当我运行程序时，出现错误java.lang.StringIndexOutOfBoundsException：字符串索引超出范围：0

Question

here is my code

import java.io.*;
import java.util.*;
public class weightOnOtherPlanets {
public static void main(String[] args) {
    Scanner kbReader = new Scanner(System.in);
    System.out.println("Enter your weight");
    double weight = kbReader.nextDouble();
    System.out.println("choose a planet by entering the corresponging letter\n");
    System.out.println("A.     Voltar");
    System.out.println("B.     Krypton");
    System.out.println("C.     Fertos");
    System.out.println("D      Servantos");
    String choice = kbReader.nextLine( );
    char p = choice.charAt(0);
    String answerPhrase = "Your weight is " + " " ;
    switch(p){
    case 'A':
    case 'a':
        System.out.println(answerPhrase +(.091*weight));
    break;
    case 'B':
    case 'b':
        System.out.println(answerPhrase + (.720*weight));
        break;
    case 'C':
    case 'c':
        System.out.println(answerPhrase + (.865*weight));
        break;
    case 'D':
    case 'd':
        System.out.println(answerPhrase + (4.612*weight));
        break;
    default:
        System.out.println("Please enter either A,B,C,or D");
        break;
    }
}
}

I have used almost the exact same code for another similar practice project and it worked just fine. When i run the program it goes to the point where it asks for a weight input, then it displays the choice list, but with the error message exception in "main":

 java.lang.StringIndexOutOfBoundsException: String index out of range: 0
    at java.lang.String.charAt(String.java:686)
    at weightOnOtherPlanets.main(weightOnOtherPlanets.java:14)

I don't know why it gives this error before allowing keyboard input for String choice.

Answer 1

This is because of the way Scanner scans lines.

After you enter a double (the weight), you press Return . This tells System.in to take all the characters you entered and pass them to the scanner. The scanner then reads the part that interests it - the double number - but leaves everything else waiting for the next operation.

This means the Return you pressed - the end-of-line - is still there. Now, the next thing is nextLine() . The scanner sees it, and it reads all the characters it has until it finds an end-of-line. But as we said, the end-of-line is right there. So it reads that, and gives you all the characters it found before it. Which is none at all, because there were no other characters between the double number and the end-of-line.

This means you get an empty string. And an empty string doesn't have a character at position 0 , because that would mean it was not empty.

So what you should do is, after receiving the double, you should add a kbReader.nextLine(); - just like that, without putting the value anywhere. This will skip the end-of-line you entered for the double, and then you'll get the next line properly.

When you do your menu reading, though, you should be checking that the string is not empty before you call charAt(0) . After all, the user can decide to press Return rather than make a valid choice. So your system should either ignore that or tell him that it's not a legal input, rather than fail with an exception.

Answer 2

You call nextDouble(); and after that you call nextLine() to get your answer phrase. But that call to nextLine(); will only consume the rest of the line on which you entered your double and it will be empty, therefore choice.charAt(0); will throw an exception.

Try doing something like this to consume the rest of the line and then call the nextLine() to get the answer phrase.

System.out.println("Enter your weight");
double weight = kbReader.nextDouble();
kbReader.nextLine(); // Consume the rest of the line

// ...

String choice = kbReader.nextLine(); // Get the actual input
char p = choice.charAt(0);

Answer 3

try this:

public static void main(String[] args) {
    Scanner kbReader = new Scanner(System.in);
    System.out.println("Enter your weight");
    double weight = kbReader.nextDouble();
    System.out.println("choose a planet by entering the corresponging letter\n");
    System.out.println("A.     Voltar");
    System.out.println("B.     Krypton");
    System.out.println("C.     Fertos");
    System.out.println("D      Servantos");
    String choice = kbReader.nextLine();
    if (choice.isEmpty()) {
        choice = kbReader.nextLine();
    }
    char p = choice.charAt(0);
    String answerPhrase = "Your weight is " + " ";
    switch (p) {
        case 'A':
        case 'a':
            System.out.println(answerPhrase + (.091 * weight));
            break;
        case 'B':
        case 'b':
            System.out.println(answerPhrase + (.720 * weight));
            break;
        case 'C':
        case 'c':
            System.out.println(answerPhrase + (.865 * weight));
            break;
        case 'D':
        case 'd':
            System.out.println(answerPhrase + (4.612 * weight));
            break;
        default:
            System.out.println("Please enter either A,B,C,or D");
            break;
    }
}

Answer 4

You need to read for input differently. That's why you are throwing an exception.

char p = choice.charAt(0); // why not just do a string comparison instead?

is where the error comes up. This is because choice is null / has no charAt(0).

Irregardless I would use something like this instead

char p = ''
while(in.hasNext()) {
    String input = in.nextLine();
    if (p.length()>0){
        p = choice.charAt(0);
    }
    //do whatever you wanted to with p

This should give you the behavior you are looking for.

Don't forget to consider changing the double input to work roughly the same though.