Question

I am a beginner in Java and do not understand what if(!s.add(a)) means in this code excerpt:

Set<String> s = new HashSet<String>();
for(String a:args) {
    if(!s.add(a)) System.out.println("Duplicate detected:"+a);
}

Answer 1

if collection s already has item a, then the add method will return false. The ! is a “not” operator, turning that false into true. So, if the item is already in the collection, you will see the println result.