| /* |
| * Copyright (C) 2008 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.base; |
| |
| import static com.google.common.base.Preconditions.checkNotNull; |
| |
| import com.google.common.annotations.GwtCompatible; |
| import com.google.errorprone.annotations.CanIgnoreReturnValue; |
| import com.google.errorprone.annotations.ForOverride; |
| import com.google.errorprone.annotations.concurrent.LazyInit; |
| import com.google.j2objc.annotations.RetainedWith; |
| import java.io.Serializable; |
| import java.util.Iterator; |
| import org.checkerframework.checker.nullness.compatqual.NullableDecl; |
| |
| /** |
| * A function from {@code A} to {@code B} with an associated <i>reverse</i> function from {@code B} |
| * to {@code A}; used for converting back and forth between <i>different representations of the same |
| * information</i>. |
| * |
| * <h3>Invertibility</h3> |
| * |
| * <p>The reverse operation <b>may</b> be a strict <i>inverse</i> (meaning that {@code |
| * converter.reverse().convert(converter.convert(a)).equals(a)} is always true). However, it is very |
| * common (perhaps <i>more</i> common) for round-trip conversion to be <i>lossy</i>. Consider an |
| * example round-trip using {@link com.google.common.primitives.Doubles#stringConverter}: |
| * |
| * <ol> |
| * <li>{@code stringConverter().convert("1.00")} returns the {@code Double} value {@code 1.0} |
| * <li>{@code stringConverter().reverse().convert(1.0)} returns the string {@code "1.0"} -- |
| * <i>not</i> the same string ({@code "1.00"}) we started with |
| * </ol> |
| * |
| * <p>Note that it should still be the case that the round-tripped and original objects are |
| * <i>similar</i>. |
| * |
| * <h3>Nullability</h3> |
| * |
| * <p>A converter always converts {@code null} to {@code null} and non-null references to non-null |
| * references. It would not make sense to consider {@code null} and a non-null reference to be |
| * "different representations of the same information", since one is distinguishable from |
| * <i>missing</i> information and the other is not. The {@link #convert} method handles this null |
| * behavior for all converters; implementations of {@link #doForward} and {@link #doBackward} are |
| * guaranteed to never be passed {@code null}, and must never return {@code null}. |
| * |
| * |
| * <h3>Common ways to use</h3> |
| * |
| * <p>Getting a converter: |
| * |
| * <ul> |
| * <li>Use a provided converter implementation, such as {@link Enums#stringConverter}, {@link |
| * com.google.common.primitives.Ints#stringConverter Ints.stringConverter} or the {@linkplain |
| * #reverse reverse} views of these. |
| * <li>Convert between specific preset values using {@link |
| * com.google.common.collect.Maps#asConverter Maps.asConverter}. For example, use this to |
| * create a "fake" converter for a unit test. It is unnecessary (and confusing) to <i>mock</i> |
| * the {@code Converter} type using a mocking framework. |
| * <li>Extend this class and implement its {@link #doForward} and {@link #doBackward} methods. |
| * <li><b>Java 8 users:</b> you may prefer to pass two lambda expressions or method references to |
| * the {@link #from from} factory method. |
| * </ul> |
| * |
| * <p>Using a converter: |
| * |
| * <ul> |
| * <li>Convert one instance in the "forward" direction using {@code converter.convert(a)}. |
| * <li>Convert multiple instances "forward" using {@code converter.convertAll(as)}. |
| * <li>Convert in the "backward" direction using {@code converter.reverse().convert(b)} or {@code |
| * converter.reverse().convertAll(bs)}. |
| * <li>Use {@code converter} or {@code converter.reverse()} anywhere a {@link |
| * java.util.function.Function} is accepted (for example {@link java.util.stream.Stream#map |
| * Stream.map}). |
| * <li><b>Do not</b> call {@link #doForward} or {@link #doBackward} directly; these exist only to |
| * be overridden. |
| * </ul> |
| * |
| * <h3>Example</h3> |
| * |
| * <pre> |
| * return new Converter<Integer, String>() { |
| * protected String doForward(Integer i) { |
| * return Integer.toHexString(i); |
| * } |
| * |
| * protected Integer doBackward(String s) { |
| * return parseUnsignedInt(s, 16); |
| * } |
| * };</pre> |
| * |
| * <p>An alternative using Java 8: |
| * |
| * <pre>{@code |
| * return Converter.from( |
| * Integer::toHexString, |
| * s -> parseUnsignedInt(s, 16)); |
| * }</pre> |
| * |
| * @author Mike Ward |
| * @author Kurt Alfred Kluever |
