While playing with boxing/unboxing in Java 5, I found an interesting case. If my collection has a null value, the unboxing fails with a NullPointerException at runtime. Here is an example Set s = new HashSet (); for (int i=0; i < 10; i++) { s.add(i); } s.add(null); System.out.println("s: " + s); // this is a problem. The unboxing does not handle null for (int sx : s) { System.out.println("sx: " + sx); } The output of the progam is here: s: [2, 4, null, 9, 8, 6, 1, 3, 7, 5, 0] sx: 2 sx: 4 Exception in thread "main" java.lang.NullPointerException at com.mykola.TestSets.main(TestSets.java:21) So, unboxing is fine, unless a collection has a null element.
This is a collection of notes on programming by Mykola Dzyuba.