or who call convertAll (and for any checkers that apply @PolyNull-like semantics
* to Converter.convert). So maybe we don't want to think too hard about how to prevent our
* checkers from issuing errors related to LegacyConverter, since it turns out that
* LegacyConverter does violate the assumptions we make elsewhere.
*/
@CheckForNull
private B unsafeDoForward(@CheckForNull A a) {
return doForward(uncheckedCastNullableTToT(a));
}
@CheckForNull
private A unsafeDoBackward(@CheckForNull B b) {
return doBackward(uncheckedCastNullableTToT(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.
*/
/*
* Just as Converter could implement `Function<@Nullable A, @Nullable B>` instead of `Function`, convertAll could accept and return iterables with nullable element types. In both cases,
* we've chosen to instead use a signature that benefits existing users -- and is still safe.
*
* For convertAll, I haven't looked as closely at *how* much existing users benefit, so we should
* keep an eye out for problems that new users encounter. Note also that convertAll could support
* both use cases by using @PolyNull. (By contrast, we can't use @PolyNull for our superinterface
* (`implements Function<@PolyNull A, @PolyNull B>`), at least as far as I know.)
*/
public Iterable convertAll(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.
*
*
Note: you should not override this method. It is non-final for legacy reasons.
*/
@CheckReturnValue
public Converter reverse() {
Converter result = reverse;
return (result == null) ? reverse = new ReverseConverter<>(this) : result;
}
private static final class ReverseConverter extends Converter
implements Serializable {
final Converter 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.
*/
@Override
protected A doForward(B b) {
throw new AssertionError();
}
@Override
protected B doBackward(A a) {
throw new AssertionError();
}
@Override
@CheckForNull
A correctedDoForward(@CheckForNull B b) {
return original.correctedDoBackward(b);
}
@Override
@CheckForNull
B correctedDoBackward(@CheckForNull A a) {
return original.correctedDoForward(a);
}
@Override
public Converter reverse() {
return original;
}
@Override
public boolean equals(@CheckForNull 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.
*
* The returned converter is serializable if {@code this} converter and {@code secondConverter}
* are.
*/
public final Converter andThen(Converter secondConverter) {
return doAndThen(secondConverter);
}
/** Package-private non-final implementation of andThen() so only we can override it. */
Converter doAndThen(Converter secondConverter) {
return new ConverterComposition<>(this, checkNotNull(secondConverter));
}
private static final class ConverterComposition extends Converter
implements Serializable {
final Converter first;
final Converter 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.
*/
@Override
protected C doForward(A a) {
throw new AssertionError();
}
@Override
protected A doBackward(C c) {
throw new AssertionError();
}
@Override
@CheckForNull
C correctedDoForward(@CheckForNull A a) {
return second.correctedDoForward(first.correctedDoForward(a));
}
@Override
@CheckForNull
A correctedDoBackward(@CheckForNull C c) {
return first.correctedDoBackward(second.correctedDoBackward(c));
}
@Override
public boolean equals(@CheckForNull 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
@InlineMe(replacement = "this.convert(a)")
public final B apply(A a) {
/*
* Given that we declare this method as accepting and returning non-nullable values (because we
* implement Function, as discussed in a class-level comment), it would make some sense to
* perform runtime null checks on the input and output. (That would also make NullPointerTester
* happy!) However, since we didn't do that for many years, we're not about to start now.
* (Runtime checks could be particularly bad for users of LegacyConverter.)
*
* Luckily, our nullness checker is smart enough to realize that `convert` has @PolyNull-like
* behavior, so it knows that `convert(a)` returns a non-nullable value, and we don't need to
* perform even a cast, much less a runtime check.
*
* All that said, don't forget that everyone should call converter.convert() instead of
* converter.apply(), anyway. If clients use only converter.convert(), then their nullness
* checkers are unlikely to ever look at the annotations on this declaration.
*
* Historical note: At one point, we'd declared this method as accepting and returning nullable
* values. For details on that, see earlier revisions of this file.
*/
return convert(a);
}
/**
* Indicates whether another object is equal to this converter.
*
* 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 interchangeable with this one. "Interchangeable"
* 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 not to be
* interchangeable.
*/
@Override
public boolean equals(@CheckForNull 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.
*
*
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.
*
* @since 17.0
*/
public static Converter from(
Function forwardFunction,
Function backwardFunction) {
return new FunctionBasedConverter<>(forwardFunction, backwardFunction);
}
private static final class FunctionBasedConverter extends Converter
implements Serializable {
private final Function forwardFunction;
private final Function backwardFunction;
private FunctionBasedConverter(
Function forwardFunction,
Function 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(@CheckForNull 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 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".
*/
private static final class IdentityConverter extends Converter implements Serializable {
static final Converter INSTANCE = new IdentityConverter<>();
@Override
protected T doForward(T t) {
return t;
}
@Override
protected T doBackward(T t) {
return t;
}
@Override
public IdentityConverter reverse() {
return this;
}
@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.
*/
@Override
public String toString() {
return "Converter.identity()";
}
private Object readResolve() {
return INSTANCE;
}
private static final long serialVersionUID = 0L;
}
}