| /* |
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| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
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| * 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). |
| * |
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| |
| 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 {@link java.math.BigDecimal}, whose {@linkplain |
| * java.math.BigDecimal#compareTo natural ordering} equates {@code |
| * BigDecimal} objects with equal numerical values and different |
| * representations (such as 4.0 and 4.00). For {@link |
| * java.math.BigDecimal#equals BigDecimal.equals()} to return true, |
| * the representation and numerical value of the two {@code |
| * BigDecimal} objects must be the same.<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> |
| * |
| * In other words, when a class's natural ordering is consistent with |
| * equals, the equivalence classes defined by the equivalence relation |
| * of the {@code equals} method and the equivalence classes defined by |
| * the quotient of the {@code compareTo} method are the same. |
| * |
| * <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 {@link Integer#signum |
| * signum}{@code (x.compareTo(y)) == -signum(y.compareTo(x))} for |
| * all {@code x} and {@code y}. (This implies that {@code |
| * x.compareTo(y)} must throw an exception if and only if {@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 signum(x.compareTo(z)) |
| * == signum(y.compareTo(z))}, for all {@code z}. |
| * |
| * @apiNote |
| * 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." |
| * |
| * @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); |
| } |
