使用Java，匹配输入值的部分，按规律输出

package Interview;

import com.sun.javafx.image.BytePixelSetter;
import jdk.internal.util.xml.impl.Input;
import jdk.nashorn.internal.ir.CaseNode;

import java.util.Scanner;

/**
 * @author 关根普
 * @create 2021-02-19 10:28
 */
public class Demo01 {
    
    
    public static void main(String[] args) {
    
    
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入8位设备编号");
        String code = scanner.next();
        Rule rule = new Rule();
        System.out.println(rule.a(code));
        scanner.close();
    }
}
class Rule{
    
    
    public String a(String code){
    
    
        String b ="";
        String c ="";

        for(int i =0;i<4;i++){
    
    
            b+=code.charAt(i);
        }

        switch (b){
    
    
            case "3251":
                System.out.println("南京市");
                break;
            case "3252":
                System.out.println("无锡市");
                break;
            case "3253":
                System.out.println("徐州市");
                break;
            case "3254":
                System.out.println("常州市");
                break;
            case "3255": for(int i =4;i<6;i++){
    
    
                b+=code.charAt(i);}
                if( b.equals("325508")){
    
    
                    System.out.println("昆山市");
                    break;}
                System.out.println("苏州市（不含昆山）");
                break;
            case "3256":
                System.out.println("南通市");
                break;
            case "3257":
                System.out.println("连云港");
                break;
            case "3258":
                System.out.println("淮安市");
                break;
            case "3259":
                System.out.println("盐城市");
                break;
            case "3260":
                System.out.println("扬州市");
                break;
            case "3261":
                System.out.println("镇江市");
                break;
            case "3262":for(int i =4;i<6;i++){
    
    
                b+=code.charAt(i);}
                if( b.equals("326205")){
    
    
                    System.out.println("泰兴市");
                    break;}
                System.out.println("泰州市（不含泰兴）");
                break;
            case "3263":for(int i =4;i<6;i++){
    
    
                b+=code.charAt(i);}
                if( b.equals("326303")){
    
    
                    System.out.println("沐阳市");
                    break;}
                System.out.println("宿迁市（不含沐阳）");
                break;

            default:
                System.out.println("输入编号有误，请重新输入");

        }
        return(c);}

}

遇到的问题：如何设置输入部分的长度刚好为8，不足或超出都输出“输入号码不规范”？