Question

I found in the util.TreeSet class that one of the constructor is calling another constructor with a new TreeMap with empty generic type.

  public TreeSet(Comparator<? super E> comparator) {
         this(new TreeMap<>(comparator));
  }

What does new TreeMap<> mean ? is that equivalent to new TreeMap<?> ?

Answer 1

This is Java 7 syntax. The diamond (<>) is a short-hand to ask the Java compiler to fill generic arguments with whatever makes sense in the local context (in this case, it’ll be ? super E).