TreeMap 中的键返回 null [英] Key in TreeMap returning null

问题描述

所以我有一个非常奇怪的错误.当我最初使用 keySet() 迭代大型 TreeMap 的前 10 个键时，我偶然发现了它.其中一个键是返回 null，据我所知，这应该是不可能的.所以我写了下面的测试代码:

So I have a very odd bug. I stumbled across it when I was originally using a keySet() to iterate over the first 10 keys of a large TreeMap. One of the keys was returning null, which should not be possible as far as my understanding goes. So I wrote the test code below:

int i = 0;
        for (Map.Entry<String, Integer> es : sortedMap.entrySet()){
            if (i >= 10) {
                break;
            }

            if (sortedMap.containsKey(es.getKey())){
                System.out.println(es.getKey() + ":" + sortedMap.get(es.getKey()));
            } else {
                System.out.println("Key " + es.getKey() + " does not exist, yet...");
                System.out.println("This does work: " + es.getKey() + ":" + es.getValue());
                System.out.println("This does NOT work: " + es.getKey() + ":" + sortedMap.get(es.getKey()));
            }
            i++;
        }

并得到以下结果:

SOAP:967
'excerpt'::679
'type'::679
Key 'author_url': does not exist, yet...
This does work: 'author_url'::679
This does NOT work: 'author_url'::null
'date'::679
Android:437
TLS:295
message:283
server:230
monthly:215
<<<<<<<<<<<<<<<<<<<<DUMPING MAP!
{SOAP=967, 'excerpt':=679, 'type':=679, 'author_url':=679, 'date':=679, Android=437, TLS=295, message=283, server=230, monthly=215...

我在前十名之后切断了地图，因为那里还有很多，但所有这些都是一个带有值的键.

I cut off the map after the top ten as there is a lot more in there, but all of it is a key with a value.

所以我的问题是:为什么在使用键直接从 TreeMap 获取(键)时我得到一个空值，但 EntrySet 返回正确的键和值?

So my question is this: Why am I getting a null when using the key to directly get(key) from the TreeMap, but the EntrySet returns the correct key and value?

这是我的比较器，因为我是在整数上订购的:

Here is my comparator since I am ordering on Integer:

class ValueComparator implements Comparator<Object> {

  Map<String, Integer> base;
  public ValueComparator(Map<String, Integer> base) {
      this.base = base;
  }

  public int compare(Object a, Object b) {

    if ((Integer) base.get(a) < (Integer) base.get(b)) {
      return 1;
    } else if ((Integer) base.get(a) == (Integer) base.get(b)) {
      return 0;
    } else {
      return -1;
    }
  }
}

TreeMap 构建如下:

And the TreeMap is built as following:

ValueComparator bvc =  new ValueComparator(allMatches);
TreeMap<String, Integer> sortedMap = new TreeMap<String, Integer>(bvc);
//Sort the HashMap
sortedMap.putAll(allMatches);

其中 allMatches 是 HashMap

Where allMatches is a HashMap<String, Integer>

推荐答案

问题解决:

class ValueComparator implements Comparator<Object> {

Map<String, Integer> base;

public ValueComparator(Map<String, Integer> base) {
    this.base = base;
}

public int compare(Object a, Object b) {

    if (((Integer) base.get(a)).intValue() < ((Integer) base.get(b)).intValue()) {
        return 1;
    } else if ( ((Integer) base.get(a)).intValue() == ((Integer) base.get(b)).intValue()) {
        return ((String)a).compareTo(((String)b));
    } else {
        return -1;
    }
}
}

这带来了额外的好处，即按字母顺序返回具有相同值的键.

This comes with the additional benefit of bringing back keys with the same value in alphabetical order.

这篇关于TreeMap 中的键返回 null的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！