/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.commons.lang3.tuple;

import java.io.Serializable;
import java.util.Map;
import java.util.Objects;

import org.apache.commons.lang3.builder.CompareToBuilder;

A pair consisting of two elements.
This class is an abstract implementation defining the basic API. It refers to the elements as 'left' and 'right'. It also implements the Map.Entry interface where the key is 'left' and the value is 'right'.
Subclass implementations may be mutable or immutable.
However, there is no restriction on the type of the stored objects that may be stored.
If mutable objects are stored in the pair, then the pair itself effectively becomes mutable.
Type parameters:
<L> – the left element type
<R> – the right element type
Since:
3.0/**
 * <p>A pair consisting of two elements.</p>
 *
 * <p>This class is an abstract implementation defining the basic API.
 * It refers to the elements as 'left' and 'right'. It also implements the
 * {@code Map.Entry} interface where the key is 'left' and the value is 'right'.</p>
 *
 * <p>Subclass implementations may be mutable or immutable.
 * However, there is no restriction on the type of the stored objects that may be stored.
 * If mutable objects are stored in the pair, then the pair itself effectively becomes mutable.</p>
 *
 * @param <L> the left element type
 * @param <R> the right element type
 *
 * @since 3.0
 */
public abstract class Pair<L, R> implements Map.Entry<L, R>, Comparable<Pair<L, R>>, Serializable {

    private static final class PairAdapter<L, R> extends Pair<L, R> {

        private static final long serialVersionUID = 1L;

        @Override
        public L getLeft() {
            return null;
        }

        @Override
        public R getRight() {
            return null;
        }

        @Override
        public R setValue(final R value) {
            return null;
        }

    }

    
Serialization version /** Serialization version */
    private static final long serialVersionUID = 4954918890077093841L;

    
An empty array.
 Consider using emptyArray() to avoid generics warnings. 
Since:
3.10./**
     * An empty array.
     * <p>
     * Consider using {@link #emptyArray()} to avoid generics warnings.
     * </p>
     *
     * @since 3.10.
     */
    public static final Pair<?, ?>[] EMPTY_ARRAY = new PairAdapter[0];

    
Returns the empty array singleton that can be assigned without compiler warning.
Type parameters:
<L> – the left element type
<R> – the right element type
Returns:
the empty array singleton that can be assigned without compiler warning.
Since:
3.10./**
     * Returns the empty array singleton that can be assigned without compiler warning.
     *
     * @param <L> the left element type
     * @param <R> the right element type
     * @return the empty array singleton that can be assigned without compiler warning.
     *
     * @since 3.10.
     */
    @SuppressWarnings("unchecked")
    public static <L, R> Pair<L, R>[] emptyArray() {
        return (Pair<L, R>[]) EMPTY_ARRAY;
    }

    
Creates an immutable pair of two objects inferring the generic types.
This factory allows the pair to be created using inference to
obtain the generic types.
Params:
left –  the left element, may be null
right –  the right element, may be null
Type parameters:
<L> – the left element type
<R> – the right element type
Returns:
a pair formed from the two parameters, not null/**
     * <p>Creates an immutable pair of two objects inferring the generic types.</p>
     *
     * <p>This factory allows the pair to be created using inference to
     * obtain the generic types.</p>
     *
     * @param <L> the left element type
     * @param <R> the right element type
     * @param left  the left element, may be null
     * @param right  the right element, may be null
     * @return a pair formed from the two parameters, not null
     */
    public static <L, R> Pair<L, R> of(final L left, final R right) {
        return ImmutablePair.of(left, right);
    }

    
Creates an immutable pair from an existing pair.
This factory allows the pair to be created using inference to
obtain the generic types.
Params:
pair – the existing pair.
Type parameters:
<L> – the left element type
<R> – the right element type
Returns:
a pair formed from the two parameters, not null
Since:
3.10/**
     * <p>Creates an immutable pair from an existing pair.</p>
     *
     * <p>This factory allows the pair to be created using inference to
     * obtain the generic types.</p>
     *
     * @param <L> the left element type
     * @param <R> the right element type
     * @param pair the existing pair.
     * @return a pair formed from the two parameters, not null
     * @since 3.10
     */
    public static <L, R> Pair<L, R> of(final Map.Entry<L, R> pair) {
        return ImmutablePair.of(pair);
    }

