爲什麼我不打印對象的循環？

Question

我覺得這樣做至少可以打印出每個子類的toString方法，但由於某些原因，它不會打印任何內容。任何幫助將不勝感激。

data.txt中：

1 meter 
1 inch 
1 foot 
1 yard 
401.336 meters 
15839 inches 
1319 feet 
439 yards 

代碼

public abstract class Length implements Comparable<Length> { 

    public String toString() { 
     return this.getClass() + ": " + getLength() + " " + getUnit(); 
    } 



public static void main(String[] args){ 
     ArrayList<Length> lo = new ArrayList <Length>(); 
     Scanner in = null; 
     try { 
      in = new Scanner(new File("src/length/data.txt")); 
     } catch (FileNotFoundException exception) { 
      throw new RuntimeException("failed to open data.txt"); 
     } 
     // need more code for other parts of this assignment 
     while (in.hasNextDouble()) { 
      double value = in.nextDouble(); 
      String unit = in.next(); 
      Length length = null; 

      if(unit.equals("Meter")){ 
       length = new Meter(value); 
       lo.add(length);    
      } 

      else if(unit.equals("Foot")){ 

       length = new Foot(value); 
       lo.add(length); 
      } 

      else if(unit.equals("Inch")){ 
       length= new Inch(value); 
       lo.add(length); 
      } 

      else if(unit.equals("Yard")){ 
       length = new Yard(value); 
       lo.add(length); 
      } 

     } 
     Length[] myLengths = new Length[lo.size()]; //declares myLengths integer array 
     for(int i=0; i < lo.size(); i++){  
      myLengths[i]=lo.get(i);  //copies arrayList to lo to Length array myLengths 

     } 


    for(int i = 0; i < myLengths.length; i++) { 
      System.out.println(myLengths[i].toString()); 
     } 
    } 
} 

Answer 1

在此代碼的問題是字符串比較。相反unit.equals（「英制」），請使用unit.equalsIgnoreCase（「寸」）等修正代碼：

public abstract class Length implements Comparable<Length> { 

    public String toString() { 
     return this.getClass() + ": " + getLength() + " " + getUnit(); 
    } 

    public static void main(String[] args) { 
     ArrayList<Length> lo = new ArrayList<Length>(); 
     Scanner in = null; 
     try { 
      in = new Scanner(new File("src/length/data.txt")); 
     } catch (FileNotFoundException exception) { 
      throw new RuntimeException("failed to open data.txt"); 
     } 
     // need more code for other parts of this assignment 
     while (in.hasNextDouble()) { 
      double value = in.nextDouble(); 
      String unit = in.next(); 
      Length length = null; 

      if (unit.equalsIgnoreCase("Meter")) { 
       length = new Meter(value); 
       lo.add(length); 
      } else if (unit.equalsIgnoreCase("Foot")) { 

       length = new Foot(value); 
       lo.add(length); 
      } else if (unit.equalsIgnoreCase("Inch")) { 
       length = new Inch(value); 
       lo.add(length); 
      } else if (unit.equalsIgnoreCase("Yard")) { 
       length = new Yard(value); 
       lo.add(length); 
      } 

     } 
     Length[] myLengths = new Length[lo.size()]; //declares myLengths integer array 
     for (int i = 0; i < lo.size(); i++) { 
      myLengths[i] = lo.get(i);  //copies arrayList to lo to Length array myLengths 

     } 


     for (int i = 0; i < myLengths.length; i++) { 
      System.out.println(myLengths[i].toString()); 
     } 
    } 
} 

Answer 2

有在單位字符串比較你的代碼的兩個問題。

1. 其他答案已經提到，在代碼中，您使用的是「英寸」而文件中使用「英寸」。它可以通過使用equalsIgnoreCase()

2. 即使你做了1的變化，你仍然會面臨閱讀不完整文件的問題。原因是在文件中，你會遇到單數和複數單位名稱。它可以通過做像if (unit.equalsIgnoreCase("foot") || unit.equalsIgnoreCase("feet"))