| * @author Gregory Kick |
| * @since 16.0 |
| */ |
| @GwtCompatible |
| public abstract class Converter<A, B> implements Function<A, B> { |
| private final boolean handleNullAutomatically; |
| |
| // We lazily cache the reverse view to avoid allocating on every call to reverse(). |
| @LazyInit @RetainedWith @NullableDecl private transient Converter<B, A> reverse; |
| |
| /** Constructor for use by subclasses. */ |
| protected Converter() { |
| this(true); |
| } |
| |
| /** Constructor used only by {@code LegacyConverter} to suspend automatic null-handling. */ |
| Converter(boolean handleNullAutomatically) { |
| this.handleNullAutomatically = handleNullAutomatically; |
| } |
| |
| // SPI methods (what subclasses must implement) |
| |
| /** |
| * Returns a representation of {@code a} as an instance of type {@code B}. If {@code a} cannot be |
| * converted, an unchecked exception (such as {@link IllegalArgumentException}) should be thrown. |
| * |
| * @param a the instance to convert; will never be null |
| * @return the converted instance; <b>must not</b> be null |
| */ |
| @ForOverride |
| protected abstract B doForward(A a); |
| |
| /** |
| * Returns a representation of {@code b} as an instance of type {@code A}. If {@code b} cannot be |
| * converted, an unchecked exception (such as {@link IllegalArgumentException}) should be thrown. |
| * |
| * @param b the instance to convert; will never be null |
| * @return the converted instance; <b>must not</b> be null |
| * @throws UnsupportedOperationException if backward conversion is not implemented; this should be |
| * very rare. Note that if backward conversion is not only unimplemented but |
| * unimplement<i>able</i> (for example, consider a {@code Converter<Chicken, ChickenNugget>}), |
| * then this is not logically a {@code Converter} at all, and should just implement {@link |
| * Function}. |
| */ |
| @ForOverride |
| protected abstract A doBackward(B b); |
| |
| // API (consumer-side) methods |
| |
| /** |
| * Returns a representation of {@code a} as an instance of type {@code B}. |
| * |
| * @return the converted value; is null <i>if and only if</i> {@code a} is null |
| */ |
| @CanIgnoreReturnValue |
| @NullableDecl |
| public final B convert(@NullableDecl A a) { |
| return correctedDoForward(a); |
| } |
| |
| @NullableDecl |
| B correctedDoForward(@NullableDecl A a) { |
| if (handleNullAutomatically) { |
| // TODO(kevinb): we shouldn't be checking for a null result at runtime. Assert? |
| return a == null ? null : checkNotNull(doForward(a)); |
| } else { |
| return doForward(a); |
| } |
| } |
| |
| @NullableDecl |
| A correctedDoBackward(@NullableDecl B b) { |
| if (handleNullAutomatically) { |
| // TODO(kevinb): we shouldn't be checking for a null result at runtime. Assert? |
| return b == null ? null : checkNotNull(doBackward(b)); |
| } else { |
| return doBackward(b); |
| } |
| } |
| |
| /** |
| * Returns an iterable that applies {@code convert} to each element of {@code fromIterable}. The |
| * conversion is done lazily. |
| * |
| * <p>The returned iterable's iterator supports {@code remove()} if the input iterator does. After |
| * a successful {@code remove()} call, {@code fromIterable} no longer contains the corresponding |
| * element. |
| */ |
| @CanIgnoreReturnValue |
| public Iterable<B> convertAll(final Iterable<? extends A> fromIterable) { |
| checkNotNull(fromIterable, "fromIterable"); |
| return new Iterable<B>() { |
| @Override |
| public Iterator<B> iterator() { |
| return new Iterator<B>() { |
| private final Iterator<? extends A> fromIterator = fromIterable.iterator(); |
| |
| @Override |
| public boolean hasNext() { |
| return fromIterator.hasNext(); |
| } |
| |
| @Override |
| public B next() { |
| return convert(fromIterator.next()); |
| } |
| |
| @Override |
| public void remove() { |
| fromIterator.remove(); |
| } |
| }; |
| } |
| }; |
| } |
| |
| /** |
| * Returns the reversed view of this converter, which converts {@code this.convert(a)} back to a |
| * value roughly equivalent to {@code a}. |
| * |
| * <p>The returned converter is serializable if {@code this} converter is. |
| * |
| * <p><b>Note:</b> you should not override this method. It is non-final for legacy reasons. |
| */ |
| @CanIgnoreReturnValue |
| public Converter<B, A> reverse() { |
| Converter<B, A> result = reverse; |
| return (result == null) ? reverse = new ReverseConverter<>(this) : result; |
