为什么HashSet调用compareTo方法并导致NullPointerException

Question

我在增加值HashSet和获取NullPointerException的compareTo方法.

java.lang.NullPointerException
    at com.fiveIQ.document.Link.compareTo(Link.java:226)
    at com.fiveIQ.document.Link.compareTo(Link.java:16)
    at java.util.HashMap.compareComparables(HashMap.java:371)
    at java.util.HashMap$TreeNode.treeify(HashMap.java:1920)
    at java.util.HashMap.treeifyBin(HashMap.java:771)
    at java.util.HashMap.putVal(HashMap.java:643)
    at java.util.HashMap.put(HashMap.java:611)
    at java.util.HashSet.add(HashSet.java:219)
    at com.fiveIQ.crawlData.parser.EightyLegJsonParser.parse(EightyLegJsonParser.java:51)
    at com.fiveIQ.crawlData.processor.CrawlDataParser.process(CrawlDataParser.java:57)
    at com.fiveIQ.crawlData.processor.CrawlDataUploader.upload(CrawlDataUploader.java:31)
    at com.fiveIQ.crawlData.processor.CrawlDataUploaderExecutor$1.run(CrawlDataUploaderExecutor.java:85)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

这是我的compareTo方法.的价值updatedOn是零.但我不明白为什么HashSet要打电话compareTo？

@Override
public int compareTo(Link o)
{
    return o.updatedOn.compareTo(this.getUpdatedOn());
}

Answer 1

我假设您使用的是Java 8.自Java 8以来,HashMap实现(由其使用HashSet)被更改以防止冲突:如果相同的存储桶具有太多元素和密钥Comparable,则此存储桶将转换为RB树(类似于TreeMap)在该桶中进行对数而不是线性搜索.这还可以保护应用程序免受恶意企图使用冲突密钥破坏HashMaps,从而导致拒绝服务攻击.有关详细信息,请参阅JEP 180.