Java & Lambda: a Featherweight Story

• Published in: Logical methods in computer science Vol. 14, Issue 3
• Main Authors: Bettini, Lorenzo, Bono, Viviana, Dezani-Ciancaglini, Mariangiola, Giannini, Paola, Venneri, Betti
• Format: Journal Article
• Language: English
• Published: Logical Methods in Computer Science e.V 05.09.2018
• Subjects: 03b70, 03b15, 68q55; computer science - logic in computer science; computer science - programming languages
• ISSN: 1860-5974; 1860-5974
• DOI: 10.23638/LMCS-14(3:17)2018

We present FJ&$\lambda$, a new core calculus that extends Featherweight Java (FJ) with interfaces, supporting multiple inheritance in a restricted form, $\lambda$-expressions, and intersection types. Our main goal is to formalise how lambdas and intersection types are grafted on Java 8, by studying their properties in a formal setting. We show how intersection types play a significant role in several cases, in particular in the typecast of a $\lambda$-expression and in the typing of conditional expressions. We also embody interface \emph{default methods} in FJ&$\lambda$, since they increase the dynamism of $\lambda$-expressions, by allowing these methods to be called on $\lambda$-expressions. The crucial point in Java 8 and in our calculus is that $\lambda$-expressions can have various types according to the context requirements (target types): indeed, Java code does not compile when $\lambda$-expressions come without target types. In particular, in the operational semantics we must record target types by decorating $\lambda$-expressions, otherwise they would be lost in the runtime expressions. We prove the subject reduction property and progress for the resulting calculus, and we give a type inference algorithm that returns the type of a given program if it is well typed. The design of FJ&$\lambda$ has been driven by the aim of making it a subset of Java 8, while preserving the elegance and compactness of FJ. Indeed, FJ&$\lambda$ programs are typed and behave the same as Java programs.