Question

可能重复：
  what is an objects hashcode
  How do hashCode() and identityHashCode() work at the back end?

我不是在谈论String类或其他重写hashcode的类。假设我只是创建了Object类的新对象，那么hashcode()或者在任何情况下都是真的，identityHashCode(Object x)会返回该对象的内存地址吗？

Answer 1

不一定。来自documentation（强调我的）：

尽可能合理，Object类定义的hashCode方法确实为不同的对象返回不同的整数。（这通常通过将对象的内部地址转换为整数来实现，但JavaTM编程语言不需要此实现技术。）

Answer 2

您可以随时查看JDK附带的来源进行检查。

我的java.lang.Object将hashCode显示为原生方法。这是javadocs。

/**
 * Returns a hash code value for the object. This method is 
 * supported for the benefit of hashtables such as those provided by 
 * <code>java.util.Hashtable</code>. 
 * <p>
 * The general contract of <code>hashCode</code> is: 
 * <ul>
 * <li>Whenever it is invoked on the same object more than once during 
 *     an execution of a Java application, the <tt>hashCode</tt> method 
 *     must consistently return the same integer, provided no information 
 *     used in <tt>equals</tt> 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 <tt>equals(Object)</tt>
 *     method, then calling the <code>hashCode</code> 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 <tt>hashCode</tt> 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 hashtables.
 * </ul>
 * <p>
 * As much as is reasonably practical, the hashCode method defined by 
 * class <tt>Object</tt> 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<font size="-2"><sup>TM</sup></font> programming language.)
 *
 * @return  a hash code value for this object.
 * @see     java.lang.Object#equals(java.lang.Object)
 * @see     java.util.Hashtable
 */
public native int hashCode();

Answer 3

不，HashCode（）函数返回一个整数。如果尚未为对象定义HashCode（）函数，Java可以将对象的内存地址转码为整数并返回该值。

Answer 4

正如Object.hashCode（）上的documentation所述，

尽可能合理，Object类定义的hashCode方法确实为不同的对象返回不同的整数。（这通常通过将对象的内部地址转换为整数来实现，但JavaTM编程语言不需要此实现技术。）

因此，Java语言不要求Object类的哈希码返回对象的内存地址，因此你不应该依赖它。

hashcode是否返回内存地址？

4 个答案: