Evaluate command line argument as boolean expression

Question

I'm trying to take a string, passed as an argument, and evaluate it as a boolean expression in an if conditional statement. For example, the user invokes MyProgram like $ java MyProgram x==y.

---

Example Program

Defined variables:

int x;
int y;

Boolean expression argument:

String stringToEval = args[0];

Control program execution with user expression:

if (stringToEval) {
    ...
}

Answer 1

You'll need to use some form of expression parser to parse the value in args[0] and create an expression that can be applied to x and y. You may be able to use something like Janino, JEXL or Jeval to do this. You could also write a small parser yourself, if the inputs are well-defined.

Answer 2

What you are doing is a bit complexe, you need to evaluate the expression and extract the 2 arguments form the operation

String []arguments = extractFromArgs(args[0])

there you get x and y values in arguments

then:

if (arguments [0].equals(arguments[1]))

If x and y are integers:

int intX = new Integer(arguments[0]);
int intY = new Integer(arguments[0]);
if (intX == intY)

etc...

PS: Why use Integer, Double ..? Because in String evaluation "2" is not equal to "2.0" whereas in Integer and Double evaluaiton, they are equal

Answer 3

What ever you are taking input as a command line argument is the String type and you want to use it as a Boolean so you need to covert a String into a boolean.

For doing this you have to Option either you use valueOf(String s) or parseBoolean(String s)

so your code must look like this,

S...
int x;
int y;
...
String stringToEval = args[0];
boolean b = Boolean.valueOf(stringToEval);

boolean b1 = Boolean.parseBoolean(stringToEval); // this also works 
...
if(b){
    printSomething();
} else printNothing();
...

Answer 4

So from what I understand the string args[0] is a boolean? Why not cast it to a boolean then?

boolean boolToEval = Boolean.parseBoolean(args[0]);
//OR
boolean boolToEval = Boolean.valueOf(args[0]);    

//THEN
(boolToEval ? printSomething() : printSomethingElse());