如何在varargs中测试arg

Question

I have a method that accepts varargs:

public static void main(String... args) throws Exception {

I'm trying to set the value of a variable based upon if an argument is passed or not. In this case, I want to know if an 8th argument has been passed and if so set the variable's value to that argument's value, otherwise use an empty string. I cannot hard code in args[8] on the boolean true result portion because that results in a compile error when it's ran with only 7 arguments. So I tried the following but it also errors out:

String titlePrefix = ((args.length!=8) ?  "" : args[args.length]);

What is the correct way to test if an argument has been passed in this case and use it's value only if it has been passed? I know that the above would give me problems should this method ever exceed 8 arguments, but for now, let's just assume that it will never have more than 8.

Thank you in advance.

Answer 1

args [args.length]应为args [args.length -1]

Answer 2

String titlePrefix = ((args == null || args.length<8) ?  "" : args[7]);

And why?

First, you may need the 9th or the 10th argument in the future. Having 9 arguments means that the 8th argument is present, so you should test whether you have at least 8 arguments, not whether you have exactly 8 arguments.

Second, for the same reason, you should not be assigning the last argument, but the exact 8th argument. After all, the last argument could be the 9th or 10th or 100th.

But even if you were testing the last argument, please remember that arrays in Java (and varargs is merely a disguised array) start from index 0 to index length-1 . The first argument is args[0] , the second is args[1] and the eighth is args[7] . So to test the last argument, you have to use args[args.length-1] .

When dealing with varargs, it's usually best to also test for null first (although when we are talking about the main method, it's probably not going to happen).

Answer 3

Check this. I think that's exactly what you're looking for.

public class Test001 {

    public static void main(String... args) throws Exception {
        for (int i=0; i<8; i++){
            String val = args.length <= i ? "" : args[i];
            System.out.println("arg[" + i + "] = [" + val + "].");
        }
    }

}