/*
* Copyright (C) 2007 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.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import com.google.common.annotations.GwtCompatible;
import java.io.Serializable;
import java.util.Map;
import javax.annotation.Nullable;
/**
* Static utility methods pertaining to {@code com.google.common.base.Function} instances; see that
* class for information about migrating to {@code java.util.function}.
*
* <p>All methods return serializable functions as long as they're given serializable parameters.
*
* <p>See the Guava User Guide article on
* <a href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code
* Function}</a>.
*
* @author Mike Bostock
* @author Jared Levy
* @since 2.0
*/
@GwtCompatible
public final class Functions {
private Functions() {}
/**
* A function equivalent to the method reference {@code Object::toString}, for users not yet using
* Java 8. The function simply invokes {@code toString} on its argument and returns the result. It
* throws a {@link NullPointerException} on null input.
*
* <p><b>Warning:</b> The returned function may not be <i>consistent with equals</i> (as
* documented at {@link Function#apply}). For example, this function yields different results for
* the two equal instances {@code ImmutableSet.of(1, 2)} and {@code ImmutableSet.of(2, 1)}.
*
* <p><b>Warning:</b> as with all function types in this package, avoid depending on the specific
* {@code equals}, {@code hashCode} or {@code toString} behavior of the returned function. A
* future migration to {@code java.util.function} will not preserve this behavior.
*
* <p><b>For Java 8 users:</b> use the method reference {@code Object::toString} instead. In the
* future, when this class requires Java 8, this method will be deprecated. See {@link Function}
* for more important information about the Java 8 transition.
*/
public static Function<Object, String> toStringFunction() {
return ToStringFunction.INSTANCE;
}
// enum singleton pattern
private enum ToStringFunction implements Function<Object, String> {
INSTANCE;
@Override
public String apply(Object o) {
checkNotNull(o); // eager for GWT.
return o.toString();
}
@Override
public String toString() {
return "Functions.toStringFunction()";
}
}
/**
* Returns the identity function.
*/
// implementation is "fully variant"; E has become a "pass-through" type
@SuppressWarnings("unchecked")
public static <E> Function<E, E> identity() {
return (Function<E, E>) IdentityFunction.INSTANCE;
}
// enum singleton pattern
private enum IdentityFunction implements Function<Object, Object> {
INSTANCE;
@Override
@Nullable
public Object apply(@Nullable Object o) {
return o;
}
@Override
public String toString() {
return "Functions.identity()";
}
}
/**
* Returns a function which performs a map lookup. The returned function throws an
* {@link IllegalArgumentException} if given a key that does not exist in the map. See also
* {@link #forMap(Map, Object)}, which returns a default value in this case.
*
* <p>Note: if {@code map} is a {@link com.google.common.collect.BiMap BiMap} (or can be one), you
* can use {@link com.google.common.collect.Maps#asConverter Maps.asConverter} instead to get a
* function that also supports reverse conversion.
*
* <p><b>Java 8 users:</b> if you are okay with {@code null} being returned for an unrecognized
* key (instead of an exception being thrown), you can use the method reference {@code map::get}
* instead.
*/
public static <K, V> Function<K, V> forMap(Map<K, V> map) {
return new FunctionForMapNoDefault<K, V>(map);
}
private static class FunctionForMapNoDefault<K, V> implements Function<K, V>, Serializable {
final Map<K, V> map;
FunctionForMapNoDefault(Map<K, V> map) {
this.map = checkNotNull(map);
}
@Override
public V apply(@Nullable K key) {
V result = map.get(key);
checkArgument(result != null || map.containsKey(key), "Key '%s' not present in map", key);
return result;
}
@Override
public boolean equals(@Nullable Object o) {
if (o instanceof FunctionForMapNoDefault) {
FunctionForMapNoDefault<?, ?> that = (FunctionForMapNoDefault<?, ?>) o;
return map.equals(that.map);
}
return false;
}
@Override
public int hashCode() {
return map.hashCode();
}
@Override
public String toString() {
return "Functions.forMap(" + map + ")";
}
private static final long serialVersionUID = 0;
}
/**
* Returns a function which performs a map lookup with a default value. The function created by
* this method returns {@code defaultValue} for all inputs that do not belong to the map's key
* set. See also {@link #forMap(Map)}, which throws an exception in this case.
*
* <p><b>Java 8 users:</b> you can just write the lambda expression {@code k ->
* map.getWithDefault(k, defaultValue)} instead.
*
* @param map source map that determines the function behavior
* @param defaultValue the value to return for inputs that aren't map keys
* @return function that returns {@code map.get(a)} when {@code a} is a key, or {@code
* defaultValue} otherwise
*/
public static <K, V> Function<K, V> forMap(Map<K, ? extends V> map, @Nullable V defaultValue) {
return new ForMapWithDefault<K, V>(map, defaultValue);
}
private static class ForMapWithDefault<K, V> implements Function<K, V>, Serializable {
final Map<K, ? extends V> map;
final V defaultValue;
ForMapWithDefault(Map<K, ? extends V> map, @Nullable V defaultValue) {
this.map = checkNotNull(map);
this.defaultValue = defaultValue;
}
@Override
public V apply(@Nullable K key) {
V result = map.get(key);
return (result != null || map.containsKey(key)) ? result : defaultValue;
}
@Override
public boolean equals(@Nullable Object o) {
if (o instanceof ForMapWithDefault) {
ForMapWithDefault<?, ?> that = (ForMapWithDefault<?, ?>) o;
return map.equals(that.map) && Objects.equal(defaultValue, that.defaultValue);
}
return false;
}
@Override
public int hashCode() {
return Objects.hashCode(map, defaultValue);
}
@Override
public String toString() {
// TODO(cpovirk): maybe remove "defaultValue=" to make this look like the method call does
return "Functions.forMap(" + map + ", defaultValue=" + defaultValue + ")";
}
private static final long serialVersionUID = 0;
}
/**
* Returns the composition of two functions. For {@code f: A->B} and {@code g: B->C}, composition
* is defined as the function h such that {@code h(a) == g(f(a))} for each {@code a}.
