java - 处理传递 String 变量时出现 NullPointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 3 年前。

我正在尝试编写一个处理 Excel 工作表的愚蠢版本的项目。我允许用户执行的命令之一是将值以 = 的格式分配给特定单元格。即 A3 = 72.3%。将值分配给单元格后，它就会打印出包含更新更改的整个电子表格。

我创建了不同的类来表示不同类型的细胞。值单元格是包含任何 double 值的单元格，百分比单元格是包含任何百分比值的单元格，文本单元格是包含文本的单元格。

当我尝试运行我的程序时，我尝试为单元格分配百分比值。 (A2 = 7.25%)当程序使用新分配的 A2 百分比单元格打印出整个电子表格时，它应该截断百分比中的小数并仅显示 7%。但是，我不断收到此错误消息

Exception in thread "main" java.lang.NullPointerException
    at PercentCell.abbreviatedCellText(PercentCell.java:18)
    at Spreadsheet.getGridText(Spreadsheet.java:127)
    at Spreadsheet.processCommand(Spreadsheet.java:73)
    at TextExcel.main(TextExcel.java:20)

这是我的代码的一部分，根据用户输入的内容分配特定的单元格类型:

//assignment command   
      } else if (command.contains("=")) {
         int eqIndex = command.indexOf("=");
         if (!command.substring(eqIndex - 1, eqIndex).equals(" ") || !command.substring(eqIndex + 1, eqIndex + 2).equals(" ")) {
            return "Formatting Error: please include a space before and after the ="; //error message
         } else {
            String[] input = command.split(" ", 3);
            SpreadsheetLocation cell = new SpreadsheetLocation(input[0]);
            int col = cell.getCol();
            int row = cell.getRow();
            if (col > COLUMNS || row > ROWS) {
               return "Error: cell outside of spreadsheet"; //error message
            }
            //assigning a textcell
            if (input[2].contains("\"")) {
               String inputValue = input[2].substring(1, input[2].length() - 1);
               TextCell textCell = new TextCell(inputValue);
               spreadsheet[row][col] = textCell;
            //assigning a percent cell 
            } else if (input[2].contains("%")) {
               PercentCell percentCell = new PercentCell(input[2]);
               spreadsheet[row][col] = percentCell;

单元格扩展父类(super class)、真实单元格的百分比:

public class RealCell implements Cell {
   private String fullText;

   public RealCell(String input) {
      this.fullText = input;
   }

   //method that returns the display of the abbreviated cell 
   public String abbreviatedCellText() {
      if (fullText.length() > 10) {
         return fullText.substring(0, CELLWIDTH);
      } else {
      String output = fullText;
      while(output.length() < CELLWIDTH) {
         output += " ";
      }
         return output;
      }
   }

   //method that returns the actual value in a real cell 
   public String fullCellText() {
      return fullText;
   } 

   //method that parses the user input into a double
   public double getDoubleValue() {
      return Double.parseDouble(fullText);   
   } 
}

现在这是我的代码的问题部分，Percent Cell 类:

public class PercentCell extends RealCell {
   private String fullText;

   public PercentCell(String input) {
      super(input);
   }

   //method that returns the display of the abbreviated cell, truncating the decimal  
   public String abbreviatedCellText() {
      String value = fullText;
      if (value.contains(".")) {
         int decIndex = fullText.indexOf(".");
         value = value.substring(0, decIndex) + "%";
      }  
      if (value.length() > 10) {
         return value.substring(0, CELLWIDTH);
      } else {
      String output = value;
      while(output.length() < CELLWIDTH) {
         output += " ";
      }
         return output;
      }
   }

   //method that parses the user input into a double and returns the percent value into a decimal
   public double getDoubleValue() {
      double decimal = Double.parseDouble(fullText.substring(0, fullText.length() - 1));
      return decimal/100;   
   }
}

如何修复此错误？如果需要对我的代码进行任何说明(因为这不是我的整个项目)，请告诉我!

Answer 1

     public class PercentCell extends RealCell {

     public PercentCell(String input) {
          super(input);
       }

       //method that returns the display of the abbreviated cell, truncating the decimal  
       public String abbreviatedCellText() {
          String value = super.fullCellText();
          if (value.contains(".")) {
             int decIndex = value.indexOf(".");
             value = value.substring(0, decIndex) + "%";
          }  
          if (value.length() > 10) {
             return value.substring(0, CELLWIDTH);
          } else {
          String output = value;
          while(output.length() < CELLWIDTH) {
             output += " ";
          }
             return output;
          }
   }

您使用了 fullText 字段但没有给出值，并且 fullText 值始终为 null我认为这就是问题所在，但如果有帮助，请告诉我!