Pair.java example

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/* This file is part of VoltDB.
 * Copyright (C) 2008-2010 VoltDB L.L.C.
 *
 * VoltDB is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * VoltDB is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with VoltDB.  If not, see <http://www.gnu.org/licenses/>.
 */

package org.voltdb.utils;

/**
 * Class representing a pair of generic-ized types. Supports equality, hashing
 * and all that other nice Java stuff. Based on STL's pair class in C++.
 *
 */
public class Pair<T, U> implements Comparable<Pair<T, U>> {

    private final T m_first;
    private final U m_second;
    private transient int m_hash;

    public Pair(T first, U second, boolean precomputeHash) {
        m_first = first;
        m_second = second;
        m_hash = (precomputeHash ? this.computeHashCode() : Integer.MIN_VALUE);
    }

    public Pair(T first, U second) {
        this(first, second, true);
    }

    protected int computeHashCode() {
        return (m_first == null ? 0 : m_first.hashCode() * 31) +
               (m_second == null ? 0 : m_second.hashCode());
    }
    
    @Override
    public final int hashCode() {
        if (m_hash != Integer.MIN_VALUE) return (m_hash);
        return (this.computeHashCode());
    }

    public String toString() {
        return String.format("<%s, %s>", m_first, m_second);
    }
    
    @Override
    public int compareTo(Pair<T, U> other) {
        return (other.m_hash - this.m_hash);
    }
    
    public Object get(int idx) {
        if (idx == 0) return m_first;
        else if (idx == 1) return m_second;
        return null;
    }

    /**
     * @param o Object to compare to.
     * @return Is the object equal to a value in the pair.
     */
    public boolean contains(Object o) {
        if ((m_first != null) && (m_first.equals(o))) return true;
        if ((m_second != null) && (m_second.equals(o))) return true;
        if (o != null) return false;
        return ((m_first == null) || (m_second == null));
    }

    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || !(getClass().isInstance(o))) {
            return false;
        }

        @SuppressWarnings("unchecked")
        Pair<T, U> other = (Pair<T, U>) o;

        return (m_first == null ? other.m_first == null : m_first.equals(other.m_first))
                && (m_second == null ? other.m_second == null : m_second.equals(other.m_second));
    }

    /**
     * @return the first
     */
    public T getFirst() {
        return m_first;
    }

    /**
     * @return the second
     */
    public U getSecond() {
        return m_second;
    }

    /**
     * Convenience class method for constructing pairs using Java's generic type
     * inference.
     */
    public static <T, U> Pair<T, U> of(T x, U y) {
        return new Pair<T, U>(x, y, true);
    }
}