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1: /* Comparator.java -- Interface for objects that specify an ordering 2: Copyright (C) 1998, 2001, 2004, 2005 Free Software Foundation, Inc. 3: 4: This file is part of GNU Classpath. 5: 6: GNU Classpath is free software; you can redistribute it and/or modify 7: it under the terms of the GNU General Public License as published by 8: the Free Software Foundation; either version 2, or (at your option) 9: any later version. 10: 11: GNU Classpath is distributed in the hope that it will be useful, but 12: WITHOUT ANY WARRANTY; without even the implied warranty of 13: MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 14: General Public License for more details. 15: 16: You should have received a copy of the GNU General Public License 17: along with GNU Classpath; see the file COPYING. If not, write to the 18: Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 19: 02110-1301 USA. 20: 21: Linking this library statically or dynamically with other modules is 22: making a combined work based on this library. Thus, the terms and 23: conditions of the GNU General Public License cover the whole 24: combination. 25: 26: As a special exception, the copyright holders of this library give you 27: permission to link this library with independent modules to produce an 28: executable, regardless of the license terms of these independent 29: modules, and to copy and distribute the resulting executable under 30: terms of your choice, provided that you also meet, for each linked 31: independent module, the terms and conditions of the license of that 32: module. An independent module is a module which is not derived from 33: or based on this library. If you modify this library, you may extend 34: this exception to your version of the library, but you are not 35: obligated to do so. If you do not wish to do so, delete this 36: exception statement from your version. */ 37: 38: 39: package java.util; 40: 41: /** 42: * Interface for objects that specify an ordering between objects. The ordering 43: * should be <em>total</em>, such that any two objects of the correct type 44: * can be compared, and the comparison is reflexive, anti-symmetric, and 45: * transitive. It is also recommended that the comparator be <em>consistent 46: * with equals</em>, although this is not a strict requirement. A relation 47: * is consistent with equals if these two statements always have the same 48: * results (if no exceptions occur):<br> 49: * <code>compare((Object) e1, (Object) e2) == 0</code> and 50: * <code>e1.equals((Object) e2)</code><br> 51: * Comparators that violate consistency with equals may cause strange behavior 52: * in sorted lists and sets. For example, a case-sensitive dictionary order 53: * comparison of Strings is consistent with equals, but if it is 54: * case-insensitive it is not, because "abc" and "ABC" compare as equal even 55: * though "abc".equals("ABC") returns false. 56: * <P> 57: * In general, Comparators should be Serializable, because when they are passed 58: * to Serializable data structures such as SortedMap or SortedSet, the entire 59: * data structure will only serialize correctly if the comparator is 60: * Serializable. 61: * 62: * @author Original author unknown 63: * @author Eric Blake (ebb9@email.byu.edu) 64: * @see Comparable 65: * @see TreeMap 66: * @see TreeSet 67: * @see SortedMap 68: * @see SortedSet 69: * @see Arrays#sort(Object[], Comparator) 70: * @see java.io.Serializable 71: * @since 1.2 72: * @status updated to 1.4 73: */ 74: public interface Comparator<T> 75: { 76: /** 77: * Return an integer that is negative, zero or positive depending on whether 78: * the first argument is less than, equal to or greater than the second 79: * according to this ordering. This method should obey the following 80: * contract: 81: * <ul> 82: * <li>if compare(a, b) < 0 then compare(b, a) > 0</li> 83: * <li>if compare(a, b) throws an exception, so does compare(b, a)</li> 84: * <li>if compare(a, b) < 0 and compare(b, c) < 0 then compare(a, c) 85: * < 0</li> 86: * <li>if compare(a, b) == 0 then compare(a, c) and compare(b, c) must 87: * have the same sign</li> 88: * </ul> 89: * To be consistent with equals, the following additional constraint is 90: * in place: 91: * <ul> 92: * <li>if a.equals(b) or both a and b are null, then 93: * compare(a, b) == 0.</li> 94: * </ul><p> 95: * 96: * Although it is permissible for a comparator to provide an order 97: * inconsistent with equals, that should be documented. 98: * 99: * @param o1 the first object 100: * @param o2 the second object 101: * @return the comparison 102: * @throws ClassCastException if the elements are not of types that can be 103: * compared by this ordering. 104: */ 105: int compare(T o1, T o2); 106: 107: /** 108: * Return true if the object is equal to this object. To be 109: * considered equal, the argument object must satisfy the constraints 110: * of <code>Object.equals()</code>, be a Comparator, and impose the 111: * same ordering as this Comparator. The default implementation 112: * inherited from Object is usually adequate. 113: * 114: * @param obj The object 115: * @return true if it is a Comparator that imposes the same order 116: * @see Object#equals(Object) 117: */ 118: boolean equals(Object obj); 119: }