    001    /*
    002     * Copyright (C) 2007 The Guava Authors
    003     *
    004     * Licensed under the Apache License, Version 2.0 (the "License");
    005     * you may not use this file except in compliance with the License.
    006     * You may obtain a copy of the License at
    007     *
    008     * http://www.apache.org/licenses/LICENSE-2.0
    009     *
    010     * Unless required by applicable law or agreed to in writing, software
    011     * distributed under the License is distributed on an "AS IS" BASIS,
    012     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    013     * See the License for the specific language governing permissions and
    014     * limitations under the License.
    015     */
    016
    017    package com.google.common.base;
    018
    019    import static com.google.common.base.Preconditions.checkArgument;
    020    import static com.google.common.base.Preconditions.checkNotNull;
    021
    022    import com.google.common.annotations.Beta;
    023    import com.google.common.annotations.GwtCompatible;
    024
    025    import java.io.Serializable;
    026    import java.util.Map;
    027
    028    import javax.annotation.Nullable;
    029
    030    /**
    031     * Static utility methods pertaining to {@code Function} instances.
    032     *
    033     * <p>All methods returns serializable functions as long as they're given serializable parameters.
    034     *
    035     * @author Mike Bostock
    036     * @author Jared Levy
    037     * @since 2.0 (imported from Google Collections Library)
    038     */
    039    @GwtCompatible
    040    public final class Functions {
    041      private Functions() {}
    042
    043      /**
    044       * Returns a function that calls {@code toString()} on its argument. The function does not accept
    045       * nulls; it will throw a {@link NullPointerException} when applied to {@code null}.
    046       *
    047       * <p><b>Warning:</b> The returned function may not be <i>consistent with equals</i> (as
    048       * documented at {@link Function#apply}). For example, this function yields different results for
    049       * the two equal instances {@code ImmutableSet.of(1, 2)} and {@code ImmutableSet.of(2, 1)}.
    050       */
    051      public static Function<Object, String> toStringFunction() {
    052        return ToStringFunction.INSTANCE;
    053      }
    054
    055      // enum singleton pattern
    056      private enum ToStringFunction implements Function<Object, String> {
    057        INSTANCE;
    058
    059        @Override
    060        public String apply(Object o) {
    061          checkNotNull(o);  // eager for GWT.
    062          return o.toString();
    063        }
    064
    065        @Override public String toString() {
    066          return "toString";
    067        }
    068      }
    069
    070      /**
    071       * Returns the identity function.
    072       */
    073      @SuppressWarnings("unchecked")
    074      public static <E> Function<E, E> identity() {
    075        return (Function<E, E>) IdentityFunction.INSTANCE;
    076      }
    077
    078      // enum singleton pattern
    079      private enum IdentityFunction implements Function<Object, Object> {
    080        INSTANCE;
    081
    082        @Override
    083        public Object apply(Object o) {
    084          return o;
    085        }
    086
    087        @Override public String toString() {
    088          return "identity";
    089        }
    090      }
    091
    092      /**
    093       * Returns a function which performs a map lookup. The returned function throws an {@link
    094       * IllegalArgumentException} if given a key that does not exist in the map.
    095       */
    096      public static <K, V> Function<K, V> forMap(Map<K, V> map) {
    097        return new FunctionForMapNoDefault<K, V>(map);
    098      }
    099
    100      private static class FunctionForMapNoDefault<K, V> implements Function<K, V>, Serializable {
    101        final Map<K, V> map;
    102
    103        FunctionForMapNoDefault(Map<K, V> map) {
    104          this.map = checkNotNull(map);
    105        }
    106
    107        @Override
    108        public V apply(K key) {
    109          V result = map.get(key);
    110          checkArgument(result != null || map.containsKey(key), "Key '%s' not present in map", key);
    111          return result;
    112        }
    113
    114        @Override public boolean equals(@Nullable Object o) {
    115          if (o instanceof FunctionForMapNoDefault) {
    116            FunctionForMapNoDefault<?, ?> that = (FunctionForMapNoDefault<?, ?>) o;
    117            return map.equals(that.map);
    118          }
    119          return false;
    120        }
    121
    122        @Override public int hashCode() {
    123          return map.hashCode();
    124        }
    125
    126        @Override public String toString() {
    127          return "forMap(" + map + ")";
    128        }
    129
    130        private static final long serialVersionUID = 0;
    131      }
    132
    133      /**
    134       * Returns a function which performs a map lookup with a default value. The function created by
    135       * this method returns {@code defaultValue} for all inputs that do not belong to the map's key
    136       * set.
