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CoJava: A Unified Language for Simulation and Decision Optimization

Demonstration number: 20

We have proposed and implemented, and will demonstrate, the language CoJava, which offers both the advantages of simulation-like process modeling, and the capabilities of true decision optimization. By design, the syntax of CoJava is identical to the programming language Java, extended with special constructs to (1) make a non-deterministic choice of a numeric value, (2) assert a constraint, and (3) designate a program variable as the objective to be optmized. A CoJava program thus defines a set of nondeterministic execution paths, each being a program run with specific selection of values in the choice statements. The semantics of CoJava interprets a program as an optimal nondeterministic execution path, namely, a path that (1) satisfies the range conditions in the {\em choice} statements, (2) satisfies the assert-constraint statements, and (3) produces the optimal value in a designated program variable, among all execution paths that satisfy (1) and (2). Thus, to run a CoJava program amounts to first finding an optimal execution path, and then procedurally executing it. To find an optimal non-deterministic execution path, we have developed a reduction to a standard constraint optimization formulation. Constraint variables represent values in program variables that can be created at any state of a non-deterministic execution. Constraints encode transitions, triggered by CoJava statements, from one program state to the next. Based on the reduction, we have developed a CoJava constraint compiler. The compiler operates by first translating the Java program into a similar Java program in which the primitive numeric operators and data types are replaced by symbolic constraint operators and data types. This intermediate java program functions as a constraint generator. This program is compiled and executed to produce a symbolic decision problem. The decision problem is then submitted to an external optimization solver. Keywords: constraints, optimization, simulation