NPEX: Repairing Java Null Pointer Exceptions without Tests

We present NPEX, a new technique for repairing Java null pointer exceptions (NPEs) without tests. State-of-the-art NPE repair techniques rely on test suites written by developers for patch validation. Unfortunately, however, those are typically future test cases that are unavailable at the time bugs are reported or insufficient to identify correct patches. Unlike existing techniques, NPEX does not require test cases; instead, NPEX automatically infers the repair specification of the buggy program and uses the inferred specification to validate patches. The key idea is to learn a statistical model that predicts how developers would handle NPEs by mining null-handling patterns from existing codebases, and to use a variant of symbolic execution that can infer the repair specification from the buggy program using the model. We evaluated NPEX on real-world NPEs collected from diverse open-source projects. The results show that NPEX significantly outperforms the current state-of-the-art.