that it considers <i>interchangeable</i> with this one. "Interchangeable"
399   * <i>typically</i> means that {@code Objects.equal(this.convert(a), that.convert(a))} is true for
400   * all {@code a} of type {@code A} (and similarly for {@code reverse}). Note that a {@code false}
401   * result from this method does not imply that the converters are known <i>not</i> to be
402   * interchangeable.
403   */
404  @Override
405  public boolean equals(@Nullable Object object) {
406    return super.equals(object);
407  }
408
409  // Static converters
410
411  /**
412   * Returns a converter based on separate forward and backward functions. This is useful if the
413   * function instances already exist, or so that you can supply lambda expressions. If those
414   * circumstances don't apply, you probably don't need to use this; subclass {@code Converter} and
415   * implement its {@link #doForward} and {@link #doBackward} methods directly.
416   *
417   * <p>These functions will never be passed {@code null} and must not under any circumstances
418   * return {@code null}. If a value cannot be converted, the function should throw an unchecked
419   * exception (typically, but not necessarily, {@link IllegalArgumentException}).
420   *
421   * <p>The returned converter is serializable if both provided functions are.
422   *
423   * @since 17.0
424   */
425  public static <A, B> Converter<A, B> from(
426      Function<? super A, ? extends B> forwardFunction,
427      Function<? super B, ? extends A> backwardFunction) {
428    return new FunctionBasedConverter<>(forwardFunction, backwardFunction);
429  }
430
431  private static final class FunctionBasedConverter<A, B> extends Converter<A, B>
432      implements Serializable {
433    private final Function<? super A, ? extends B> forwardFunction;
434    private final Function<? super B, ? extends A> backwardFunction;
435
436    private FunctionBasedConverter(
437        Function<? super A, ? extends B> forwardFunction,
438        Function<? super B, ? extends A> backwardFunction) {
439      this.forwardFunction = checkNotNull(forwardFunction);
440      this.backwardFunction = checkNotNull(backwardFunction);
441    }
442
443    @Override
444    protected B doForward(A a) {
445      return forwardFunction.apply(a);
446    }
447
448    @Override
449    protected A doBackward(B b) {
450      return backwardFunction.apply(b);
451    }
452
453    @Override
454    public boolean equals(@Nullable Object object) {
455      if (object instanceof FunctionBasedConverter) {
456        FunctionBasedConverter<?, ?> that = (FunctionBasedConverter<?, ?>) object;
457        return this.forwardFunction.equals(that.forwardFunction)
458            && this.backwardFunction.equals(that.backwardFunction);
459      }
460      return false;
461    }
462
463    @Override
464    public int hashCode() {
465      return forwardFunction.hashCode() * 31 + backwardFunction.hashCode();
466    }
467
468    @Override
469    public String toString() {
470      return "Converter.from(" + forwardFunction + ", " + backwardFunction + ")";
471    }
472  }
473
474  /** Returns a serializable converter that always converts or reverses an object to itself. */
475  @SuppressWarnings("unchecked") // implementation is "fully variant"
476  public static <T> Converter<T, T> identity() {
477    return (IdentityConverter<T>) IdentityConverter.INSTANCE;
478  }
479
480  /**
481   * A converter that always converts or reverses an object to itself. Note that T is now a
482   * "pass-through type".
483   */
484  private static final class IdentityConverter<T> extends Converter<T, T> implements Serializable {
485    static final IdentityConverter<?> INSTANCE = new IdentityConverter<>();
486
487    @Override
488    protected T doForward(T t) {
489      return t;
490    }
491
492    @Override
493    protected T doBackward(T t) {
494      return t;
495    }
496
497    @Override
498    public IdentityConverter<T> reverse() {
499      return this;
500    }
501
502    @Override
503    <S> Converter<T, S> doAndThen(Converter<T, S> otherConverter) {
504      return checkNotNull(otherConverter, "otherConverter");
505    }
506
507    /*
508     * We *could* override convertAll() to return its input, but it's a rather pointless
509     * optimization and opened up a weird type-safety problem.
510     */
511
512    @Override
513    public String toString() {
514      return "Converter.identity()";
515    }
516
517    private Object readResolve() {
518      return INSTANCE;
519    }
520
521    private static final long serialVersionUID = 0L;
522  }
523}