What you need is an engine/library that can evaluate expressions, defined as string at execution time. If you wrap the evaluation code into function call (e.g. lambda function), you will get what you need.
Option 1: You can use exp4j. exp4j is a small footprint library, capable of evaluating expressions and functions at execution time. Here is an example:
Expression e = new ExpressionBuilder("3 * sin(y) - 2 / (x - 2)")
.variables("x", "y")
.build()
.setVariable("x", 2.3)
.setVariable("y", 3.14);
double result = e.evaluate();
Option 2: You can use the Java's script engine. You can use it to evaluate expressions defined, for example, in JavaScript:
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("js");
Object result = engine.eval("sin(1.25)");
Option 3: Compile to native Java. With this approach, you use template to generate .java file with a class that contains your expression. Than you call the Java compiler. This approach has the drawback that has some complexity in the implementation and some initial latency (until the class is compiled), but the performance is the best. Here are some links to explore:
Note of Caution Whatever approach you chose, have in mind that you need to think about the security. Allowing the user to enter code which can be evaluated without security restrictions could be very dangerous.
