    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.lang3.tuple;
    018
    019import java.io.Serializable;
    020import java.util.Map;
    021import java.util.Objects;
    022
    023import org.apache.commons.lang3.builder.CompareToBuilder;
    024
    025/**
    026 * <p>A pair consisting of two elements.</p>
    027 *
    028 * <p>This class is an abstract implementation defining the basic API.
    029 * It refers to the elements as 'left' and 'right'. It also implements the
    030 * {@code Map.Entry} interface where the key is 'left' and the value is 'right'.</p>
    031 *
    032 * <p>Subclass implementations may be mutable or immutable.
    033 * However, there is no restriction on the type of the stored objects that may be stored.
    034 * If mutable objects are stored in the pair, then the pair itself effectively becomes mutable.</p>
    035 *
    036 * @param <L> the left element type
    037 * @param <R> the right element type
    038 *
    039 * @since 3.0
    040 */
    041public abstract class Pair<L, R> implements Map.Entry<L, R>, Comparable<Pair<L, R>>, Serializable {
    042
    043    private static final class PairAdapter<L, R> extends Pair<L, R> {
    044
    045        private static final long serialVersionUID = 1L;
    046
    047        @Override
    048        public L getLeft() {
    049            return null;
    050        }
    051
    052        @Override
    053        public R getRight() {
    054            return null;
    055        }
    056
    057        @Override
    058        public R setValue(final R value) {
    059            return null;
    060        }
    061
    062    }
    063
    064    /** Serialization version */
    065    private static final long serialVersionUID = 4954918890077093841L;
    066
    067    /**
    068     * An empty array.
    069     * <p>
    070     * Consider using {@link #emptyArray()} to avoid generics warnings.
    071     * </p>
    072     *
    073     * @since 3.10.
    074     */
    075    public static final Pair<?, ?>[] EMPTY_ARRAY = new PairAdapter[0];
    076
    077    /**
    078     * Returns the empty array singleton that can be assigned without compiler warning.
    079     *
    080     * @param <L> the left element type
    081     * @param <R> the right element type
    082     * @return the empty array singleton that can be assigned without compiler warning.
    083     *
    084     * @since 3.10.
    085     */
    086    @SuppressWarnings("unchecked")
    087    public static <L, R> Pair<L, R>[] emptyArray() {
    088        return (Pair<L, R>[]) EMPTY_ARRAY;
    089    }
    090
    091    /**
    092     * <p>Creates an immutable pair of two objects inferring the generic types.</p>
    093     *
    094     * <p>This factory allows the pair to be created using inference to
    095     * obtain the generic types.</p>
    096     *
    097     * @param <L> the left element type
    098     * @param <R> the right element type
    099     * @param left  the left element, may be null
    100     * @param right  the right element, may be null
    101     * @return a pair formed from the two parameters, not null
    102     */
    103    public static <L, R> Pair<L, R> of(final L left, final R right) {
    104        return ImmutablePair.of(left, right);
    105    }
    106
    107    /**
    108     * <p>Creates an immutable pair from an existing pair.</p>
    109     *
    110     * <p>This factory allows the pair to be created using inference to
    111     * obtain the generic types.</p>
    112     *
    113     * @param <L> the left element type
    114     * @param <R> the right element type
    115     * @param pair the existing pair.
    116     * @return a pair formed from the two parameters, not null
    117     * @since 3.10
    118     */
    119    public static <L, R> Pair<L, R> of(final Map.Entry<L, R> pair) {
    120        return ImmutablePair.of(pair);
    121    }
    122
    123    //-----------------------------------------------------------------------
    124    /**
    125     * <p>Compares the pair based on the left element followed by the right element.
    126     * The types must be {@code Comparable}.</p>
    127     *
    128     * @param other  the other pair, not null
    129     * @return negative if this is less, zero if equal, positive if greater
    130     */
    131    @Override
    132    public int compareTo(final Pair<L, R> other) {
    133      return new CompareToBuilder().append(getLeft(), other.getLeft())
    134              .append(getRight(), other.getRight()).toComparison();
    135    }
    136
    137    /**
    138     * <p>Compares this pair to another based on the two elements.</p>
    139     *
    140     * @param obj  the object to compare to, null returns false
    141     * @return true if the elements of the pair are equal
    142     */
    143    @Override
    144    public boolean equals(final Object obj) {
    145        if (obj == this) {
    146            return true;
    147        }
    148        if (obj instanceof Map.Entry<?, ?>) {
    149            final Map.Entry<?, ?> other = (Map.Entry<?, ?>) obj;
    150            return Objects.equals(getKey(), other.getKey())
    151                    && Objects.equals(getValue(), other.getValue());
    152        }
    153        return false;
    154    }
    155
    156    /**
    157     * <p>Gets the key from this pair.</p>
    158     *
    159     * <p>This method implements the {@code Map.Entry} interface returning the
    160     * left element as the key.</p>
    161     *
    162     * @return the left element as the key, may be null
    163     */
    164    @Override
    165    public final L getKey() {
    166        return getLeft();
    167    }
    168
    169    //-----------------------------------------------------------------------
    170    /**
    171     * <p>Gets the left element from this pair.</p>
    172     *
    173     * <p>When treated as a key-value pair, this is the key.</p>
    174     *
    175     * @return the left element, may be null
    176     */
    177    public abstract L getLeft();
    178
    179    /**
    180     * <p>Gets the right element from this pair.</p>
    181     *
    182     * <p>When treated as a key-value pair, this is the value.</p>
    183     *
    184     * @return the right element, may be null
    185     */
    186    public abstract R getRight();
    187
    188    /**
    189     * <p>Gets the value from this pair.</p>
    190     *
    191     * <p>This method implements the {@code Map.Entry} interface returning the
    192     * right element as the value.</p>
    193     *
    194     * @return the right element as the value, may be null
    195     */
    196    @Override
    197    public R getValue() {
    198        return getRight();
    199    }
    200
    201    /**
    202     * <p>Returns a suitable hash code.
    203     * The hash code follows the definition in {@code Map.Entry}.</p>
    204     *
    205     * @return the hash code
    206     */
    207    @Override
    208    public int hashCode() {
    209        // see Map.Entry API specification
    210        return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
    211    }
    212
    213    /**
    214     * <p>Returns a String representation of this pair using the format {@code ($left,$right)}.</p>
    215     *
    216     * @return a string describing this object, not null
    217     */
    218    @Override
    219    public String toString() {
    220        return "(" + getLeft() + ',' + getRight() + ')';
    221    }
    222
    223    /**
    224     * <p>Formats the receiver using the given format.</p>
    225     *
    226     * <p>This uses {@link java.util.Formattable} to perform the formatting. Two variables may
    227     * be used to embed the left and right elements. Use {@code %1$s} for the left
    228     * element (key) and {@code %2$s} for the right element (value).
    229     * The default format used by {@code toString()} is {@code (%1$s,%2$s)}.</p>
    230     *
    231     * @param format  the format string, optionally containing {@code %1$s} and {@code %2$s}, not null
    232     * @return the formatted string, not null
    233     */
    234    public String toString(final String format) {
    235        return String.format(format, getLeft(), getRight());
    236    }
    237
    238}