Read String as Java Code

Question

I am creating a Graphing Calculator as our project in Calculus. The only thing left in my program is the input from the user.

For example, the user placed an input in the text field named 'userInput':

y=sin(x)

Next, converting it to string will have

String equation = userInput.getText();

And the string variable 'equation' will have a value which is:

"y=sin(x)"

Is there a way to read a String as a Java code?

http://stackoverflow.com/questions/2605032/using-eval-in-java — Leon, Sep 23 '13 at 10:41
Be careful when using eval()! It's the go-to way to ruin the security of any system you run it on. — Timothy Groote, Sep 23 '13 at 10:42
Might be better to read it like a mathematical equation rather than arbitrary java code — nitegazer2003, Sep 23 '13 at 10:45
Such easy exmples you can parse on you own. More complex values you can compile in runtime (but this isnt secure) — Oleksandr Samsonov, Sep 23 '13 at 10:51

fvu · Answer 1 · 2013-09-23T10:51:26.370

Janino can do that for you, assuming that you want these expressions to be valid Java, I think the ExpressionDemo would be a good starting point for what you want.

The basic code will look +- like this (quickly adapted from the basic example):

// Compile the expression once; relatively slow.
ExpressionEvaluator ee = new ExpressionEvaluator(
    "sin(x)",                    // expression
    double.class,                // expressionType
    new String[] { "x" },        // parameterNames
    new Class[] { double.class } // parameterTypes
);

// Evaluate it with varying parameter values; very fast.
Double res = (Integer) ee.evaluate(
    new Object[] {          // parameterValues
        new Double(12.34)
    }
);
System.out.println("result = " + res);

score 0 · Answer 2 · answered Sep 23 '13 at 10:51

I had the same problem before (while creating a graphing calculator in C). I converted the expression in its Reverse Polish notation and then parsed it using Shunting-yard algorithm. You can also check this GeeksForGeeks tutorial on expression evaluation.

score 0 · Accepted Answer · answered Sep 23 '13 at 10:57

0

This does not exactly answers your question, but is probably a much better way to achieve your goal. See Javaluator.

answered Sep 23 '13 at 10:57

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Read String as Java Code

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