    137       *
    138       * @param map source map that determines the function behavior
    139       * @param defaultValue the value to return for inputs that aren't map keys
    140       * @return function that returns {@code map.get(a)} when {@code a} is a key, or {@code
    141       *         defaultValue} otherwise
    142       */
    143      public static <K, V> Function<K, V> forMap(Map<K, ? extends V> map, @Nullable V defaultValue) {
    144        return new ForMapWithDefault<K, V>(map, defaultValue);
    145      }
    146
    147      private static class ForMapWithDefault<K, V> implements Function<K, V>, Serializable {
    148        final Map<K, ? extends V> map;
    149        final V defaultValue;
    150
    151        ForMapWithDefault(Map<K, ? extends V> map, @Nullable V defaultValue) {
    152          this.map = checkNotNull(map);
    153          this.defaultValue = defaultValue;
    154        }
    155
    156        @Override
    157        public V apply(K key) {
    158          V result = map.get(key);
    159          return (result != null || map.containsKey(key)) ? result : defaultValue;
    160        }
    161
    162        @Override public boolean equals(@Nullable Object o) {
    163          if (o instanceof ForMapWithDefault) {
    164            ForMapWithDefault<?, ?> that = (ForMapWithDefault<?, ?>) o;
    165            return map.equals(that.map) && Objects.equal(defaultValue, that.defaultValue);
    166          }
    167          return false;
    168        }
    169
    170        @Override public int hashCode() {
    171          return Objects.hashCode(map, defaultValue);
    172        }
    173
    174        @Override public String toString() {
    175          return "forMap(" + map + ", defaultValue=" + defaultValue + ")";
    176        }
    177
    178        private static final long serialVersionUID = 0;
    179      }
    180
    181      /**
    182       * Returns the composition of two functions. For {@code f: A->B} and {@code g: B->C}, composition
    183       * is defined as the function h such that {@code h(a) == g(f(a))} for each {@code a}.
    184       *
    185       * @param g the second function to apply
    186       * @param f the first function to apply
    187       * @return the composition of {@code f} and {@code g}
    188       * @see <a href="//en.wikipedia.org/wiki/Function_composition">function composition</a>
    189       */
    190      public static <A, B, C> Function<A, C> compose(Function<B, C> g, Function<A, ? extends B> f) {
    191        return new FunctionComposition<A, B, C>(g, f);
    192      }
    193
    194      private static class FunctionComposition<A, B, C> implements Function<A, C>, Serializable {
    195        private final Function<B, C> g;
    196        private final Function<A, ? extends B> f;
    197
    198        public FunctionComposition(Function<B, C> g, Function<A, ? extends B> f) {
    199          this.g = checkNotNull(g);
    200          this.f = checkNotNull(f);
    201        }
    202
    203        @Override
    204        public C apply(A a) {
    205          return g.apply(f.apply(a));
    206        }
    207
    208        @Override public boolean equals(@Nullable Object obj) {
    209          if (obj instanceof FunctionComposition) {
    210            FunctionComposition<?, ?, ?> that = (FunctionComposition<?, ?, ?>) obj;
    211            return f.equals(that.f) && g.equals(that.g);
    212          }
    213          return false;
    214        }
    215
    216        @Override public int hashCode() {
    217          return f.hashCode() ^ g.hashCode();
    218        }
    219
    220        @Override public String toString() {
    221          return g.toString() + "(" + f.toString() + ")";
    222        }
    223
    224        private static final long serialVersionUID = 0;
    225      }
    226
    227      /**
    228       * Creates a function that returns the same boolean output as the given predicate for all inputs.
