Java tutorial
/* * Copyright (c) 1997, 2018, 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.lang; import java.util.*; /** * This interface imposes a total ordering on the objects of each class that * implements it. This ordering is referred to as the class's <i>natural * ordering</i>, and the class's {@code compareTo} method is referred to as * its <i>natural comparison method</i>.<p> * * Lists (and arrays) of objects that implement this interface can be sorted * automatically by {@link Collections#sort(List) Collections.sort} (and * {@link Arrays#sort(Object[]) Arrays.sort}). Objects that implement this * interface can be used as keys in a {@linkplain SortedMap sorted map} or as * elements in a {@linkplain SortedSet sorted set}, without the need to * specify a {@linkplain Comparator comparator}.<p> * * The natural ordering for a class {@code C} is said to be <i>consistent * with equals</i> if and only if {@code e1.compareTo(e2) == 0} has * the same boolean value as {@code e1.equals(e2)} for every * {@code e1} and {@code e2} of class {@code C}. Note that {@code null} * is not an instance of any class, and {@code e.compareTo(null)} should * throw a {@code NullPointerException} even though {@code e.equals(null)} * returns {@code false}.<p> * * It is strongly recommended (though not required) that natural orderings be * consistent with equals. This is so because sorted sets (and sorted maps) * without explicit comparators behave "strangely" when they are used with * elements (or keys) whose natural ordering is inconsistent with equals. In * particular, such a sorted set (or sorted map) violates the general contract * for set (or map), which is defined in terms of the {@code equals} * method.<p> * * For example, if one adds two keys {@code a} and {@code b} such that * {@code (!a.equals(b) && a.compareTo(b) == 0)} to a sorted * set that does not use an explicit comparator, the second {@code add} * operation returns false (and the size of the sorted set does not increase) * because {@code a} and {@code b} are equivalent from the sorted set's * perspective.<p> * * Virtually all Java core classes that implement {@code Comparable} have natural * orderings that are consistent with equals. One exception is * {@code java.math.BigDecimal}, whose natural ordering equates * {@code BigDecimal} objects with equal values and different precisions * (such as 4.0 and 4.00).<p> * * For the mathematically inclined, the <i>relation</i> that defines * the natural ordering on a given class C is:<pre>{@code * {(x, y) such that x.compareTo(y) <= 0}. * }</pre> The <i>quotient</i> for this total order is: <pre>{@code * {(x, y) such that x.compareTo(y) == 0}. * }</pre> * * It follows immediately from the contract for {@code compareTo} that the * quotient is an <i>equivalence relation</i> on {@code C}, and that the * natural ordering is a <i>total order</i> on {@code C}. When we say that a * class's natural ordering is <i>consistent with equals</i>, we mean that the * quotient for the natural ordering is the equivalence relation defined by * the class's {@link Object#equals(Object) equals(Object)} method:<pre> * {(x, y) such that x.equals(y)}. </pre><p> * * This interface is a member of the * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> * Java Collections Framework</a>. * * @param <T> the type of objects that this object may be compared to * * @author Josh Bloch * @see java.util.Comparator * @since 1.2 */ public interface Comparable<T> { /** * Compares this object with the specified object for order. Returns a * negative integer, zero, or a positive integer as this object is less * than, equal to, or greater than the specified object. * * <p>The implementor must ensure * {@code sgn(x.compareTo(y)) == -sgn(y.compareTo(x))} * for all {@code x} and {@code y}. (This * implies that {@code x.compareTo(y)} must throw an exception iff * {@code y.compareTo(x)} throws an exception.) * * <p>The implementor must also ensure that the relation is transitive: * {@code (x.compareTo(y) > 0 && y.compareTo(z) > 0)} implies * {@code x.compareTo(z) > 0}. * * <p>Finally, the implementor must ensure that {@code x.compareTo(y)==0} * implies that {@code sgn(x.compareTo(z)) == sgn(y.compareTo(z))}, for * all {@code z}. * * <p>It is strongly recommended, but <i>not</i> strictly required that * {@code (x.compareTo(y)==0) == (x.equals(y))}. Generally speaking, any * class that implements the {@code Comparable} interface and violates * this condition should clearly indicate this fact. The recommended * language is "Note: this class has a natural ordering that is * inconsistent with equals." * * <p>In the foregoing description, the notation * {@code sgn(}<i>expression</i>{@code )} designates the mathematical * <i>signum</i> function, which is defined to return one of {@code -1}, * {@code 0}, or {@code 1} according to whether the value of * <i>expression</i> is negative, zero, or positive, respectively. * * @param o the object to be compared. * @return a negative integer, zero, or a positive integer as this object * is less than, equal to, or greater than the specified object. * * @throws NullPointerException if the specified object is null * @throws ClassCastException if the specified object's type prevents it * from being compared to this object. */ public int compareTo(T o); }