Why does it assign the value only to one random thread?

I'm intentionally left out first line of the run method not to violate the rule to post complete solution. t is currentThread. When I run this code, I found that all the inputs are stored only one of the thread instead of being randomly distributed - that's what I thought after reading the code. Could anyone explain why it stores only in a single thread?

while(!t.isInterrupted()){
    try{
        String s = reader.readLine();
        if(s != null){
            readStringCount.incrementAndGet();
            result.add(s);
        }
    } catch(IOException e){}
}