method in main read string expression in java

Question

I need to create a method in main to read string expression and parse it with parenthesis first.

the expression is:
(3+4)*7/2

It has to be done in java. But i don't know how to do it.

Any ideas on that!

Answer 1

Try this:

ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine engine = mgr.getEngineByName("JavaScript");
String foo = "(3+4)*7/2";
double result=0;
try {
    /*
    It seems that (on some platforms) this results in a java.lang.RuntimeException 
    because of converting Object to double, so let's replace it with
    Double.doubleValue()
    */
    //result = (double) (engine.eval(foo));
    result = ((Double) (engine.eval(foo))).doubleValue(); //result = 24.5
} catch (ScriptException e) {
    //handle exception here
}

UPDATE

Before you try to evaluate the expression, you should test if the "JavaScript" ScriptEngine is registered on your system, you can do it like this:

ScriptEngineManager manager = new ScriptEngineManager();
List<ScriptEngineFactory> factories = manager.getEngineFactories();
for (ScriptEngineFactory factory : factories) {
    System.out.println(factory.getNames());
}

My output is

[js, rhino, JavaScript, javascript, ECMAScript, ecmascript]

If your output doesn't contain JavaScript, then try with js and javascript, like

ScriptEngine engine = mgr.getEngineByName("js");

or

ScriptEngine engine = mgr.getEngineByName("javascript");

If neither of them exist you can't use js as script engine to evaluate your expression