Java tutorial
/* * Copyright (c) 2009, 2016, Oracle and/or its affiliates. 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. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.util; import jdk.internal.util.Preconditions; import jdk.internal.vm.annotation.ForceInline; import java.util.function.Supplier; /** * This class consists of {@code static} utility methods for operating * on objects, or checking certain conditions before operation. 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, comparing two * objects, and checking if indexes or sub-range values are out of bounds. * * @apiNote * Static methods such as {@link Objects#checkIndex}, * {@link Objects#checkFromToIndex}, and {@link Objects#checkFromIndexSize} are * provided for the convenience of checking if values corresponding to indexes * and sub-ranges are out of bounds. * Variations of these static methods support customization of the runtime * exception, and corresponding exception detail message, that is thrown when * values are out of bounds. Such methods accept a functional interface * argument, instances of {@code BiFunction}, that maps out-of-bound values to a * runtime exception. Care should be taken when using such methods in * combination with an argument that is a lambda expression, method reference or * class that capture values. In such cases the cost of capture, related to * functional interface allocation, may exceed the cost of checking bounds. * * @since 1.7 */ public final 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. Otherwise, if the first argument is not {@code * null}, equality is determined by calling the {@link * Object#equals equals} method of the first argument with the * second argument of this method. Otherwise, {@code false} is * returned. * * @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 {@code true} if the arguments are deeply equal to each other * and {@code false} otherwise. * * Two {@code null} values are deeply equal. If both arguments are * arrays, the algorithm in {@link Arrays#deepEquals(Object[], * Object[]) Arrays.deepEquals} is used to determine equality. * 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 deep equality * @return {@code true} if the arguments are deeply equal to each other * and {@code false} otherwise * @see Arrays#deepEquals(Object[], Object[]) * @see Objects#equals(Object, Object) */ public static boolean deepEquals(Object a, Object b) { if (a == b) return true; else if (a == null || b == null) return false; else return Arrays.deepEquals0(a, 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; } /** * Generates a hash code for a sequence of input values. The hash * code is generated as if all the input values were placed into an * array, and that array were hashed by calling {@link * Arrays#hashCode(Object[])}. * * <p>This method is useful for implementing {@link * Object#hashCode()} on objects containing multiple fields. For * example, if an object that has three fields, {@code x}, {@code * y}, and {@code z}, one could write: * * <blockquote><pre> * @Override public int hashCode() { * return Objects.hash(x, y, z); * } * </pre></blockquote> * * <b>Warning: When a single object reference is supplied, the returned * value does not equal the hash code of that object reference.</b> This * value can be computed by calling {@link #hashCode(Object)}. * * @param values the values to be hashed * @return a hash value of the sequence of input values * @see Arrays#hashCode(Object[]) * @see List#hashCode */ public static int hash(Object... values) { return Arrays.hashCode(values); } /** * 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 the result of calling {@code toString} on the first * argument if the first argument is not {@code null} and returns * the second argument otherwise. * * @param o an object * @param nullDefault string to return if the first argument is * {@code null} * @return the result of calling {@code toString} on the first * argument if it is not {@code null} and the second argument * otherwise. * @see Objects#toString(Object) */ public static String toString(Object o, String nullDefault) { return (o != null) ? o.toString() : nullDefault; } /** * 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.requireNonNull(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 requireNonNull(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.requireNonNull(bar, "bar must not be null"); * this.baz = Objects.requireNonNull(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 requireNonNull(T obj, String message) { if (obj == null) throw new NullPointerException(message); return obj; } /** * Returns {@code true} if the provided reference is {@code null} otherwise * returns {@code false}. * * @apiNote This method exists to be used as a * {@link java.util.function.Predicate}, {@code filter(Objects::isNull)} * * @param obj a reference to be checked against {@code null} * @return {@code true} if the provided reference is {@code null} otherwise * {@code false} * * @see java.util.function.Predicate * @since 1.8 */ public static boolean isNull(Object obj) { return obj == null; } /** * Returns {@code true} if the provided reference is non-{@code null} * otherwise returns {@code false}. * * @apiNote This method exists to be used as a * {@link java.util.function.Predicate}, {@code filter(Objects::nonNull)} * * @param obj a