Duplicate Keys in HashMap under Concurrent Update

Background

I met this weird issue during work. It took me quit a while to debug to realize duplicate keys could exist in a map! It might sound obvious since everyone learned this in textbooks, but I was still a little shocked that one day it finally occurred in real life.

Reproduce

The issue is pretty transient. No guarantee you will see duplicate keys every time you run the code. Here is a simple code in Kotlin to reproduce the issue:

import java.util.stream.Collectors

val lst = (1..2).toList()

(1..10000).forEach {
    val mm = mutableMapOf<String, Int>()
    lst.parallelStream()
        .map {
            mm.put("k", it)
        }
        .collect(Collectors.toList())

    if (mm.size != 1) {
        println(mm)
        println(mm.size)
    }
}

You will find sometimes the size of mm is not 1, which is impossible theoretically.

Solution

1. Use ConcurrentHashMap instead of HashMap which is thread-safe.
2. Do not update the map concurrently if possible.