Is there an eval() function in Java? [duplicate]

Answer 1

As previous answers, there is no standard API in Java for this.

You can add groovy jar files to your path and groovy.util.Eval.me("4*5") gets your job done.

Answer 2

There are some perfectly capable answers here. However for non-trivial script it may be desirable to retain the code in a cache, or for debugging purposes, or even to have dynamically self-updating code.

To that end, sometimes it's simpler or more robust to interact with Java via command line. Create a temporary directory, output your script and any assets, create the jar. Finally import your new code.

It's a bit beyond the scope of normal eval() use in most languages, though you could certainly implement eval by returning the result from some function in your jar.

Still, thought I'd mention this method as it does fully encapsulate everything Java can do without 3rd party tools, in case of desperation. This method allows me to turn HTML templates into objects and save them, avoiding the need to parse a template at runtime.

Answer 3

You can use the ScriptEngine class and evaluate it as a Javascript string.

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("js");
Object result = engine.eval("4*5");

There may be a better way, but this one works.

Answer 4

There is no standard Java class or method that will do what you want. Your options include:

• Select and use some third-party expression evaluation library. For example JEL or any of the half dozen libraries listed here.

• Wrap the expression in the Java source code for a class with an eval method, send that to the Java compiler, and then load the resulting compiled class.

• Use some scripting language that can be called from Java as an expression evaluator. Possibilities include Javascript, BeanShell, and so on.

• Write your own expression evaluator from scratch.

The first approach is probably simplest. The second and third approaches are a potential security risk if you get the expression to be evaluated from an untrusted user. (Think code injection.)

Answer 5

There are very few real use cases in which being able to evaluate a String as a fragment of Java code is necessary or desirable. That is, asking how to do this is really an XY problem: you actually have a different problem, which can be solved a different way.

First ask yourself, where did this String that you wish to evaluate come from? Did another part of your program generate it, or was it input provided by the user?

• Another part of my program generated it: so, you want one part of your program to decide the kind of operation to perform, but not perform the operation, and a second part that performs the chosen operation. Instead of generating and then evaluating a String, use the Strategy, Command or Builder design pattern, as appropriate for your particular case.

• It is user input: the user could input anything, including commands that, when executed, could cause your program to misbehave, crash, expose information that should be secret, damage persistent information (such as the content of a database), and other such nastiness. The only way to prevent that would be to parse the String yourself, check it was not malicious, and then evaluate it. But parsing it yourself is much of the work that the requested evalfunction would do, so you have saved yourself nothing. Worse still, checking that arbitrary Java was not malicious is impossible, because checking that is the halting problem.

• It is user input, but the syntax and semantics of permitted text to evaluate is greatly restricted: No general purpose facility can easily implement a general purpose parser and evaluator for whatever restricted syntax and semantics you have chosen. What you need to do is implement a parser and evaluator for your chosen syntax and semantics. If the task is simple, you could write a simple recursive-descent or finite-state-machine parser by hand. If the task is difficult, you could use a compiler-compiler (such as ANTLR) to do some of the work for you.

• I just want to implement a desktop calculator!: A homework assignment, eh? If you could implement the evaluation of the input expression using a provided eval function, it would not be much of a homework assignment, would it? Your program would be three lines long. Your instructor probably expects you to write the code for a simple arithmetic parser/evaluator. There is well known algorithm, shunting-yard, which you might find useful.

Answer 6

I could advise you to use Exp4j. It is easy to understand as you can see from the following example code:

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();