如何在沒有類實例的情況下訪問枚舉類中的靜態變量？

Question

我有一些處理固定長度數據記錄的代碼。我用java枚舉定義了記錄結構。我把它簡化成最簡單的例子來說明我現在必須跳過的框架才能訪問枚舉中的靜態變量。有沒有更好的方法來獲得我忽略的這個變量？如果您編譯並運行代碼，它會將每個字符串轉換爲XML塊。我已經手動完成了XML，因此該示例更簡單，但我的真實代碼使用Java DOM。

編輯：我已經更新了一個更完整的例子，我認爲更好地顯示了我想要做的事。

class EnumTest 
{ 
    public static void main(String args[]) 
    { 
     System.out.println(parse("ABC", RecordType1.class)); 
     System.out.println(parse("ABCDEF", RecordType2.class)); 
    } 

    private interface RecordLayout { 
     public int length(); 
    } 

    private enum RecordType1 implements RecordLayout { 
     FIELD1  (2), 
     FIELD2  (1), 
     ; 
     private int length; 
     private RecordType1(int length) { this.length = length; } 
     public int length() { return length; } 

     public static int RECORD_LEN = 3; 
    } 

    private enum RecordType2 implements RecordLayout { 
     FIELD1  (5), 
     FIELD2  (1), 
     ; 
     private int length; 
     private RecordType2(int length) { this.length = length; } 
     public int length() { return length; } 

     public static int RECORD_LEN = 6; 
    } 

    private static <E extends Enum<E> & RecordLayout> String parse(String data, Class<E> record) { 

     // ugly hack to get at RECORD_LEN... 
     int len = 0; 
     try { 
      len = record.getField("RECORD_LEN").getInt(record); 
     } 
     catch (Exception e) { System.out.println(e); } 

     String results = "<RECORD length=\"" + len + "\">\n"; 
     int curPos = 0; 
     for (E field: record.getEnumConstants()) { 
      int endPos = curPos + field.length(); 
      results += " <" + field.toString() + ">" 
        + data.substring(curPos, endPos) 
        + "</" + field.toString() + ">\n"; 
      curPos = endPos; 
     } 
     results += "</RECORD>\n"; 
     return results; 
    } 
} 

Answer 1

這個代碼在Java 8中怎麼樣？

// Java 8 
public class EnumTest { 
    public static void main(String args[]) { 
     System.out.println(parse("ABC", RecordType1.class)); 
     System.out.println(parse("ABCDEF", RecordType2.class)); 
    } 

    private interface RecordLayout { 
     public int length(); 

     public static <E extends Enum<E> & RecordLayout> int getTotalLength(Class<E> e) { 
      int total = 0; 
      for (E e1 : e.getEnumConstants()) { 
       total += e1.length(); 
      } 
      return total; 
     } 
    } 

    private enum RecordType1 implements RecordLayout { 
     FIELD1(2), 
     FIELD2(1),; 
     private int length; 

     private RecordType1(int length) { 
      this.length = length; 
     } 

     public int length() { 
      return length; 
     } 
    } 

    private enum RecordType2 implements RecordLayout { 
     FIELD1(5), 
     FIELD2(1),; 
     private int length; 

     private RecordType2(int length) { 
      this.length = length; 
     } 

     public int length() { 
      return length; 
     } 
    } 

    private static <E extends Enum<E> & RecordLayout> String parse(String data, Class<E> record) { 
     int len = RecordLayout.getTotalLength(record); 
     String results = "<RECORD length=\"" + len + "\">\n"; 
     int curPos = 0; 
     for (E field : record.getEnumConstants()) { 
      int endPos = curPos + field.length(); 
      results += " <" + field.toString() + ">" 
        + data.substring(curPos, endPos) 
        + "</" + field.toString() + ">\n"; 
      curPos = endPos; 
     } 
     results += "</RECORD>\n"; 
     return results; 
    } 
} 

Answer 2

RecordType1.LEN不工作？

我現在手邊沒有Java環境，所以我準備從我的臉上抹蛋，但因爲enum Foo等於class Foo extends Enum<Foo>我看不到爲什麼你不能只訪問靜態成員標準的方式。

編輯：如果你想動態地訪問靜態成員，那麼這是不會工作的，因爲你有有基於類而不是實例訪問靜態成員，這就是要點。你或許應該做這樣的事情，而不是，因爲如果根據實例的亞型行爲的變化，這是正是的那種東西的接口是：

class EnumTest 
{ 

    private interface RecordLayout { 
     public int length(); 
     public int staticLength(); // give this a more meaningful name 
    } 

    private enum RecordType1 implements RecordLayout { 
     ... 

     public static int LEN = 3; 
     public int staticLength() { 
      return LEN; 
     } 
    } 

    private enum RecordType2 implements RecordLayout { 
     ... 

     public static int LEN = 5; 
     public int staticLength() { 
      return LEN; 
     } 
    } 

    ... 

    private static String parse(String data, RecordLayout record) { 
     int len = record.getStaticLength(); 
     System.out.println(len); 

     ... 
    } 
} 

Answer 3

只要你是裏面的EnumTest類，你可以通過簡單的寫訪問LEN領域：

int len = RecordType1.LEN; 

從外面看，它仍然是困難的。