我拼命试图找出一种方法来阻止“字符串索引超出范围：0”错误……只要我不输入任何内容然后继续执行,就会发生这种情况：

static String getRef(Scanner Keyboard)

{

Scanner keyboard = new Scanner(System.in);

String ref= "";

boolean valid = false;

int errors = 0;

boolean problem = false;

while(valid==false)

{

System.out.println("Please enter a reference number which is two letters followed by three digits and a letter(B for business accounts and N for non business accounts)");

ref = keyboard.nextLine();

for (int i=0; i<6; i++)

{

if (ref.charAt(i)=='\0')

{

problem = true;

}

}

if(problem == true)

{

System.out.println("The reference must consist of 6 Characters");

}

else

{

if ((Character.isDigit(ref.charAt(0))== true) || (Character.isDigit(ref.charAt(1))== true))

{

System.out.println("The first 2 characters must be letters");

errors = errors + 1;

}

if ((Character.isDigit(ref.charAt(2))== false) || (Character.isDigit(ref.charAt(3))== false)||(Character.isDigit(ref.charAt(4))== false))

{

System.out.println("The 3rd,4th and 5th characters must be numbers");

errors = errors + 1;

}

if ((!ref.toUpperCase().endsWith("B"))&&(!ref.toUpperCase().endsWith("N")))

{

System.out.println("The 6th character must be either B(for business accounts) or N(for non business accounts) ");

errors = errors + 1;

}

if (errors==0)

{

valid=true;

}

}

}

return ref;

}

我想要的是,如果字符串不包含特定索引处的字符,则能够输出错误消息,例如ref.charAt(0).

这将有所帮助(例子)：

if(ref.charAt(aNumber)==null) {

// display error message

}