java中所有的类都默认继承Object,hashcode是在Object中定义出来的,有时我们需要重写hashCode方法。
jdk文档中是这样写的hashCode
public int hashCode()
Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable.
The general contract of hashCode is:
Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in 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.
If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the 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 hashtables.
As much as is reasonably practical, the hashCode method defined by class 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 JavaTM programming language.)
Returns:
a hash code value for this object.
See Also:
equals(java.lang.Object), Hashtable
该方法在 Object 类中是这样定义的 public native int hashCode();
我觉着是系统定义的,返回的值就是一个标号,有利于hashMap,hashTable等等结合框架或其他,在内存中找到他。具体为什么这个方法是native的还有他的值是如何来的 我觉着应该是系统里面的产生的,我只能按自己的想法解答到这。 |