com.abubusoft.kripton.common.Converter Maven / Gradle / Ivy
/*******************************************************************************
* Copyright 2016-2019 Francesco Benincasa ([email protected])
*
* 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.abubusoft.kripton.common;
import static com.abubusoft.kripton.common.Preconditions.checkNotNull;
import java.io.Serializable;
import java.util.Iterator;
/**
* A function from {@code A} to {@code B} with an associated reverse
* function from {@code B} to {@code A}; used for converting back and forth
* between different representations of the same information.
*
* Invertibility
*
*
* 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 more common) for round-trip
* conversion to be lossy. Consider an example round-trip using
* com.google.common.primitives.Doubles#stringConverter:
*
*
* - {@code stringConverter().convert("1.00")} returns the {@code Double}
* value {@code 1.0}
*
- {@code stringConverter().reverse().convert(1.0)} returns the string
* {@code "1.0"} -- not the same string ({@code "1.00"}) we started with
*
*
*
* Note that it should still be the case that the round-tripped and original
* objects are similar.
*
*
Nullability
*
*
* 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 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}.
*
*
Common ways to use
*
*
* Getting a converter:
*
*
* - Use a provided converter implementation, such as Enums#stringConverter,
* com.google.common.primitives.Ints#stringConverter Ints.stringConverter or the
* {@linkplain #reverse reverse} views of these.
*
- Convert between specific preset values using
* 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 mock the {@code Converter} type using a
* mocking framework.
*
- Otherwise, extend this class and implement its {@link #doForward} and
* {@link #doBackward} methods.
*
*
*
* Using a converter:
*
*
* - Convert one instance in the "forward" direction using
* {@code converter.convert(a)}.
*
- Convert multiple instances "forward" using
* {@code converter.convertAll(as)}.
*
- Convert in the "backward" direction using
* {@code converter.reverse().convert(b)} or {@code
* converter.reverse().convertAll(bs)}.
*
- Use {@code converter} or {@code converter.reverse()} anywhere a
* {@link Function} is accepted
*
- Do not call {@link #doForward} or {@link #doBackward} directly;
* these exist only to be overridden.
*
*
* @author Mike Ward
* @author Kurt Alfred Kluever
* @author Gregory Kick
* @param
* the generic type
* @param
* the generic type
* @since 16.0
*/
public abstract class Converter implements Function {
/** The handle null automatically. */
private final boolean handleNullAutomatically;
/** The reverse. */
// We lazily cache the reverse view to avoid allocating on every call to
// reverse().
private transient Converter reverse;
/** Constructor for use by subclasses. */
protected Converter() {
this(true);
}
/**
* Constructor used only by {@code LegacyConverter} to suspend automatic
* null-handling.
*
* @param handleNullAutomatically
* the handle null automatically
*/
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; 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; 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 unimplementable (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}.
*
* @param a
* the a
* @return the converted value; is null if and only if {@code a} is
* null
*/
public final B convert(A a) {
return correctedDoForward(a);
}
/**
* Corrected do forward.
*
* @param a
* the a
* @return the b
*/
B correctedDoForward(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);
}
}
/**
* Corrected do backward.
*
* @param b
* the b
* @return the a
*/
A correctedDoBackward(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.
*
*
* 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.
*
* @param fromIterable
* the from iterable
* @return the iterable
*/
public Iterable convertAll(final Iterable fromIterable) {
checkNotNull(fromIterable, "fromIterable");
return new Iterable() {
@Override
public Iterator iterator() {
return new Iterator() {
private final Iterator 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}.
*
*
* The returned converter is serializable if {@code this} converter is.
*
* @return the converter
*/
// TODO(kak): Make this method final
public Converter reverse() {
Converter result = reverse;
return (result == null) ? reverse = new ReverseConverter(this) : result;
}
/**
* The Class ReverseConverter.
*
* @param
* the generic type
* @param
* the generic type
*/
private static final class ReverseConverter extends Converter implements Serializable {
/** The Constant serialVersionUID. */
private static final long serialVersionUID = 5073708032168078367L;
/** The original. */
final Converter original;
/**
* Instantiates a new reverse converter.
*
* @param original
* the original
*/
ReverseConverter(Converter 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.
*/
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#doForward(java.lang.Object)
*/
@Override
protected A doForward(B b) {
throw new AssertionError();
}
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#doBackward(java.lang.Object)
*/
@Override
protected B doBackward(A a) {
throw new AssertionError();
}
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#correctedDoForward(java.lang.
* Object)
*/
@Override
A correctedDoForward(B b) {
return original.correctedDoBackward(b);
}
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#correctedDoBackward(java.lang.
* Object)
*/
@Override
B correctedDoBackward(A a) {
return original.correctedDoForward(a);
}
/*
* (non-Javadoc)
*
* @see com.abubusoft.kripton.common.Converter#reverse()
*/
@Override
public Converter reverse() {
return original;
}
/*
* (non-Javadoc)
*
* @see com.abubusoft.kripton.common.Converter#equals(java.lang.Object)
*/
@Override
public boolean equals(Object object) {
if (object instanceof ReverseConverter) {
ReverseConverter that = (ReverseConverter) object;
return this.original.equals(that.original);
}
return false;
}
/*
* (non-Javadoc)
*
* @see java.lang.Object#hashCode()
*/
@Override
public int hashCode() {
return ~original.hashCode();
}
/*
* (non-Javadoc)
*
* @see java.lang.Object#toString()
*/
@Override
public String toString() {
return original + ".reverse()";
}
}
/**
* 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.
*
*
* The returned converter is serializable if {@code this} converter and
* {@code secondConverter} are.
*
* @param
* the generic type
* @param secondConverter
* the second converter
* @return the converter
*/
public final Converter andThen(Converter secondConverter) {
return doAndThen(secondConverter);
}
/**
* Package-private non-final implementation of andThen() so only we can
* override it.
*
* @param
* the generic type
* @param secondConverter
* the second converter
* @return the converter
*/
Converter doAndThen(Converter secondConverter) {
return new ConverterComposition(this, checkNotNull(secondConverter));
}
/**
* The Class ConverterComposition.
*
* @param
* the generic type
* @param
* the generic type
* @param
* the generic type
*/
private static final class ConverterComposition extends Converter implements Serializable {
/** The first. */
final Converter first;
/** The second. */
final Converter second;
/**
* Instantiates a new converter composition.
*
* @param first
* the first
* @param second
* the second
*/
ConverterComposition(Converter 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.
*/
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#doForward(java.lang.Object)
*/
@Override
protected C doForward(A a) {
throw new AssertionError();
}
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#doBackward(java.lang.Object)
*/
@Override
protected A doBackward(C c) {
throw new AssertionError();
}
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#correctedDoForward(java.lang.
* Object)
*/
@Override
C correctedDoForward(A a) {
return second.correctedDoForward(first.correctedDoForward(a));
}
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#correctedDoBackward(java.lang.
* Object)
*/
@Override
A correctedDoBackward(C c) {
return first.correctedDoBackward(second.correctedDoBackward(c));
}
/*
* (non-Javadoc)
*
* @see com.abubusoft.kripton.common.Converter#equals(java.lang.Object)
*/
@Override
public boolean equals(Object object) {
if (object instanceof ConverterComposition) {
ConverterComposition that = (ConverterComposition) object;
return this.first.equals(that.first) && this.second.equals(that.second);
}
return false;
}
/*
* (non-Javadoc)
*
* @see java.lang.Object#hashCode()
*/
@Override
public int hashCode() {
return 31 * first.hashCode() + second.hashCode();
}
/*
* (non-Javadoc)
*
* @see java.lang.Object#toString()
*/
@Override
public String toString() {
return first + ".andThen(" + second + ")";
}
/** The Constant serialVersionUID. */
private static final long serialVersionUID = 0L;
}
/**
* Apply.
*
* @param a
* the a
* @return the b
* @deprecated Provided to satisfy the {@code Function} interface; use
* {@link #convert} instead.
