hashcode方法返回该对象的哈希码值。
hashCode()方法可以用来来提高Map里面的搜索效率的,Map会根据不同的hashCode()来放在不同的位置,Map在搜索一个对象的时候先通过hashCode()找到相应的位置，然后再根据equals()方法判断这个位置上的对象与当前要插入的对象是不是同一个。若两个对象equals相等，但不在一个区间，根本没有机会进行比较，会被认为是不同的对象。
所以，Java对于eqauls方法和hashCode方法是这样规定的：
1、如果两个对象相同，那么它们的hashCode值一定要相同，也告诉我们重写equals方法，一定要重写hashCode方法；
2、如果两个对象的hashCode相同，它们并不一定相同
hashCode()和equals()定义在Object类中，这个类是所有java类的基类，所以所有的java类都继承这两个方法。
注意到hashCode方法前面有个native的修饰符，这表示hashCode方法是由非java语言实现的，说明是一个本地方法，它的实现是根据本地机器相关的。具体的方法实现在外部，返回内存对象的地址。
/**
* 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();
在java类中可以重写这两个方法，下面是String类中这两个类的实现。
/**
* Compares this string to the specified object. The result is {@code
* true} if and only if the argument is not {@code null} and is a {@code
* String} object that represents the same sequence of characters as this
* object.
*
* @param anObject
* The object to compare this {@code String} against
*
* @return {@code true} if the given object represents a {@code String}
* equivalent to this string, {@code false} otherwise
*
* @see #compareTo(String)
* @see #equalsIgnoreCase(String)
*/
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = count;
if (n == anotherString.count) {
char v1[] = value;
char v2[] = anotherString.value;
int i = offset;
int j = anotherString.offset;
while (n-- != 0) {
if (v1[i++] != v2[j++])
return false;
}
return true;
}
}
return false;
}