1 /*
2 * Copyright 2015 The Netty Project
3 *
4 * The Netty Project licenses this file to you under the Apache License,
5 * version 2.0 (the "License"); you may not use this file except in compliance
6 * with the License. You may obtain a copy of the License at:
7 *
8 * https://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
12 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
13 * License for the specific language governing permissions and limitations
14 * under the License.
15 */
16 package io.netty.util;
17
18 /**
19 * Abstraction for hash code generation and equality comparison.
20 */
21 public interface HashingStrategy<T> {
22 /**
23 * Generate a hash code for {@code obj}.
24 * <p>
25 * This method must obey the same relationship that {@link java.lang.Object#hashCode()} has with
26 * {@link java.lang.Object#equals(Object)}:
27 * <ul>
28 * <li>Calling this method multiple times with the same {@code obj} should return the same result</li>
29 * <li>If {@link #equals(Object, Object)} with parameters {@code a} and {@code b} returns {@code true}
30 * then the return value for this method for parameters {@code a} and {@code b} must return the same result</li>
31 * <li>If {@link #equals(Object, Object)} with parameters {@code a} and {@code b} returns {@code false}
32 * then the return value for this method for parameters {@code a} and {@code b} does <strong>not</strong> have to
33 * return different results results. However this property is desirable.</li>
34 * <li>if {@code obj} is {@code null} then this method return {@code 0}</li>
35 * </ul>
36 */
37 int hashCode(T obj);
38
39 /**
40 * Returns {@code true} if the arguments are equal to each other and {@code false} otherwise.
41 * This method has the following restrictions:
42 * <ul>
43 * <li><i>reflexive</i> - {@code equals(a, a)} should return true</li>
44 * <li><i>symmetric</i> - {@code equals(a, b)} returns {@code true} if {@code equals(b, a)} returns
45 * {@code true}</li>
46 * <li><i>transitive</i> - if {@code equals(a, b)} returns {@code true} and {@code equals(a, c)} returns
47 * {@code true} then {@code equals(b, c)} should also return {@code true}</li>
48 * <li><i>consistent</i> - {@code equals(a, b)} should return the same result when called multiple times
49 * assuming {@code a} and {@code b} remain unchanged relative to the comparison criteria</li>
50 * <li>if {@code a} and {@code b} are both {@code null} then this method returns {@code true}</li>
51 * <li>if {@code a} is {@code null} and {@code b} is non-{@code null}, or {@code a} is non-{@code null} and
52 * {@code b} is {@code null} then this method returns {@code false}</li>
53 * </ul>
54 */
55 boolean equals(T a, T b);
56
57 /**
58 * A {@link HashingStrategy} which delegates to java's {@link Object#hashCode()}
59 * and {@link Object#equals(Object)}.
60 */
61 @SuppressWarnings("rawtypes")
62 HashingStrategy JAVA_HASHER = new HashingStrategy() {
63 @Override
64 public int hashCode(Object obj) {
65 return obj != null ? obj.hashCode() : 0;
66 }
67
68 @Override
69 public boolean equals(Object a, Object b) {
70 return (a == b) || (a != null && a.equals(b));
71 }
72 };
73 }