How can I use the Solver with Java language?

[giuseppeboezio]

I am trying to use the Solver with Java language, I have a custom Library called ClpFDLibrary and I want to activate the flag for tracking interesting variables.
My code is the following:

Solver solver = ClassicSolverFactory.solverOf(
            ClpFDLibrary.toRuntime(),
            FlagStore.EMPTY + TrackVariables { ON },
            theory
);

How can I fix the code in order to work with Java language?

[gciatto]

Quick answer:

import it.unibo.tuprolog.solve.Solver;
import it.unibo.tuprolog.solve.flags.TrackVariables;
import it.unibo.tuprolog.solve.library.Runtime;
import package.of.MyLibrary; // assuming this is a Kotlin object

Solver solver = Solver.prolog().newBuilder()
        .runtime(Runtime.of(MyLibrary.INSTANCE))
        .flag(TrackVariables.INSTANCE, TrackVariables.ON)
        .staticKb(theory)
        .build();

An explanation will follow.

[gciatto]

Explanation:

1. ClassicSolverFactory is somewhat an implementation detail. The same factory can and should be achieved via Solver.prolog(), in Java (or simply Solver.prolog in Kotlin)

2. Since version 0.30.x, the builder pattern can be exploited to build solvers. While this is somewhat equivalent to Solver.factory().solver*Of(...) methods in theory, in practice, builders are far handier in Java. The reason is that SolverFactorys rely on default values for their methods, which only exist in Kotlin, while Java does not support them.

3. So, the code I provided above is actually equivalent to the following Kotlin code:

   import it.unibo.tuprolog.solve.Solver
   import it.unibo.tuprolog.solve.flags.TrackVariables
   import it.unibo.tuprolog.solve.library.toRuntime
   import package.of.MyLibrary; // assuming this is a Kotlin object
   
   val solver: Solver = Solver.prolog.newBuilder()
       .runtime(MyLibrary.toRuntime())
       .flag(TrackVariables) { ON }
       .staticKb(theory)
       .build()

   but keeping into account that:

   1. Kotlin objects are compiled as ordinary Java classes with a public static field named INSTANCE and referencing the only admissible instance of that class
   2. the Library.toRuntime() method is actually an extension method, which cannot be invoked via infix notation in Java. In Java, one can attain the same result via Runtime.of(library)
   3. the .flag(TrackVariables) { ON } is not possible in Java ('cause it does not support the dangling lambda convention, so the best override is .flag(library, value)
   4. notice that notable flags such as TrackVariables are actually implemented as Kotlin objects too, so in Java they must be referenced as TrackVariables.INSTANCE