java - java读写数据报错

Question

线程“main”中的异常 java.lang.NullPointerException 在 MovieSeating.assignCustomerAt(MovieSeating.java:27) 在 Assignment8.main(Assignment8.java:84)

在我从包含名字和姓氏列表的 txt 文件中提取数据后，在我尝试将名称分配给二维数组范围内的某个元素后，它给出了错误

谢谢!

---

    public class Customer
     {
       private String lastName;
       private String firstName;

       // This constructor sets the first name and last name to "???�
       public Customer()
       {
              lastName = "???";
              firstName = "???";
       }

      // This constructor constructs a Customer object  given the last name and first name
       public Customer(String customerInfo)
       {
             int space = customerInfo.indexOf(" ");
             firstName = customerInfo.substring(0, space).trim();
             lastName = customerInfo.substring(space+1).trim();

       }

       // This constructor cConstructs a Customer object using the string containing customer's info.
       // It uses the StringTokenizer to extract first name, last name, id, the number of matinee tickets,
       // and the number of normal tickets.
       public Customer(String lName, String fName)
       {
             lastName = lName;
             firstName = fName;

       }

       // This method sets the last name.
       public void setLastName(String lName)
       {
             lastName = lName;
       }
       // This method sets the first name.
       public void setFirstName(String fName)
       {
             firstName = fName;
       }

       // This method returns the last name.
       public String getLastName()
       {
             return lastName;
        }
       // This method returns the first name.
       public String getFirstName()
       {
             return firstName;
       }

       // This method checks if a customer object passed as a parameter and itself (customer object)
       // are same using their last names and first names.
       public boolean equals(Customer other)
       {
             if (lastName.equals(other.lastName) && firstName.equals(other.firstName))
                 return true;
             else
                 return false;
       }

       // This method returns a string containing a customer's initials
       // (first characters of firstName and lastName.)
       public String toString()
       {
               String result = firstName.charAt(0) + "." + lastName.charAt(0) + ".";
               return result;
       }


     } // end of the class Customer

---

    class MovieSeating 
    {
        private String[][] Seats;
        public MovieSeating(int rowNum, int columnNum)
        {
            String [][] Seats = new String[rowNum][columnNum];
            for (int r = 0; r < rowNum; r++)
            {
                for (int c = 0; c < columnNum; c++)
                {
                    Seats[r][c] = "?.?";
                }
            }
        }

        private Customer getCustomerAt(int row, int col)
        {
            System.out.println("Customer at row " + row + " and col " + col + "." );
            System.out.println(Seats[row][col]);

        }

        public boolean assignCustomerAt(int row, int col, Customer tempCustomer)
        {
            if (Seats[row][col].equals("?.?"))
            {
                tempCustomer = Seats[row][col];
                return true;
            }
            else {
                System.out.println("Seat taken..");
                return false;
            }

        }

        public boolean checkBoundaries(int row, int col)
        {
            if (col < 0 || row < 0)
            {
                return false;
            }
            else {
                return true;
            }
        }
    }

---

    import java.io.*;
    import java.util.*;

    public class Assignment8
    {
       public static void main(String[] args) throws IOException
       {

           MovieSeating theatreSeating;
           Customer tempCustomer;
           int requestedRow, requestedCol, row, col, rowNum, columnNum;
           String line, fileName;

           // to read input from a KEYBOARD.
           Scanner stdin = new Scanner(System.in);

           // Ask a user to enter a number of rows for a movie theatre seating from a KEYBOARD.
           System.out.println("Please enter a number of rows for a movie theatre seating.");
           rowNum = stdin.nextInt();

           // Ask a user to enter a number of columns for a movie theatre seating from a KEYBOARD.
           System.out.println("Please enter a number of columns for a movie theatre seating.");
           columnNum = stdin.nextInt();

           // instantiate a MovieSeating object
           theatreSeating = new MovieSeating(rowNum, columnNum);

           // get a file name read from a KEYBOARD.
           System.out.println("Please enter a file name");
           fileName = stdin.next();

           // create FileReader and BufferedReader object to
           // read from a file.
           FileReader fr = new FileReader (fileName);
           BufferedReader inFile = new BufferedReader (fr);

           /*** reading a customer's information from a FILE ***/
           line = inFile.readLine();

           /*** we will read line by line until we read the end of a given file ***/
           while (line != null)
           {
               System.out.println("\nA customer information is read from a file.");
               // printing information read from a file.
               System.out.println(line);

               // creating a customer object using information from a file
               tempCustomer = new Customer(line);

               // Ask a user to decide where to seat a customer by asking for row and column of a seat
               System.out.println("Please enter a row number where the customer wants to sit.");
               requestedRow = stdin.nextInt();
               row = requestedRow -1;

               System.out.println("Please enter a column number where the customer wants to set.");
               requestedCol =  stdin.nextInt();
               col = requestedCol -1;

               // Checking if the row number and column number are valid (exist in the theatre that we created.)
               if (theatreSeating.checkBoundaries(row, col) == false)
               {
                    System.out.println("\nrow or column number is not valid.");
                    System.out.println("A customer " + tempCustomer.getFirstName() + " " + tempCustomer.getLastName() + " is not assigned a seat.");
               }
               else
               {
                  // Assigning a seat for a customer
                  if (theatreSeating.assignCustomerAt(row, col, tempCustomer) == true)
                  {
                    System.out.println("\nThe seat at row " + row + " and column " + col + " is assigned to the customer " + tempCustomer.toString());
                    System.out.println(theatreSeating);
                  }
                  else
                  {
                    System.out.println("\nThe seat is taken.");
                  }
                }
               // Read next line in a FILE
               line = inFile.readLine();

           }//end of the while loop
             // Closing the file
           inFile.close();

         }

      }

Answer 1

好的，这是你的问题:

您有一个名为seats 的数组。它是 String 类型。所以它可以容纳像“？？？”这样的字符串。或“免费座位”。但是，您尝试将客户放入其中。这正是错误消息所说的:error: incompatible types tempCustomer = Seats[row][col]; ^ 必需:客户发现:字符串 1 错误

现在您有几个选择。一个非常简单的方法是将数组的类型更改为 Customers。而不是输入“？？？”在空闲座位上，什么也不放。创建数组后，每个“槽”都是null，其实就是“空”的意思。

顺便说一句:您违反了许多编写 Java 代码的建议规则。这只是一些简单的事情，不会影响您的代码(无论如何它都会运行；)。但尊重这些不成文的规则是一个很好的做法(比如将 { 与函数头放在同一行，或者用大写字母命名变量 no)。