*/
@Deprecated
@Override
public final B apply(A a) {
return convert(a);
}
/* (non-Javadoc)
* @see java.lang.Object#equals(java.lang.Object)
*/
@Override
public boolean equals(Object object) {
return super.equals(object);
}
/* (non-Javadoc)
* @see java.lang.Object#hashCode()
*/
@Override
public int hashCode() {
return super.hashCode();
}
/**
* Returns a converter based on existing forward and backward
* functions. Note that it is unnecessary to create 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.
*
*
* 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}).
*
*
* The returned converter is serializable if both provided functions are.
*
* @param
* the generic type
* @param
* the generic type
* @param forwardFunction
* the forward function
* @param backwardFunction
* the backward function
* @return the converter
* @since 17.0
*/
public static Converter from(Function forwardFunction, Function backwardFunction) {
return new FunctionBasedConverter(forwardFunction, backwardFunction);
}
/**
* The Class FunctionBasedConverter.
*
* @param
* the generic type
* @param
* the generic type
*/
private static final class FunctionBasedConverter extends Converter implements Serializable {
/** The Constant serialVersionUID. */
private static final long serialVersionUID = 1519401989279200113L;
/** The forward function. */
private final Function forwardFunction;
/** The backward function. */
private final Function backwardFunction;
/**
* Instantiates a new function based converter.
*
* @param forwardFunction
* the forward function
* @param backwardFunction
* the backward function
*/
private FunctionBasedConverter(Function forwardFunction, Function backwardFunction) {
this.forwardFunction = checkNotNull(forwardFunction);
this.backwardFunction = checkNotNull(backwardFunction);
}
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#doForward(java.lang.Object)
*/
@Override
protected B doForward(A a) {
return forwardFunction.apply(a);
}
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#doBackward(java.lang.Object)
*/
@Override
protected A doBackward(B b) {
return backwardFunction.apply(b);
}
/*
* (non-Javadoc)
*
* @see com.abubusoft.kripton.common.Converter#equals(java.lang.Object)
*/
@Override
public boolean equals(Object object) {
if (object instanceof FunctionBasedConverter) {
FunctionBasedConverter that = (FunctionBasedConverter) object;
return this.forwardFunction.equals(that.forwardFunction) && this.backwardFunction.equals(that.backwardFunction);
}
return false;
}
/*
* (non-Javadoc)
*
* @see java.lang.Object#hashCode()
*/
@Override
public int hashCode() {
return forwardFunction.hashCode() * 31 + backwardFunction.hashCode();
}
/*
* (non-Javadoc)
*
* @see java.lang.Object#toString()
*/
@Override
public String toString() {
return "Converter.from(" + forwardFunction + ", " + backwardFunction + ")";
}
}
/**
* Returns a serializable converter that always converts or reverses an
* object to itself.
*
* @param
* the generic type
* @return the converter
*/
@SuppressWarnings("unchecked") // implementation is "fully variant"
public static Converter identity() {
return (IdentityConverter) IdentityConverter.INSTANCE;
}
/**
* A converter that always converts or reverses an object to itself. Note
* that T is now a "pass-through type".
*
* @param
* the generic type
*/
private static final class IdentityConverter extends Converter implements Serializable {
/** The Constant serialVersionUID. */
private static final long serialVersionUID = -163613376588335881L;
/** The Constant INSTANCE. */
@SuppressWarnings("rawtypes")
static final IdentityConverter INSTANCE = new IdentityConverter();
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#doForward(java.lang.Object)
*/
@Override
protected T doForward(T t) {
return t;
}
/*
* (non-Javadoc)
*
* @see
* com.abubusoft.kripton.common.Converter#doBackward(java.lang.Object)
*/
@Override
protected T doBackward(T t) {
return t;
}
/*
* (non-Javadoc)
*
* @see com.abubusoft.kripton.common.Converter#reverse()
*/
@Override
public IdentityConverter reverse() {
return this;
}
/*
* (non-Javadoc)
*
* @see com.abubusoft.kripton.common.Converter#doAndThen(com.abubusoft.
* kripton.common.Converter)
*/
@Override
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.
*/
/*
* (non-Javadoc)
*
* @see java.lang.Object#toString()
*/
@Override
public String toString() {
return "Converter.identity()";
}
/**
* Read resolve.
*
* @return the object
*/
private Object readResolve() {
return INSTANCE;
}
}
}