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());
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。