    //-----------------------------------------------------------------------
    
Compares the pair based on the left element followed by the right element. The types must be Comparable.
Params:
other –  the other pair, not null
Returns:
negative if this is less, zero if equal, positive if greater/**
     * <p>Compares the pair based on the left element followed by the right element.
     * The types must be {@code Comparable}.</p>
     *
     * @param other  the other pair, not null
     * @return negative if this is less, zero if equal, positive if greater
     */
    @Override
    public int compareTo(final Pair<L, R> other) {
      return new CompareToBuilder().append(getLeft(), other.getLeft())
              .append(getRight(), other.getRight()).toComparison();
    }

    
Compares this pair to another based on the two elements.
Params:
obj –  the object to compare to, null returns false
Returns:
true if the elements of the pair are equal/**
     * <p>Compares this pair to another based on the two elements.</p>
     *
     * @param obj  the object to compare to, null returns false
     * @return true if the elements of the pair are equal
     */
    @Override
    public boolean equals(final Object obj) {
        if (obj == this) {
            return true;
        }
        if (obj instanceof Map.Entry<?, ?>) {
            final Map.Entry<?, ?> other = (Map.Entry<?, ?>) obj;
            return Objects.equals(getKey(), other.getKey())
                    && Objects.equals(getValue(), other.getValue());
        }
        return false;
    }

    
Gets the key from this pair.
This method implements the Map.Entry interface returning the left element as the key.
Returns:
the left element as the key, may be null/**
     * <p>Gets the key from this pair.</p>
     *
     * <p>This method implements the {@code Map.Entry} interface returning the
     * left element as the key.</p>
     *
     * @return the left element as the key, may be null
     */
    @Override
    public final L getKey() {
        return getLeft();
    }

    //-----------------------------------------------------------------------
    
Gets the left element from this pair.
When treated as a key-value pair, this is the key.
Returns:
the left element, may be null/**
     * <p>Gets the left element from this pair.</p>
     *
     * <p>When treated as a key-value pair, this is the key.</p>
     *
     * @return the left element, may be null
     */
    public abstract L getLeft();

    
Gets the right element from this pair.
When treated as a key-value pair, this is the value.
Returns:
the right element, may be null/**
     * <p>Gets the right element from this pair.</p>
     *
     * <p>When treated as a key-value pair, this is the value.</p>
     *
     * @return the right element, may be null
     */
    public abstract R getRight();

    
Gets the value from this pair.
This method implements the Map.Entry interface returning the right element as the value.
Returns:
the right element as the value, may be null/**
     * <p>Gets the value from this pair.</p>
     *
     * <p>This method implements the {@code Map.Entry} interface returning the
     * right element as the value.</p>
     *
     * @return the right element as the value, may be null
     */
    @Override
    public R getValue() {
        return getRight();
    }

    
Returns a suitable hash code. The hash code follows the definition in Map.Entry.
Returns:
the hash code/**
     * <p>Returns a suitable hash code.
     * The hash code follows the definition in {@code Map.Entry}.</p>
     *
     * @return the hash code
     */
    @Override
    public int hashCode() {
        // see Map.Entry API specification
        return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
    }

    
Returns a String representation of this pair using the format ($left,$right).
Returns:
a string describing this object, not null/**
     * <p>Returns a String representation of this pair using the format {@code ($left,$right)}.</p>
     *
     * @return a string describing this object, not null
     */
    @Override
    public String toString() {
        return "(" + getLeft() + ',' + getRight() + ')';
    }

    
Formats the receiver using the given format.
This uses Formattable to perform the formatting. Two variables may be used to embed the left and right elements. Use %1$s for the left element (key) and %2$s for the right element (value). The default format used by toString() is (%1$s,%2$s).
Params:
format –  the format string, optionally containing %1$s and %2$s, not null
Returns:
the formatted string, not null/**
     * <p>Formats the receiver using the given format.</p>
     *
     * <p>This uses {@link java.util.Formattable} to perform the formatting. Two variables may
     * be used to embed the left and right elements. Use {@code %1$s} for the left
     * element (key) and {@code %2$s} for the right element (value).
     * The default format used by {@code toString()} is {@code (%1$s,%2$s)}.</p>
     *
     * @param format  the format string, optionally containing {@code %1$s} and {@code %2$s}, not null
     * @return the formatted string, not null
     */
    public String toString(final String format) {
        return String.format(format, getLeft(), getRight());
    }

}