String物件的equals（）與 = =

Diy_os發表於2015-11-06
物件
最近看了網上有人說，String物件的equals()與==的區別，不知道兩者到底有什麼區別，如果真的要徹底搞清楚，我們還是從“萬類之源”說起，http://blog.itpub.net/29876893/viewspace-1819489/這是之前寫關於Object的一篇文章，現在我們還是從原始碼入手（當然結合API更好）。
下面還是分析Object類中的equals()方法，關於原始碼如何獲取上篇文章已經說的很清楚。

下面是Object類中的equals（）：

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  1. public boolean equals(Object obj) {
  2.         return (this == obj);
  3.     }


  6.     /**
  7.      * Creates and returns a copy of this object. The precise meaning
  8.      * of "copy" may depend on the class of the object. The general
  9.      * intent is that, for any object {@code x}, the expression:
  10.      *

  11.      *
  12.      * x.clone() != x
  13.      * will be true, and that the expression:
  14.      *

  15.      *
  16.      * x.clone().getClass() == x.getClass()
  17.      * will be {@code true}, but these are not absolute requirements.
  18.      * While it is typically the case that:
  19.      *

  20.      *
  21.      * x.clone().equals(x)
  22.      * will be {@code true}, this is not an absolute requirement.
  23.      *


  24.      * By convention, the returned object should be obtained by calling
  25.      * {@code super.clone}. If a class and all of its superclasses (except
  26.      * {@code Object}) obey this convention, it will be the case that
  27.      * {@code x.clone().getClass() == x.getClass()}.
  28.      *


  29.      * By convention, the object returned by this method should be independent
  30.      * of this object (which is being cloned). To achieve this independence,
  31.      * it may be necessary to modify one or more fields of the object returned
  32.      * by {@code super.clone} before returning it. Typically, this means
  33.      * copying any mutable objects that comprise the internal "deep structure"
  34.      * of the object being cloned and replacing the references to these
  35.      * objects with references to the copies. If a class contains only
  36.      * primitive fields or references to immutable objects, then it is usually
  37.      * the case that no fields in the object returned by {@code super.clone}
  38.      * need to be modified.
  39.      *


  40.      * The method {@code clone} for class {@code Object} performs a
  41.      * specific cloning operation. First, if the class of this object does
  42.      * not implement the interface {@code Cloneable}, then a
  43.      * {@code CloneNotSupportedException} is thrown. Note that all arrays
  44.      * are considered to implement the interface {@code Cloneable} and that
  45.      * the return type of the {@code clone} method of an array type {@code T[]}
  46.      * is {@code T[]} where T is any reference or primitive type.
  47.      * Otherwise, this method creates a new instance of the class of this
  48.      * object and initializes all its fields with exactly the contents of
  49.      * the corresponding fields of this object, as if by assignment; the
  50.      * contents of the fields are not themselves cloned. Thus, this method
  51.      * performs a "shallow copy" of this object, not a "deep copy" operation.
  52.      *


  53.      * The class {@code Object} does not itself implement the interface
  54.      * {@code Cloneable}, so calling the {@code clone} method on an object
  55.      * whose class is {@code Object} will result in throwing an
  56.      * exception at run time.
  57.      *
  58.      * @return a clone of this instance.
  59.      * @exception CloneNotSupportedException if the object's class does not
  60.      * support the {@code Cloneable} interface. Subclasses
  61.      * that override the {@code clone} method can also
  62.      * throw this exception to indicate that an instance cannot
  63.      * be cloned.
  64.      * @see java.lang.Cloneable
  65.      */
從上面我們可以看出來，比較的是兩個物件的地址值，更全面的介紹，自己看下上面的 註釋。

下面看下toString()方法，當然和本篇沒有多少關係。

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  1. public String toString() {
  2.         return getClass().getName() + "@" + Integer.toHexString(hashCode());
  3.     }

  5.     
返回的是包名.類名@十六進位制的雜湊值




下面看一小程式：

上面結果也許你會問，不都是字串麼！為什麼結果不一樣呢！？這個要結合java記憶體來分析，對於new出來的物件，是在堆中分配記憶體空間，而對於String類很特殊，
如果對於直接String ss = xxxx,是在常量池中分配記憶體空間，並不是在棧中分配。但是對於上述的a,b在棧中分配空間，指向了堆中不同的記憶體空間，當然兩者不等！

再看一個小程式：



咦，equals（）不是比較的是地址麼？還是true，不能有這種定向思維，應該查閱API或者看下原始碼：

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  1. */
  2.     public boolean equals(Object anObject) {
  3.         if (this == anObject) {
  4.             return true;
  5.         }
  6.         if (anObject instanceof String) {
  7.             String anotherString = (String) anObject;
  8.             int n = value.length;
  9.             if (n == anotherString.value.length) {
  10.                 char v1[] = value;
  11.                 char v2[] = anotherString.value;
  12.                 int i = 0;
  13.                 while (n-- != 0) {
  14.                     if (v1[i] != v2[i])
  15.                             return false;
  16.                     i++;
  17.                 }
  18.                 return true;
  19.             }
  20.         }
  21.         return false;
  22.     }
上面很清楚，String類重寫了Object中的toString()方法，如果不是指向同一物件的話，那就比較內容是不是一樣，所以上文中a.equals(b),b.equals(a)返回的是true.
在繼承父類時，重寫某個方法很重要。上文中沒有說Object類中一個也很重要的方法hashcode(),這也是後續文章會寫。文章作為本人學習總結，如有錯誤之處，請指正！謝謝。

來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/29876893/viewspace-1823819/，如需轉載，請註明出處，否則將追究法律責任。

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