我不斷收到'線程異常'main「java.lang.NullPointerException'

Question

錯誤與writeToFile（）方法有關。我可以對它進行註釋並且它可以在沒有Exception錯誤的情況下運行。所以基本上，我試圖讓這些營養成分在文本文件上打印出來。 錯誤看起來像主 「顯示java.lang.NullPointerException在 Environment.writeToFile（Java1001.java:71）在 環境這個

「在線程異常」。（Java1001.java:54）在 Java1001 。主要（Java1001.java:43）」

import static java.lang.System.*; 
    import java.io.*; 

    public class Java1001 
    { 
     public static void main(String[] args) throws IOException 
     { 
      new Environment(); 
     } 
    } 

    class Environment 
    { 
     private PrintStream outFile; 

     public Environment() 
     { 
      createOutputStream(); 
      writeToFile(); 
     } 

     public void createOutputStream() 
     { 
      // create outFile stream here 
      File outFile = null; 
      try 
      { 
       outFile = new File("Nutrition.txt"); 
       outFile.createNewFile(); 
      } 
      catch (Exception e){} 
     } 

     public void writeToFile() 
     { 
      outFile.println("NUTRITION FACTS"); 
      outFile.println("======================================"); 
      outFile.println("Serving Size 1 tbsp (6g)"); 
      outFile.println("Servings Per Container about 4"); 
      outFile.println("======================================"); 
      outFile.println("Amount Per Serving"); 
      outFile.println("Calories   20"); 
      outFile.println("Calories from Fat 5"); 
      outFile.println("======================================"); 
      outFile.println("Total Fat 0.5g"); 
      outFile.println("Sodium 280mg"); 
      outFile.println("Total Carbohydrate 4g"); 
      outFile.println("Sugars 0g"); 
      outFile.println("Protein less than 1 g"); 
      outFile.println("======================================"); 
     } 
    } 

Answer 1

約翰尼MOPP指出你在createOutputStream方法既是不過outFile以及實例不過outFile。嘗試在createOutputStream方法中刪除File outFile = null;，然後在嘗試中寫入：

this.outFile = new File("Nutrition.txt"); 
this.outFile.createNewFile();