Enforcing correct array indexes with a type system

Abstract

We have built the Index Checker, a type checker that issues warnings about array, list, and string accesses that are potentially unsafe. An example is shown in Figure 1. As with any sound tool, some of its warnings may be false positives. If the Index Checker issues no warning, then the programmer is guaranteed that no array access will cause an IndexOutOfBoundsException at run time (modulo suppressed warnings and unchecked code). The Index Checker ships with knowledge of Java APIs. The developer can optionally write a few type annotations in the program to make the Index Checker more precise. Our system includes five new type qualifiers, defined in Figure 2, that can be applied to integral types such as Java int. These are dependent types that indicate the relationship between the int and given arrays. Figures 3 and 4 show the relationship among these type qualifiers. The type system also contains a type qualifier for arrays, @MinLen, which is a lower bound on its length and permits use of literal integers to access the array or to construct a new array. The Index Checker is built upon the Checker Framework (http://CheckerFramework.org/).