我们首先来看一下 HashCode 的源码:
- /**
- * Returns a hash code value for the object. This method is
- * supported for the benefit of hash tables such as those provided by
- * {@link java.util.HashMap}.
- * <p>
- * The general contract of {@code hashCode} is:
- * <ul>
- * <li>Whenever it is invoked on the same object more than once during
- * an execution of a Java application, the {@code hashCode} method
- * must consistently return the same integer, provided no information
- * used in {@code equals} comparisons on the object is modified.
- * This integer need not remain consistent from one execution of an
- * application to another execution of the same application.
- * <li>If two objects are equal according to the {@code equals(Object)}
- * method, then calling the {@code hashCode} method on each of
- * the two objects must produce the same integer result.
- * <li>It is <em>not</em> required that if two objects are unequal
- * according to the {@link java.lang.Object#equals(java.lang.Object)}
- * method, then calling the {@code hashCode} method on each of the
- * two objects must produce distinct integer results. However, the
- * programmer should be aware that producing distinct integer results
- * for unequal objects may improve the performance of hash tables.
- * </ul>
- * <p>
- * As much as is reasonably practical, the hashCode method defined by
- * class {@code Object} does return distinct integers for distinct
- * objects. (This is typically implemented by converting the internal
- * address of the object into an integer, but this implementation
- * technique is not required by the
- * Java™ programming language.)
- *
- * @return a hash code value for this object.
- * @see java.lang.Object#equals(java.lang.Object)
- * @see java.lang.System#identityHashCode
- */
- public native int hashCode();
刨到这种地步, 也差不多了, 我们看一下上面的注释是啥意思?
返回对象的 Hash Code 值, 支持此方法的好处是可以使用{@link java.util.HashMap}. 提供的哈希表
每当在 Java 应用程序执行期间在同一个对象上多次调用该方法时,
{@code hashCode}方法必须一致地返回相同的整数,
前提是不修改对象上的 {@code =} 比较中使用的信息.
从应用程序的一次执行到同一应用程序的另一次执行, 该整数不必保持一致.
- String string = new String();
- string="bbbb";
- System.out.println(string.hashCode());
- string="dddd";
- System.out.println(string.hashCode());
HashCode 结果 1.PNG
如果根据 {@code equals(Object)} 方法两个对象相等,
那么在两个对象上调用 {@code hashCode} 方法必须产生相同的整数结果.
如果根据 {@link java.lang.Object#equals(java.lang.Object)} 方法两个对象不相等,
那么在两个对象上调用 {@code hashCode} 方法必须产生不同的整数结果,
这是不必要的. 但是, 程序员应该意识到, 为不相等的对象生成不同的整数结果可能会提高哈希表的性能.
在相当实际的情况下, 类 {@code Object} 定义的 hashCode 方法确实为不同的对象返回不同的整数.
(这通常是通过将对象的内部地址转换为整数来实现的,
但是 Java&trade 并不需要这种实现技术; 编程语言).
- /**
- * Indicates whether some other object is "equal to" this one.
- * <p>
- * The {@code equals} method implements an equivalence relation
- * on non-null object references:
- * <ul>
- * <li>It is <i>reflexive</i>: for any non-null reference value
- * {@code x}, {@code x.equals(x)} should return
- * {@code true}.
- * <li>It is <i>symmetric</i>: for any non-null reference values
- * {@code x} and {@code y}, {@code x.equals(y)}
- * should return {@code true} if and only if
- * {@code y.equals(x)} returns {@code true}.
- * <li>It is <i>transitive</i>: for any non-null reference values
- * {@code x}, {@code y}, and {@code z}, if
- * {@code x.equals(y)} returns {@code true} and
- * {@code y.equals(z)} returns {@code true}, then
- * {@code x.equals(z)} should return {@code true}.
- * <li>It is <i>consistent</i>: for any non-null reference values
- * {@code x} and {@code y}, multiple invocations of
- * {@code x.equals(y)} consistently return {@code true}
- * or consistently return {@code false}, provided no
- * information used in {@code equals} comparisons on the
- * objects is modified.
- * <li>For any non-null reference value {@code x},
- * {@code x.equals(null)} should return {@code false}.
- * </ul>
- * <p>
- * The {@code equals} method for class {@code Object} implements
- * the most discriminating possible equivalence relation on objects;
- * that is, for any non-null reference values {@code x} and
- * {@code y}, this method returns {@code true} if and only
- * if {@code x} and {@code y} refer to the same object
- * ({@code x == y} has the value {@code true}).
- * <p>
- * Note that it is generally necessary to override the {@code hashCode}
- * method whenever this method is overridden, so as to maintain the
- * general contract for the {@code hashCode} method, which states
- * that equal objects must have equal hash codes.
- *
- * @param obj the reference object with which to compare.
- * @return {@code true} if this object is the same as the obj
- * argument; {@code false} otherwise.
- * @see #hashCode()
- * @see java.util.HashMap
- */
- public boolean equals(Object obj) {
- return (this == obj);
- }
1,{@code =}方法在非空对象引用上实现等价关系:
2, 它是自反的: 对于任何非空引用值{@code x},
{@code x.equals(x)}应该返回{@code true}.
3, 它是对称的: 对于任何非空的引用值 {@code x} 和{@code y},
当且仅当 {@code y = (x)} 返回 {@code true} 时,
{@code x.equals(y)}应该返回{@code true}.
4, 它是传递的: 对于任何非空的参考值 {@code x}, {@code y}, {@code z}, 如果{@code x.equals(y)} 返回{@code true},
{@code y.equals(z)}返回 {@code true}, 那么{@code x.equals(z)} 应该返回{@code true}.
5, 它是一致的: 对于任何非空引用值 {@code x} 和{@code y}, 对 {@code x.equals(y)} 的多次调用
一致返回 {@code true} 或一致返回{@code false}, 但在对象被修改.
6, 对于任何非空引用值 {@code x}, {@code x.equals(null)} 应该返回{@code false}.
7,{@code eauals}方法类 {@codeObject} 实现最歧视可能等价关系对象, 也就是说, 对于任何非空引用值 {@code x} 和
{@code y}, 该方法返回 {@code true} 当且仅当 {@code x} 和{@code y}引用同一个对象({@code x = = y}
{@code true})的值.
8, 注意, 通常需要在重写该方法时重写 {@code hashCode} 方法, 以便维护 {@code hashCode} 方法的通用契约,
该契约声明相等的对象必须具有相等的散列代码.
来源: http://www.jianshu.com/p/d9c1ec0ae523