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
? ? * 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 {@link java.lang.Object#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
? ? * Java^TM 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;
? ? }

?

?

/**
? ? * Returns a hash code for this string. The hash code for a
? ? * String object is computed as
? ? *

? ? * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
? ? *