The Converter.java Java example source code
/*
* 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.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.Serializable;
import java.util.Iterator;
import javax.annotation.Nullable;
/**
* A function from {@code A} to {@code B} with an associated <i>reverse function from {@code B}
* to {@code A}; used for converting back and forth between <i>different representations of the same
* information</i>.
*
* <h3>Invertibility
*
* <p>The reverse operation may be a strict inverse (meaning that {@code
* converter.reverse().convert(converter.convert(a)).equals(a)} is always true). However, it is very
* common (perhaps <i>more common) for round-trip conversion to be lossy. 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 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.
*
* <h3>Nullability
*
* <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 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
*
* <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
* the {@code Converter} type using a mocking framework.
* <li>Extend this class and implement its {@link #doForward} and {@link #doBackward} methods.
* <li>If using Java 8, 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 Function} is accepted
* <li>Do not call {@link #doForward} or {@link #doBackward} directly; these exist only to be
* overridden.
* </ul>
*
* <h3>Example
*
* <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: {@code
* return Converter.from(
* Integer::toHexString,
* s -> parseUnsignedInt(s, 16));}</pre>
*
* @author Mike Ward
* @author Kurt Alfred Kluever
* @author Gregory Kick
* @since 16.0
*/
@Beta
@GwtCompatible
public abstract class Converter<A, B> implements Function {
private final boolean handleNullAutomatically;
// We lazily cache the reverse view to avoid allocating on every call to reverse().
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 be null
*/
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 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 (for example, consider a {@code Converter}),
* then this is not logically a {@code Converter} at all, and should just implement {@link
* Function}.
*/
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 {@code a} is null
*/
@Nullable
@CanIgnoreReturnValue
public final B convert(@Nullable A a) {
return correctedDoForward(a);
}
@Nullable
B correctedDoForward(@Nullable 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);
}
}
@Nullable
A correctedDoBackward(@Nullable 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 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.
*/
// TODO(kak): Make this method final
@CanIgnoreReturnValue
public Converter<B, A> reverse() {
Converter<B, A> result = reverse;
return (result == null) ? reverse = new ReverseConverter<A, B>(this) : result;
}
private static final class ReverseConverter<A, B> extends Converter
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
@Nullable
A correctedDoForward(@Nullable B b) {
return original.correctedDoBackward(b);
}
@Override
@Nullable
B correctedDoBackward(@Nullable A a) {
return original.correctedDoForward(a);
}
@Override
public Converter<A, B> reverse() {
return original;
}
@Override
public boolean equals(@Nullable 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 andThen(Converter secondConverter) {
return doAndThen(secondConverter);
}
/**
* Package-private non-final implementation of andThen() so only we can override it.
*/
<C> Converter doAndThen(Converter secondConverter) {
return new ConverterComposition<A, B, C>(this, checkNotNull(secondConverter));
}
private static final class ConverterComposition<A, B, C> extends Converter
implements Serializable {
final Converter<A, B> first;
final Converter<B, C> second;
ConverterComposition(Converter<A, B> first, Converter 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
@Nullable
C correctedDoForward(@Nullable A a) {
return second.correctedDoForward(first.correctedDoForward(a));
}
@Override
@Nullable
A correctedDoBackward(@Nullable C c) {
return first.correctedDoBackward(second.correctedDoBackward(c));
}
@Override
public boolean equals(@Nullable 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
@Nullable
@CanIgnoreReturnValue
public final B apply(@Nullable 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 with this one. "Interchangeable"
* <i>typically 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 to be
* interchangeable.
*/
@Override
public boolean equals(@Nullable Object object) {
return super.equals(object);
}
// Static converters
/**
* Returns a converter based on <i>existing forward and backward functions. Note that it is
* unnecessary to create <i>new classes implementing {@code Function} just to pass them in
* here. Instead, simply 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 from(
Function<? super A, ? extends B> forwardFunction,
Function<? super B, ? extends A> backwardFunction) {
return new FunctionBasedConverter<A, B>(forwardFunction, backwardFunction);
}
private static final class FunctionBasedConverter<A, B> extends Converter
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(@Nullable 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 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 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 doAndThen(Converter 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;
}
}
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