java - How do I validate what I input?

Question

This is what I need to achieve:

The user can try to input a valid license plate. The valid license plate must be in the following format: [XXXX][YYY] Where X is a number between 0 and 9 and Y is an alphabet between A and Z. The user can input word ‘random’ (case insensitive). After that, the program will generate a new random valid license plate.

This is my current code:

for (int i = 0; i < 25; i++) 
    System.out.println("");  
String plat = "";  
do { 
    System.out.print("License Plate Format: XXXXYYY "); 
    plat = scan.nextLine();  
    for (int i = 0; i < plate.size(); i++) {
        if(plat.matches(plate.get(i)))  { 
            System.out.println("not unique!"); 
            scan.nextLine();  plat = " "; break; 
        }  
    } 
while (plat.length() < 7 ); 
plate.add(plat);

Answer 1

You can use regular expressions for input validation (see here). For this particular task,assuming the plate must have upper case characters, you can match the input with this regular expression to perform validation.

// assuming input is a String and it is already defined
if (input.matches("[0-9]{4}[A-Z]{3}")) {
 return "Plate valid";
} else {
 return "Plate invalid";
}

This regular expression explicitly tells java to look for strings formed by 4 digits followed by 3 characters.