编写一个公共方法firstFifteen()，返回一个由longNumber中的前十五个字符组成的String

Question

我开始学习Java，我们学习的内容我无法继续，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，我需要去掉"longNumber"字符串并去掉它的最后一个字符。

* 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);
}
}```

Answer 1

我不完全确定您是否需要firstFifteen作为字段。你可以简单地获取前15个字符的

return longNumber.substring(0, 15);

该方法也不需要参数，因为longNumber是一个字段