这个布尔数组如何工作？

Question

I don't understand how this code works. The problem is find the next year after a given year with distinct digits.

public Main(){
    try{
        BufferedReader in;
        in = new BufferedReader (new InputStreamReader (System.in)); //Used for CCC
        String year = (in.readLine());
        for(int i = Integer.parseInt(year)+1;; i++){
            String f = Integer.toString(i);
            boolean [] characters = new boolean[10];
            boolean unique = true;
            for(int x = 0; x < f.length(); x++){
                if (characters[f.charAt(x) - '0']){
                    unique = false;
                    break;
                }
                else{
                    characters[f.charAt(x)-'0'] = true; 
                }
            }   
            if (unique){
                System.out.println(f);
                return; 
            }

Specifically how does this if statement work?

if (characters[f.charAt(x) - '0']){

what does checking a boolean do here? Shouldn't they all be defaulted to false? and why is the -'0' there? Sorry for the noob question.

Answer 1

The expression

f.charAt(x) - '0'

will return a number between 0 and 9 depending on the digit at position x in the String f . Hence the code will set a boolean flag at each of the 10 digit positions in the array, and if a digit occurs more than once, then it will report false and break out of the inner loop to consider the following year.

Answer 2

characters is a boolean array of 10 elements and stores whether a given input character (which are supposed to be limited to decimal digits) appears more than once in the input.

Character codes for digits are in the 0x30 - 0x39 range; f.charAt(x) returns the character at index x in the input and then - '0' transforms it into a number in the range of 0 - 9 - an index into the boolean array.

When a character is first found, its slot is set to true ; when it's found again, unique is set to false to indicate that the input has duplicate characters.