java - 在迭代期间替换/更改 HashMap 项

Question

当我尝试从 HashMap 中删除项目时，出现并发修改异常。我知道在通过 HashMap 迭代期间删除项目会触发此异常，但我需要用新项目替换旧项目。我怎样才能做到这一点 ？也许创建 countNumberOfEachCharacter HashMap 的副本，然后迭代原始 HashMap 以从副本 HashMap 中删除项目？

countNumberOfEachCharacter = new HashMap<Character,Character>();
if (countNumberOfEachCharacter.containsKey(word.charAt(i))) {
    System.out.println("This character already exists");                     
    for (Iterator it = countNumberOfEachCharacter.entrySet().iterator(); it.hasNext();) {

      Map.Entry entry = (Map.Entry) it.next();

      Object key = entry.getKey();
      Object value = entry.getValue();

      if (key.equals(word.charAt(i))) { 

        int toIncrease = Integer.parseInt(value.toString());
        toIncrease++;

        System.out.println("key  "+key);                                                     
        System.out.println("increased  "+toIncrease);                                                       
        countNumberOfEachCharacter.remove(word.charAt(i));

        char c = Character.forDigit(toIncrease, 10);                                                     
        countNumberOfEachCharacter.put(word.charAt(i),c);                                                                                                                                                                               
    }                                                                                                                                                                                                           
  }                                                                           
}
else {    

   System.out.println("First time found this character");

   char c = Character.forDigit(1, 10);                                
   countNumberOfEachCharacter.put(word.charAt(i),c);                            
   System.out.println("Stored "+word.charAt(i)+" with count "+c);                                                               
}

Answer 1

在Collection上进行迭代时，您只能使用Iterator#remove方法删除元素。这也记录在 HashMap

的类 javadoc 中

The iterators returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future

此外，对于您想要执行的操作(=更新值)，您不必删除它。只需使用该键和更新后的值调用 put 即可更新该值，如 HashMap#put 方法的 javadoc 中所述

Associates the specified value with the specified key in this map. If the map previously contained a mapping for the key, the old value is replaced.