/* * Copyright 2016 DiffPlug * * 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.diffplug.common.base; import java.util.Map; import java.util.function.Function; import java.util.function.Predicate; import java.util.function.Supplier; import javax.annotation.Nullable; /** * Helper functions for manipulating {@link Function}, copied from Guava. * <p> * The function signatures below are identical to Google's Guava 18.0, except that guava's functional * interfaces have been swapped with Java 8's. It is tested against the same test suite * as Google Guava to ensure functional compatibility. * <p> * Most of the implementation has been replaced with lambdas, which means that the following * functionality has been removed: equals(), hashCode(), toString(), GWT, and serialization. * <p> * Lambdas don't support these methods, and there isn't much reason why they should, so we * removed them in Durian. */ public final class Functions { private Functions() {} /** * Returns the identity function. */ // implementation is "fully variant"; E has become a "pass-through" type public static <E> Function<E, E> identity() { return Function.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. */ public static <K, V> Function<K, V> forMap(Map<K, V> map) { return key -> { if (map.containsKey(key)) { return map.get(key); } else { throw new IllegalArgumentException("Key '" + key + "' not present in map"); } }; } /** * 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. * * @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 key -> map.containsKey(key) ? map.get(key) : defaultValue; } /** * 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}. * * @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 g.compose(f); } /** * 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. */ public static <T> Function<T, Boolean> forPredicate(Predicate<T> predicate) { return predicate::test; } /** * Creates a function that returns {@code value} for any input. * * @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 input -> value; } /** * Returns a function that always returns the result of invoking {@link Supplier#get} on {@code * supplier}, regardless of its input. * * @since 10.0 */ public static <T> Function<Object, T> forSupplier(Supplier<T> supplier) { return input -> supplier.get(); } }