Counting words with a missing entry
A medium Java puzzle in the null handling family. One line below contains a real bug. Try to spot it first, then read why it fails and how to fix it.
Highlighted: line 7. Play this one as a puzzle without the answer.
What is wrong
On the first occurrence of a word counts.get returns null, and assigning it to an int unboxes it, which throws NullPointerException. The line looks like plain arithmetic, so the unboxing is easy to miss.
The fix
How to avoid it next time
Use merge or getOrDefault when counting. Beware of unboxing a value that can be null.
Want the background? Read Autoboxing and wrapper types in the Java concepts library.
About null handling
NullPointerException is still the most common production exception. Chained calls, auto-unboxing, maps that return null for missing keys and Optional used the wrong way all hide a null until the worst moment.
In code review: Trace each dereference back to where the value comes from and decide whether it can be null. Be wary of Optional.get, getOrDefault on null values and unboxing of Integer fields.
More puzzles in this family
Frequently asked questions
What is the bug in "Counting words with a missing entry"?
On the first occurrence of a word counts.get returns null, and assigning it to an int unboxes it, which throws NullPointerException. The line looks like plain arithmetic, so the unboxing is easy to miss. The faulty line is line 7: int c = counts.get(w);
How do I fix it?
Replace it with: counts.merge(w, 1, Integer::sum);. Use merge or getOrDefault when counting. Beware of unboxing a value that can be null.
How do I catch null handling in code review?
Trace each dereference back to where the value comes from and decide whether it can be null. Be wary of Optional.get, getOrDefault on null values and unboxing of Integer fields.
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