java - 程序编译但无法运行，无法使用构造函数

Question

谁能告诉我为什么我无法在“交互”面板中创建新对象，或者为什么该程序即使已编译也无法运行？

   import java.util.Random;
/**
 * This program allows a user to enter their first and last name
 * and generate a random user id and default password.
 * 
 * @author Brian Drake
 * @version 9/14/11
 */

   public class UserID {

      private String firstName;

      private String lastName;

      private String userId;

      private String password;


      public UserID(String first, String last) {
         Random generator = new Random();

         firstName = first;

         lastName = last;

         userId = first.substring(0, 3) + last.substring(0, 3)
            + generator.nextInt(1) + (generator.nextInt(7) + 3)
            + generator.nextInt(10);

         password = Integer.toString(generator.nextInt(10) + generator.nextInt(10)
            + generator.nextInt(10) + generator.nextInt(10)
            + generator.nextInt(10) + generator.nextInt(10));

      } 

      public String getId() {
         return userId;
      }

      public String getPassword() {
         return password;
      }

      public boolean setPassword(String randomPass) {

         if (password.length() < 6 || password.length() > 6) {
            randomPass = password;

            return false;       
         }

         else {
            password = randomPass;
         }
         return true;
      }

      public void generateNewPassword() {

         Random generator = new Random();

         password = Integer.toString(generator.nextInt(10) + generator.nextInt(10)
            + generator.nextInt(10) + generator.nextInt(10)
            + generator.nextInt(10) + generator.nextInt(10));

      }

      public String toString(String first, String last) {
         String output = firstName + " " + lastName + "\n";
         output += userId + "\n";
         output += password;
         return output;
      }

   }

Answer 1

它不会运行，因为没有 main 方法