java.lang.NumberFormatException：对于输入字符串：“”解析字符串表达式

Question

I'm trying to parse a string expression (ie "6+10*2-5") and calculate the answer as a double using a for-case statement. However I'm getting the error:

Exception in thread "main" java.lang.NumberFormatException: For input string: ""
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
    at java.lang.Integer.parseInt(Integer.java:470)
    at java.lang.Integer.parseInt(Integer.java:499)
    at Calculator.random(Calculator.java:50)
    at Calculator.main(Calculator.java:210)

any ideas?

public static double random(String exp) {
    double primeResult = 0;
    for (int i=0; i<exp.length();i++) {
        double left = Integer.parseInt(exp.substring(0,i));
        double right = Integer.parseInt(exp.substring(i+1,i+2));
        switch(exp.charAt(i)) {
            case '*':
            primeResult = left * right;
            break;
        case '/':
            primeResult = left / right;
            break;
        case '+':
            primeResult = left + right;
            break;
        case '-':
            primeResult = left - right;
            break;
        default:
            return 0;
        }
        System.out.println(primeResult);
        }
        return primeResult;
    }
}

Answer 1

Your problem has nothing to do with switch / case . The problem is simply that you're trying to parse an empty string as a number. Work out why you've got an empty string, then fix it. (Hint: what do you expect exp.substring(0, 0) to return?)

You should also consider what you expect exp.substring(i+1,i+2) to return when i is exp.length() - 1 (or even exp.length() - 2 ).

Also, take a step back and consider why you thought this was a problem with the switch / case part. Look at the stack trace - it's not even within the switch / case statement, but before it. It's very important to be able to diagnose where errors are actually occurring, so you can focus on that and not get distracted by irrelevant parts.

Answer 2

Jon Skeet has answered your question with very good diagnostic training.

But... I do think your solution to your problem will not work.

If you mentally trace through your code, assuming a proper input of "5+5", the first thing your loop will do is this:

double left = Integer.parseInt(exp.substring(0,i));
double right = Integer.parseInt(exp.substring(i+1,i+2));

With i==0 (first time through the loop), this will translate to:

double left = Integer.parseInt(exp.substring(0,0));
double right = Integer.parseInt(exp.substring(0+1,0+2));

This will IMMEDIATELY get an "empty string" for exp.substring(0,0) , which throws the exception in your question.

Oh, and why is your function called "random" if you're evaluating an expression?