Objects.java

/*
 * Copyright 2009 Sun Microsystems, Inc.  All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Sun designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Sun in the LICENSE file that accompanied this code.
 *
 * This code 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
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 */

package java.util;

/**
 * This class consists of {@code static} utility methods for operating
 * on objects.  These utilities include {@code null}-safe or {@code
 * null}-tolerant methods for computing the hash code of an object,
 * returning a string for an object, and comparing two objects.
 *
 * @since 1.7
 */
public class Objects {
    private Objects() {
        throw new AssertionError("No java.util.Objects instances for you!");
    }

    /**
     * Returns {@code true} if the arguments are equal to each other
     * and {@code false} otherwise.
     * Consequently, if both arguments are {@code null}, {@code true}
     * is returned and if exactly one argument is {@code null}, {@code
     * false} is returned.  Otherwise, equality is determined by using
     * the {@link Object#equals equals} method of the first
     * argument.
     *
     * @param a an object
     * @param b an object to be compared with {@code a} for equality
     * @return {@code true} if the arguments are equal to each other
     * and {@code false} otherwise
     * @see Object#equals(Object)
     */
    public static boolean equals(Object a, Object b) {
        return (a == b) || (a != null && a.equals(b));
    }

    /**
     * Returns the hash code of a non-{@code null} argument and 0 for
     * a {@code null} argument.
     *
     * @param o an object
     * @return the hash code of a non-{@code null} argument and 0 for
     * a {@code null} argument
     * @see Object#hashCode
     */
    public static int hashCode(Object o) {
        return o != null ? o.hashCode() : 0;
    }

    /**
     * Returns the result of calling {@code toString} for a non-{@code
     * null} argument and {@code "null"} for a {@code null} argument.
     *
     * @param o an object
     * @return the result of calling {@code toString} for a non-{@code
     * null} argument and {@code "null"} for a {@code null} argument
     * @see Object#toString
     * @see String#valueOf(Object)
     */
    public static String toString(Object o) {
        return String.valueOf(o);
    }

    /**
     * Returns 0 if the arguments are identical and {@code
     * c.compare(a, b)} otherwise.
     * Consequently, if both arguments are {@code null} 0
     * is returned.
     *
     * <p>Note that if one of the arguments is {@code null}, a {@code
     * NullPointerException} may or may not be thrown depending on
     * what ordering policy, if any, the {@link Comparator Comparator}
     * chooses to have for {@code null} values.
     *
     * @param <T> the type of the objects being compared
     * @param a an object
     * @param b an object to be compared with {@code a}
     * @param c the {@code Comparator} to compare the first two arguments
     * @return 0 if the arguments are identical and {@code
     * c.compare(a, b)} otherwise.
     * @see Comparable
     * @see Comparator
     */
    public static <T> int compare(T a, T b, Comparator<? super T> c) {
        return (a == b) ? 0 :  c.compare(a, b);
    }

    /**
     * Checks that the specified object reference is not {@code null}. This
     * method is designed primarily for doing parameter validation in methods
     * and constructors, as demonstrated below:
     * <blockquote><pre>
     * public Foo(Bar bar) {
     *     this.bar = Objects.nonNull(bar);
     * }
     * </pre></blockquote>
     *
     * @param obj the object reference to check for nullity
     * @param <T> the type of the reference
     * @return {@code obj} if not {@code null}
     * @throws NullPointerException if {@code obj} is {@code null}
     */
    public static <T> T nonNull(T obj) {
        if (obj == null)
            throw new NullPointerException();
        return obj;
    }

    /**
     * Checks that the specified object reference is not {@code null} and
     * throws a customized {@link NullPointerException} if it is. This method
     * is designed primarily for doing parameter validation in methods and
     * constructors with multiple parameters, as demonstrated below:
     * <blockquote><pre>
     * public Foo(Bar bar, Baz baz) {
     *     this.bar = Objects.nonNull(bar, "bar must not be null");
     *     this.baz = Objects.nonNull(baz, "baz must not be null");
     * }
     * </pre></blockquote>
     *
     * @param obj     the object reference to check for nullity
     * @param message detail message to be used in the event that a {@code
     *                NullPointerException} is thrown
     * @param <T> the type of the reference
     * @return {@code obj} if not {@code null}
     * @throws NullPointerException if {@code obj} is {@code null}
     */
    public static <T> T nonNull(T obj, String message) {
        if (obj == null)
            throw new NullPointerException(message);
        return obj;
    }
}