*
* <p><b>Java 8 users:</b> use {@code g.compose(f)} or (probably clearer) {@code f.andThen(g)}
* instead.
*
* @param g the second function to apply
* @param f the first function to apply
* @return the composition of {@code f} and {@code g}
* @see <a href="//en.wikipedia.org/wiki/Function_composition">function composition</a>
*/
public static <A, B, C> Function<A, C> compose(Function<B, C> g, Function<A, ? extends B> f) {
return new FunctionComposition<A, B, C>(g, f);
}
private static class FunctionComposition<A, B, C> implements Function<A, C>, Serializable {
private final Function<B, C> g;
private final Function<A, ? extends B> f;
public FunctionComposition(Function<B, C> g, Function<A, ? extends B> f) {
this.g = checkNotNull(g);
this.f = checkNotNull(f);
}
@Override
public C apply(@Nullable A a) {
return g.apply(f.apply(a));
}
@Override
public boolean equals(@Nullable Object obj) {
if (obj instanceof FunctionComposition) {
FunctionComposition<?, ?, ?> that = (FunctionComposition<?, ?, ?>) obj;
return f.equals(that.f) && g.equals(that.g);
}
return false;
}
@Override
public int hashCode() {
return f.hashCode() ^ g.hashCode();
}
@Override
public String toString() {
// TODO(cpovirk): maybe make this look like the method call does ("Functions.compose(...)")
return g + "(" + f + ")";
}
private static final long serialVersionUID = 0;
}
/**
* Creates a function that returns the same boolean output as the given predicate for all inputs.
*
* <p>The returned function is <i>consistent with equals</i> (as documented at
* {@link Function#apply}) if and only if {@code predicate} is itself consistent with equals.
*
* <p><b>Java 8 users:</b> use the method reference {@code predicate::test} instead.
*/
public static <T> Function<T, Boolean> forPredicate(Predicate<T> predicate) {
return new PredicateFunction<T>(predicate);
}
/** @see Functions#forPredicate */
private static class PredicateFunction<T> implements Function<T, Boolean>, Serializable {
private final Predicate<T> predicate;
private PredicateFunction(Predicate<T> predicate) {
this.predicate = checkNotNull(predicate);
}
@Override
public Boolean apply(@Nullable T t) {
return predicate.apply(t);
}
@Override
public boolean equals(@Nullable Object obj) {
if (obj instanceof PredicateFunction) {
PredicateFunction<?> that = (PredicateFunction<?>) obj;
return predicate.equals(that.predicate);
}
return false;
}
@Override
public int hashCode() {
return predicate.hashCode();
}
@Override
public String toString() {
return "Functions.forPredicate(" + predicate + ")";
}
private static final long serialVersionUID = 0;
}
/**
* Returns a function that ignores its input and always returns {@code value}.
*
* <p><b>Java 8 users:</b> use the lambda expression {@code o -> value} instead.
*
* @param value the constant value for the function to return
* @return a function that always returns {@code value}
*/
public static <E> Function<Object, E> constant(@Nullable E value) {
return new ConstantFunction<E>(value);
}
private static class ConstantFunction<E> implements Function<Object, E>, Serializable {
private final E value;
public ConstantFunction(@Nullable E value) {
this.value = value;
}
@Override
public E apply(@Nullable Object from) {
return value;
}
@Override
public boolean equals(@Nullable Object obj) {
if (obj instanceof ConstantFunction) {
ConstantFunction<?> that = (ConstantFunction<?>) obj;
return Objects.equal(value, that.value);
}
return false;
}
@Override
public int hashCode() {
return (value == null) ? 0 : value.hashCode();
}
@Override
public String toString() {
return "Functions.constant(" + value + ")";
}
private static final long serialVersionUID = 0;
}
/**
* Returns a function that ignores its input and returns the result of {@code supplier.get()}.
*
* <p><b>Java 8 users:</b> use the lambda expression {@code o -> supplier.get()} instead.
*
* @since 10.0
*/
public static <T> Function<Object, T> forSupplier(Supplier<T> supplier) {
return new SupplierFunction<T>(supplier);
}
/** @see Functions#forSupplier */
private static class SupplierFunction<T> implements Function<Object, T>, Serializable {
private final Supplier<T> supplier;
private SupplierFunction(Supplier<T> supplier) {
this.supplier = checkNotNull(supplier);
}
@Override
public T apply(@Nullable Object input) {
return supplier.get();
}
@Override
public boolean equals(@Nullable Object obj) {
if (obj instanceof SupplierFunction) {
SupplierFunction<?> that = (SupplierFunction<?>) obj;
return this.supplier.equals(that.supplier);
}
return false;
}
@Override
public int hashCode() {
return supplier.hashCode();
}
@Override
public String toString() {
return "Functions.forSupplier(" + supplier + ")";
}
private static final long serialVersionUID = 0;
}
}