DNA test with "if"

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        System.out.println("Введите свой днк. ");
        Scanner sc = new Scanner (System.in);
        String DNA = sc.nextLine();
        String RevDNA = "";
        for (int i = 1; i <= DNA.length(); i++) {
            char a = DNA.charAt(DNA.length() - i);
            if (a == 'A') {
                char b = 'T';
                RevDNA += b;
            } else if (a == 'T') {
                char b = 'A';
                RevDNA += b;
            } else if (a == 'C') {
                char b = 'G';
                RevDNA += b;
            } else if (a == 'G') {
                char b = 'C';
                RevDNA += b;
            }

        }
        System.out.printf("Ваш ДНК - %s", RevDNA);
    }
}