java - 在java中使用索引访问 map

Question

为什么 Java HashMap 中没有 map.get(index) ？

要访问 HashMap 中第一个键的值，我必须将键放入列表中，然后 list.get(0) ，然后将其传递给 map.get(list.get(0));我觉得这很笨拙。为什么会这样？

从 HashMap 访问“第一个或第二个元素”的正确方法是什么？

(请不要建议“将 HashMap 转换为 LinkedHashMap 并通过 迭代 访问它)。

Answer 1

要求 HashMap 的“第一个元素”没有意义，因为顺序可以(并且通常会)随着元素的添加和删除而改变。

HashMap 的文档对此进行了解释

"... makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time."

您可以通过以下实验对其进行测试:

import java.util.HashMap;
public class M {
public static void main(String[] args) {
   HashMap<String, Integer> m = new HashMap<String, Integer>();
   for (int i=1, p=1; i<100000; i++) {
      m.put(""+i, i);
      if (i == p) { 
         p <<= 1;  // keys are reordered at power-of-two multiples
         System.out.println("at " + p + ": first is now " 
            + m.keySet().iterator().next());
      }
   }
}
}    

输出是(JDK 1.7):

at 2: first is now 1
at 4: first is now 2
at 8: first is now 3
at 16: first is now 3
at 32: first is now 15
at 64: first is now 19
at 128: first is now 35
at 256: first is now 35
at 512: first is now 35
at 1024: first is now 338
at 2048: first is now 338
at 4096: first is now 1411
at 8192: first is now 3280
at 16384: first is now 6873
at 32768: first is now 10997
at 65536: first is now 10997
at 131072: first is now 10997

如果您需要可重复的排序(= 迭代元素将始终为您提供插入顺序)，您始终可以使用 LinkedHashMap。请注意，它们需要更多的内存，并且速度比标准品种稍慢。