Type-based Object Immutability with Flexible Initialization

We present a type system for checking object immutability, reference immutability, and class immutability in an open or closed world. In order to separate object initialization from object constructors (which is often needed in practice), immutable objects are initialized in lexically scoped regions. The system is simple and direct; its only type qualifiers specify immutability properties. No auxiliary annotations, e.g., ownership types, are needed, yet good support for deep immutability is provided. To express object confinement, as required for class immutability in an open world, we make use of qualifier polymorphism. The system has two versions: one version with explicit specification commands that delimit the object initialization phase, and one version where such commands are implicit and inferred. In the latter version, all annotations are compatible with Java's extended annotation syntax, as proposed in JSR 308.