我开始学习java，我们学习的内容我就是无法继续，它没有解释太多，只是给你一个温和的例子，告诉你自己用完全不同的东西来做。如上所述，我需要采取'长数字'字符串，并采取最后一个字符了。

* Write a description of class CreditCardChecker here.

 * 
 * @author Craig Beverley 
 * @version 03/12/2020
 */
public class CreditCardChecker
{
   // Variable for long numbers to be checked
   public String longNumber;
   public StringBuilder firstFifteen;

   /**
    * Constructor for objects of class CreditCardChecker
    * including long number and first fifteen
    */
   public CreditCardChecker(String longNumber)
   {
      // initialise long number variable
      this.longNumber=longNumber;
   }
   /**
    * Sets the value of long number
    */
   public void setLongNumber(String aLongNumber)
   {
      this.longNumber=aLongNumber;
   }

   /**
    * method to get the long number
    */
   public String getLongNumber()
   {
      return this.longNumber;
   } 
  /**
   * method to check that long number has exactly 16 digits
   */    
  public boolean isCorrectLength()
  {
     if (longNumber.length() == 16)
     {
        return (true);
      }
      else
      {
         return (false);
      }
   }
   /**
    * Method to get the first 15 characters of long number
    */
   public String firstFifteen(String longNumber)
   {
    firstFifteen=longNumber.deleteCharAt(16);
   }
}```