String Vs Stringbuffer作为HashMap键 [英] String Vs Stringbuffer as HashMap key

问题描述

我想了解为什么String和Stringbuilder / StringBuffer在作为Hashmap键使用时被区别对待。以下插图让我更加清楚：

示例＃1，使用字符串：

  String s1 = new String（abc）; 
 String s2 = new String（abc）; 
 HashMap hm = new HashMap（）; 
 hm.put（s1，1）; 
 hm.put（s2，2）; 
 System.out.println（hm.size（））; 
  

上面的代码段打印出'1'。

<使用StringBuilder（或StringBuffer）的例子＃2：

  StringBuilder sb1 = new StringBuilder（abc）; 
 StringBuilder sb2 = new StringBuilder（abc）; 
 HashMap hm = new HashMap（）; 
 hm.put（sb1，1）; 
 hm.put（sb2，2）; 
 System.out.println（hm.size（））; 
  

以上代码片段打印出'2'。

任何人都可以解释为什么行为有所不同。

StringBuilder / Buffer不会覆盖hashCode和equals。这意味着对象的每个实例都应该是唯一的哈希代码，并且其值或状态不重要。您应该使用字符串作为键。

StringBuilder / Buffer也是可变的，这通常不是一个好主意，因为将它作为HashMap的键存储它可能会导致修改后无法访问该值。

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14 /java/lang/StringBuilder.java

I am trying to understand why String and Stringbuilder/StringBuffer are treated differently when used as Hashmap keys. Let me make my confusion clearer with the following illustrations:

Example #1, using String:

String s1 = new String("abc");
String s2 = new String("abc");
HashMap hm = new HashMap();
hm.put(s1, 1);
hm.put(s2, 2);
System.out.println(hm.size());

Above code snippet prints '1'.

Example #2, using StringBuilder(or StringBuffer):

StringBuilder sb1 = new StringBuilder("abc");
StringBuilder sb2 = new StringBuilder("abc");
HashMap hm = new HashMap();
hm.put(sb1, 1);
hm.put(sb2, 2);
System.out.println(hm.size());

The above code snippet prints '2'.

Could anyone please explain why the difference in behaviour.

解决方案

StringBuilder/Buffer do not override hashCode and equals. This means each instance of the object should be a unique hash code and the value or state of it does not matter. You should use the String for a key.

StringBuilder/Buffer is also mutable which is generally not a good idea to use as a key for a HashMap since storing the value under it can cause the value to be inaccessible after modification.

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/lang/StringBuilder.java

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