| } |
| |
| private static final class ReverseConverter<A, B> extends Converter<B, A> |
| implements Serializable { |
| final Converter<A, B> original; |
| |
| ReverseConverter(Converter<A, B> original) { |
| this.original = original; |
| } |
| |
| /* |
| * These gymnastics are a little confusing. Basically this class has neither legacy nor |
| * non-legacy behavior; it just needs to let the behavior of the backing converter shine |
| * through. So, we override the correctedDo* methods, after which the do* methods should never |
| * be reached. |
| */ |
| |
| @Override |
| protected A doForward(B b) { |
| throw new AssertionError(); |
| } |
| |
| @Override |
| protected B doBackward(A a) { |
| throw new AssertionError(); |
| } |
| |
| @Override |
| @NullableDecl |
| A correctedDoForward(@NullableDecl B b) { |
| return original.correctedDoBackward(b); |
| } |
| |
| @Override |
| @NullableDecl |
| B correctedDoBackward(@NullableDecl A a) { |
| return original.correctedDoForward(a); |
| } |
| |
| @Override |
| public Converter<A, B> reverse() { |
| return original; |
| } |
| |
| @Override |
| public boolean equals(@NullableDecl Object object) { |
| if (object instanceof ReverseConverter) { |
| ReverseConverter<?, ?> that = (ReverseConverter<?, ?>) object; |
| return this.original.equals(that.original); |
| } |
| return false; |
| } |
| |
| @Override |
| public int hashCode() { |
| return ~original.hashCode(); |
| } |
| |
| @Override |
| public String toString() { |
| return original + ".reverse()"; |
| } |
| |
| private static final long serialVersionUID = 0L; |
| } |
| |
| /** |
| * Returns a converter whose {@code convert} method applies {@code secondConverter} to the result |
| * of this converter. Its {@code reverse} method applies the converters in reverse order. |
| * |
| * <p>The returned converter is serializable if {@code this} converter and {@code secondConverter} |
| * are. |
| */ |
| public final <C> Converter<A, C> andThen(Converter<B, C> secondConverter) { |
| return doAndThen(secondConverter); |
| } |
| |
| /** Package-private non-final implementation of andThen() so only we can override it. */ |
| <C> Converter<A, C> doAndThen(Converter<B, C> secondConverter) { |
| return new ConverterComposition<>(this, checkNotNull(secondConverter)); |
| } |
| |
| private static final class ConverterComposition<A, B, C> extends Converter<A, C> |
| implements Serializable { |
| final Converter<A, B> first; |
| final Converter<B, C> second; |
| |
| ConverterComposition(Converter<A, B> first, Converter<B, C> second) { |
| this.first = first; |
| this.second = second; |
| } |
| |
| /* |
| * These gymnastics are a little confusing. Basically this class has neither legacy nor |
| * non-legacy behavior; it just needs to let the behaviors of the backing converters shine |
| * through (which might even differ from each other!). So, we override the correctedDo* methods, |
| * after which the do* methods should never be reached. |
| */ |
| |
| @Override |
| protected C doForward(A a) { |
| throw new AssertionError(); |
| } |
| |
| @Override |
| protected A doBackward(C c) { |
| throw new AssertionError(); |
| } |
| |
| @Override |
| @NullableDecl |
| C correctedDoForward(@NullableDecl A a) { |
| return second.correctedDoForward(first.correctedDoForward(a)); |
| } |
| |
| @Override |
| @NullableDecl |
| A correctedDoBackward(@NullableDecl C c) { |
| return first.correctedDoBackward(second.correctedDoBackward(c)); |
| } |
| |
| @Override |
| public boolean equals(@NullableDecl Object object) { |
| if (object instanceof ConverterComposition) { |
| ConverterComposition<?, ?, ?> that = (ConverterComposition<?, ?, ?>) object; |
| return this.first.equals(that.first) && this.second.equals(that.second); |
| } |
| return false; |
| } |
| |
| @Override |
| public int hashCode() { |
| return 31 * first.hashCode() + second.hashCode(); |
| } |
| |
| @Override |
| public String toString() { |
| return first + ".andThen(" + second + ")"; |
| } |
| |
| private static final long serialVersionUID = 0L; |
| } |
| |
| /** |
| * @deprecated Provided to satisfy the {@code Function} interface; use {@link #convert} instead. |
| */ |
| @Deprecated |
| @Override |
| @CanIgnoreReturnValue |
| @NullableDecl |
| public final B apply(@NullableDecl A a) { |
| return convert(a); |
| } |
| |
| /** |
| * Indicates whether another object is equal to this converter. |
| * |
| * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}. |