reference to be checked against {@code null} * @return {@code true} if the provided reference is non-{@code null} * otherwise {@code false} * * @see java.util.function.Predicate * @since 1.8 */ public static boolean nonNull(Object obj) { return obj != null; } /** * Returns the first argument if it is non-{@code null} and * otherwise returns the non-{@code null} second argument. * * @param obj an object * @param defaultObj a non-{@code null} object to return if the first argument * is {@code null} * @param <T> the type of the reference * @return the first argument if it is non-{@code null} and * otherwise the second argument if it is non-{@code null} * @throws NullPointerException if both {@code obj} is null and * {@code defaultObj} is {@code null} * @since 9 */ public static <T> T requireNonNullElse(T obj, T defaultObj) { return (obj != null) ? obj : requireNonNull(defaultObj, "defaultObj"); } /** * Returns the first argument if it is non-{@code null} and otherwise * returns the non-{@code null} value of {@code supplier.get()}. * * @param obj an object * @param supplier of a non-{@code null} object to return if the first argument * is {@code null} * @param <T> the type of the first argument and return type * @return the first argument if it is non-{@code null} and otherwise * the value from {@code supplier.get()} if it is non-{@code null} * @throws NullPointerException if both {@code obj} is null and * either the {@code supplier} is {@code null} or * the {@code supplier.get()} value is {@code null} * @since 9 */ public static <T> T requireNonNullElseGet(T obj, Supplier<? extends T> supplier) { return (obj != null) ? obj : requireNonNull(requireNonNull(supplier, "supplier").get(), "supplier.get()"); } /** * Checks that the specified object reference is not {@code null} and * throws a customized {@link NullPointerException} if it is. * * <p>Unlike the method {@link #requireNonNull(Object, String)}, * this method allows creation of the message to be deferred until * after the null check is made. While this may confer a * performance advantage in the non-null case, when deciding to * call this method care should be taken that the costs of * creating the message supplier are less than the cost of just * creating the string message directly. * * @param obj the object reference to check for nullity * @param messageSupplier supplier of the 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} * @since 1.8 */ public static <T> T requireNonNull(T obj, Supplier<String> messageSupplier) { if (obj == null) throw new NullPointerException(messageSupplier == null ? null : messageSupplier.get()); return obj; } /** * Checks if the {@code index} is within the bounds of the range from * {@code 0} (inclusive) to {@code length} (exclusive). * * <p>The {@code index} is defined to be out of bounds if any of the * following inequalities is true: * <ul> * <li>{@code index < 0}</li> * <li>{@code index >= length}</li> * <li>{@code length < 0}, which is implied from the former inequalities</li> * </ul> * * @param index the index * @param length the upper-bound (exclusive) of the range * @return {@code index} if it is within bounds of the range * @throws IndexOutOfBoundsException if the {@code index} is out of bounds * @since 9 */ @ForceInline public static int checkIndex(int index, int length) { return Preconditions.checkIndex(index, length, null); } /** * Checks if the sub-range from {@code fromIndex} (inclusive) to * {@code toIndex} (exclusive) is within the bounds of range from {@code 0} * (inclusive) to {@code length} (exclusive). * * <p>The sub-range is defined to be out of bounds if any of the following * inequalities is true: * <ul> * <li>{@code fromIndex < 0}</li> * <li>{@code fromIndex > toIndex}</li> * <li>{@code toIndex > length}</li> * <li>{@code length < 0}, which is implied from the former inequalities</li> * </ul> * * @param fromIndex the lower-bound (inclusive) of the sub-range * @param toIndex the upper-bound (exclusive) of the sub-range * @param length the upper-bound (exclusive) the range * @return {@code fromIndex} if the sub-range within bounds of the range * @throws IndexOutOfBoundsException if the sub-range is out of bounds * @since 9 */ public static int checkFromToIndex(int fromIndex, int toIndex, int length) { return Preconditions.checkFromToIndex(fromIndex, toIndex, length, null); } /** * Checks if the sub-range from {@code fromIndex} (inclusive) to * {@code fromIndex + size} (exclusive) is within the bounds of range from * {@code 0} (inclusive) to {@code length} (exclusive). * * <p>The sub-range is defined to be out of bounds if any of the following * inequalities is true: * <ul> * <li>{@code fromIndex < 0}</li> * <li>{@code size < 0}</li> * <li>{@code fromIndex + size > length}, taking into account integer overflow</li> * <li>{@code length < 0}, which is implied from the former inequalities</li> * </ul> * * @param fromIndex the lower-bound (inclusive) of the sub-interval * @param size the size of the sub-range * @param length the upper-bound (exclusive) of the range * @return {@code fromIndex} if the sub-range within bounds of the range * @throws IndexOutOfBoundsException if the sub-range is out of bounds * @since 9 */ public static int checkFromIndexSize(int fromIndex, int size, int length) { return Preconditions.checkFromIndexSize(fromIndex, size, length, null); } }