Java equals 和 hashcode 方法 - JAVA

问题
面试时经常会问起字符串比较相关的问题,

总结一下,大体是如下几个:

1.字符串比较时用的什么方法,内部实现如何

2.hashcode的作用,以及重写equal方法,为什么要重写hashcode方法

现在对以上几个问题,彻底做一个研究和小结.

PS:本文使用jdk1.7

解析
1.Object类的equals 方法

 * Indicates whether some other object is "equal to" this one.
  * 
  * The {@code equals} method implements an equivalence relation
  * on non-null object references:
  * 

  * It is reflexive: for any non-null reference value
  *     {@code x}, {@code x.equals(x)} should return
  *     {@code true}.
  * 
It is symmetric: 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}.
  * 
It is transitive: 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}.
  * 
It is consistent: 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.
  * 
For any non-null reference value {@code x},
  *     {@code x.equals(null)} should return {@code false}.
  * 
  * 
  * 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}).
  * 

  * 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); 
 } 

   /**
     * Indicates whether some other object is "equal to" this one.
     * 

     * The {@code equals} method implements an equivalence relation
     * on non-null object references:
     * 

     * It is reflexive: for any non-null reference value
     *     {@code x}, {@code x.equals(x)} should return
     *     {@code true}.
     * 
It is symmetric: 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}.
     * 
It is transitive: 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}.
     * 
It is consistent: 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.
     * 
For any non-null reference value {@code x},
     *     {@code x.equals(null)} should return {@code false}.
     * 
     * 
     * The {@code equals} method for class {@code Object} implements
     * the most discriminating possible equivalence relation on objects;

Java equals 和 hashcode 方法 (一)