    229       *
    230       * <p>The returned function is <i>consistent with equals</i> (as documented at {@link
    231       * Function#apply}) if and only if {@code predicate} is itself consistent with equals.
    232       */
    233      public static <T> Function<T, Boolean> forPredicate(Predicate<T> predicate) {
    234        return new PredicateFunction<T>(predicate);
    235      }
    236
    237      /** @see Functions#forPredicate */
    238      private static class PredicateFunction<T> implements Function<T, Boolean>, Serializable {
    239        private final Predicate<T> predicate;
    240
    241        private PredicateFunction(Predicate<T> predicate) {
    242          this.predicate = checkNotNull(predicate);
    243        }
    244
    245        @Override
    246        public Boolean apply(T t) {
    247          return predicate.apply(t);
    248        }
    249
    250        @Override public boolean equals(@Nullable Object obj) {
    251          if (obj instanceof PredicateFunction) {
    252            PredicateFunction<?> that = (PredicateFunction<?>) obj;
    253            return predicate.equals(that.predicate);
    254          }
    255          return false;
    256        }
    257
    258        @Override public int hashCode() {
    259          return predicate.hashCode();
    260        }
    261
    262        @Override public String toString() {
    263          return "forPredicate(" + predicate + ")";
    264        }
    265
    266        private static final long serialVersionUID = 0;
    267      }
    268
    269      /**
    270       * Creates a function that returns {@code value} for any input.
    271       *
    272       * @param value the constant value for the function to return
    273       * @return a function that always returns {@code value}
    274       */
    275      public static <E> Function<Object, E> constant(@Nullable E value) {
    276        return new ConstantFunction<E>(value);
    277      }
    278
    279      private static class ConstantFunction<E> implements Function<Object, E>, Serializable {
    280        private final E value;
    281
    282        public ConstantFunction(@Nullable E value) {
    283          this.value = value;
    284        }
    285
    286        @Override
    287        public E apply(@Nullable Object from) {
    288          return value;
    289        }
    290
    291        @Override public boolean equals(@Nullable Object obj) {
    292          if (obj instanceof ConstantFunction) {
    293            ConstantFunction<?> that = (ConstantFunction<?>) obj;
    294            return Objects.equal(value, that.value);
    295          }
    296          return false;
    297        }
    298
    299        @Override public int hashCode() {
    300          return (value == null) ? 0 : value.hashCode();
    301        }
    302
    303        @Override public String toString() {
    304          return "constant(" + value + ")";
    305        }
    306
    307        private static final long serialVersionUID = 0;
    308      }
    309
    310      /**
    311       * Returns a function that always returns the result of invoking {@link Supplier#get} on {@code
    312       * supplier}, regardless of its input.
    313       *
    314       * @since 10.0
    315       */
    316      @Beta
    317      public static <T> Function<Object, T> forSupplier(Supplier<T> supplier) {
    318        return new SupplierFunction<T>(supplier);
    319      }
    320
    321      /** @see Functions#forSupplier*/
    322      private static class SupplierFunction<T> implements Function<Object, T>, Serializable {
    323
    324        private final Supplier<T> supplier;
    325
    326        private SupplierFunction(Supplier<T> supplier) {
    327          this.supplier = checkNotNull(supplier);
    328        }
    329
    330        @Override public T apply(@Nullable Object input) {
    331          return supplier.get();
    332        }
    333
    334        @Override public boolean equals(@Nullable Object obj) {
    335          if (obj instanceof SupplierFunction) {
    336            SupplierFunction<?> that = (SupplierFunction<?>) obj;
    337            return this.supplier.equals(that.supplier);
    338          }
    339          return false;
    340        }
    341
    342        @Override public int hashCode() {
    343          return supplier.hashCode();
    344        }
    345
    346        @Override public String toString() {
    347          return "forSupplier(" + supplier + ")";
    348        }
    349
    350        private static final long serialVersionUID = 0;
    351      }
    352    }