What causes javac to issue the "uses unchecked or unsafe operations" warning

before

 Set<Integer> set = new TreeSet(Collections.reverseOrder());

after

 Set<Integer> set = new TreeSet<>(Collections.reverseOrder());

This comes up in Java 5 and later if you're using collections without type specifiers (e.g., Arraylist() instead of ArrayList()). It means that the compiler can't check that you're using the collection in a type-safe way, using generics.

자바 5에 등장, <>없이 사용할 경우 나타난다.

To get rid of the warning, you need to be specific about what type of objects you're storing in the collection. So, instead of