Converting from string to lambda

Question

I'm trying to use ScriptEngine to convert from a string to a lambda function for input. This is what I've come up so far, but I only experience errors:

ScripEngine engine = new ScriptEngineManager().getEngineByName("javascript");
someFunctionThatTakesInALambdaEquation((Function<Double, Double>)engine.eval("x -> x + 3")); 

Answer 1

JavaScript's syntax for a lambda expression is x => x + 3, or equivalently you can write it as an anonymous function function(x) { return x + 3; }. Take note that it is slightly different from Java's syntax for a lambda expression x -> x + 3.

So I tried engine.eval("x => x + 3");, but got an exception from the script engine. I don't understand why.
But with

AbstractJSObject obj = (AbstractJSObject) engine.eval("function(x) { return x + 3; }");

you can get a JavaScript function object. Then you need to convert this to a Java function object (aka lambda expression).

Function<Double, Double> f = x -> (Double) obj.call(null, x);

and then do what ever you like

someFunctionThatTakesInALambdaExpression(f);