| * However, an implementation may also choose to return {@code true} whenever {@code object} is a |
| * {@link Converter} that it considers <i>interchangeable</i> with this one. "Interchangeable" |
| * <i>typically</i> means that {@code Objects.equal(this.convert(a), that.convert(a))} is true for |
| * all {@code a} of type {@code A} (and similarly for {@code reverse}). Note that a {@code false} |
| * result from this method does not imply that the converters are known <i>not</i> to be |
| * interchangeable. |
| */ |
| @Override |
| public boolean equals(@NullableDecl Object object) { |
| return super.equals(object); |
| } |
| |
| // Static converters |
| |
| /** |
| * Returns a converter based on separate forward and backward functions. This is useful if the |
| * function instances already exist, or so that you can supply lambda expressions. If those |
| * circumstances don't apply, you probably don't need to use this; subclass {@code Converter} and |
| * implement its {@link #doForward} and {@link #doBackward} methods directly. |
| * |
| * <p>These functions will never be passed {@code null} and must not under any circumstances |
| * return {@code null}. If a value cannot be converted, the function should throw an unchecked |
| * exception (typically, but not necessarily, {@link IllegalArgumentException}). |
| * |
| * <p>The returned converter is serializable if both provided functions are. |
| * |
| * @since 17.0 |
| */ |
| public static <A, B> Converter<A, B> from( |
| Function<? super A, ? extends B> forwardFunction, |
| Function<? super B, ? extends A> backwardFunction) { |
| return new FunctionBasedConverter<>(forwardFunction, backwardFunction); |
| } |
| |
| private static final class FunctionBasedConverter<A, B> extends Converter<A, B> |
| implements Serializable { |
| private final Function<? super A, ? extends B> forwardFunction; |
| private final Function<? super B, ? extends A> backwardFunction; |
| |
| private FunctionBasedConverter( |
| Function<? super A, ? extends B> forwardFunction, |
| Function<? super B, ? extends A> backwardFunction) { |
| this.forwardFunction = checkNotNull(forwardFunction); |
| this.backwardFunction = checkNotNull(backwardFunction); |
| } |
| |
| @Override |
| protected B doForward(A a) { |
| return forwardFunction.apply(a); |
| } |
| |
| @Override |
| protected A doBackward(B b) { |
| return backwardFunction.apply(b); |
| } |
| |
| @Override |
| public boolean equals(@NullableDecl Object object) { |
| if (object instanceof FunctionBasedConverter) { |
| FunctionBasedConverter<?, ?> that = (FunctionBasedConverter<?, ?>) object; |
| return this.forwardFunction.equals(that.forwardFunction) |
| && this.backwardFunction.equals(that.backwardFunction); |
| } |
| return false; |
| } |
| |
| @Override |
| public int hashCode() { |
| return forwardFunction.hashCode() * 31 + backwardFunction.hashCode(); |
| } |
| |
| @Override |
| public String toString() { |
| return "Converter.from(" + forwardFunction + ", " + backwardFunction + ")"; |
| } |
| } |
| |
| /** Returns a serializable converter that always converts or reverses an object to itself. */ |
| @SuppressWarnings("unchecked") // implementation is "fully variant" |
| public static <T> Converter<T, T> identity() { |
| return (IdentityConverter<T>) IdentityConverter.INSTANCE; |
| } |
| |
| /** |
| * A converter that always converts or reverses an object to itself. Note that T is now a |
| * "pass-through type". |
| */ |
| private static final class IdentityConverter<T> extends Converter<T, T> implements Serializable { |
| static final IdentityConverter<?> INSTANCE = new IdentityConverter<>(); |
| |
| @Override |
| protected T doForward(T t) { |
| return t; |
| } |
| |
| @Override |
| protected T doBackward(T t) { |
| return t; |
| } |
| |
| @Override |
| public IdentityConverter<T> reverse() { |
| return this; |
| } |
| |
| @Override |
| <S> Converter<T, S> doAndThen(Converter<T, S> otherConverter) { |
| return checkNotNull(otherConverter, "otherConverter"); |
| } |
| |
| /* |
| * We *could* override convertAll() to return its input, but it's a rather pointless |
| * optimization and opened up a weird type-safety problem. |
| */ |
| |
| @Override |
| public String toString() { |
| return "Converter.identity()"; |
| } |
| |
| private Object readResolve() { |
| return INSTANCE; |
| } |
| |
| private static final long serialVersionUID = 0L; |
| } |
| } |