Error Prone

It’s common for even the best programmers to make simple mistakes. And sometimes a refactoring which seems safe can leave behind code which will never do what’s intended.

We’re used to getting help from the compiler, but it doesn’t do much beyond static type checking. Using Error Prone to augment the compiler’s type analysis, you can catch more mistakes before they cost you time, or end up as bugs in production. We use Error Prone in Google’s Java build system to eliminate classes of serious bugs from entering our code, and we’ve open-sourced it, so you can too!

Error Prone …

• hooks into your standard build, so all developers run it without thinking
• tells you about mistakes immediately after they’re made
• produces suggested fixes, allowing you to build tooling on it

How it works

import java.util.Set;
import java.util.HashSet;

public class ShortSet {
  public static void main (String[] args) {
    Set<Short> s = new HashSet<>();
    for (short i = 0; i < 100; i++) {
      s.add(i);
      s.remove(i - 1);
    }
    System.out.println(s.size());
  }
}

$ bazel build :hello
ERROR: example/myproject/BUILD:29:1: Java compilation in rule '//example/myproject:hello'
ShortSet.java:6: error: [CollectionIncompatibleType] Argument 'i - 1' should not be passed to this method;
its type int is not compatible with its collection's type argument Short
      s.remove(i - 1);
              ^
    (see http://errorprone.info/bugpattern/CollectionIncompatibleType)
1 error

Testimonials

Doug Lea, on learning of a bug we discovered in ConcurrentHashMap:

Definitely embarrassing. I guess I’m back to liking Error Prone even though it sometimes annoys me :-)