String Vs Stringbuffer 作为 HashMap 键

问题描述

我试图理解为什么 String 和 Stringbuilder/StringBuffer 在用作 Hashmap 键时会被区别对待.让我通过以下插图更清楚地说明我的困惑:

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:

示例 #1，使用字符串:

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());

上面的代码片段打印1".

Above code snippet prints '1'.

示例 #2，使用 StringBuilder(或 StringBuffer):

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());

上面的代码片段打印2".

The above code snippet prints '2'.

谁能解释一下为什么会出现这种行为差异.

Could anyone please explain why the difference in behaviour.

推荐答案

StringBuilder/Buffer 不覆盖 hashCode 和 equals.这意味着对象的每个实例都应该是唯一的哈希码，并且它的值或状态无关紧要.您应该使用字符串作为键.

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 也是可变的，这通常不是用作 HashMap 的键的好主意，因为将值存储在其下可能会导致修改后无法访问该值.

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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