Java鑄造（動態）

Question

我敢肯定這是一個非常愚蠢的問題，但我仍然想知道，是否可以動態地強制使用全局變量cause，換句話說，不使用instanceof運算符？

的原因的問題是，我覺得instanceof運營商沒有做任何事情大在這裏，它只是鑄造cause靜態，但在任何情況下，它是創造一個new IOException(cause)

因爲cause是Object類型，我不得不打字輸入String或Throwable。

private Object cause; // global variable 
//... 
if (failed) 
    throw cause instanceof String ? new IOException((String) cause) : new IOException((Throwable) cause); 

下面是實際的代碼片段，其中兩個重寫的方法將被異步調用。

public class Command implements ResponseListener { 
    private Object cause; 

    // ... 
    @Override 
    public void messageReceived(String message, String status) { 
     // ... 
     if (!status.equals(MyConstants.COMPLD_MSG)) { 
      this.cause = status + " received for " + command.split(":")[0] + message; 
      this.failed = true; 
     } 
     doNotify(); 
    } 

    @Override 
    public void exceptionCaught(Throwable cause) { 
     this.cause = cause; 
     this.failed = true; 
     doNotify(); 
    } 

    public void waitForResponse(int cmdTimeout) throws IOException, InterruptedException { 
     // ... 
     if (failed) 
      throw cause instanceof String ? new IOException((String) cause) : new IOException((Throwable) cause); 
    } 
} 

Answer 1

爲什麼不總是一個Throwable爲您的原因變量？ Throwable似乎更適合表示失敗比字符串。此外，它避免了您使用「醜陋」的運營商instanceof。

public class Command implements ResponseListener { 
    private Throwable cause; 

    // ... 
    @Override 
    public void messageReceived(String message, String status) { 
     // ... 
     if (!status.equals(MyConstants.COMPLD_MSG)) { 
      this.cause = new Throwable(status + " received for " + command.split(":")[0] + message); 
      this.failed = true; 
     } 
     doNotify(); 
    } 

    @Override 
    public void exceptionCaught(Throwable cause) { 
     this.cause = cause; 
     this.failed = true; 
     doNotify(); 
    } 

    public void waitForResponse(int cmdTimeout) throws IOException, InterruptedException { 
     // ... 
     if (failed) 
      throw new IOException(cause); 
    } 
} 

---

更新下面的討論後：

public class Command implements ResponseListener { 
    private String cause; 

    // ... 
    @Override 
    public void messageReceived(String message, String status) { 
     // ... 
     if (!status.equals(MyConstants.COMPLD_MSG)) { 
      this.cause = status + " received for " + command.split(":")[0] + message; 
      this.failed = true; 
     } 
     doNotify(); 
    } 

    @Override 
    public void exceptionCaught(Throwable cause) { 
     if(cause.getMessage().isEmpty()) { 
      this.cause = cause.toString(); 
     } 
     else { 
      this.cause = cause.getMessage(); 
     } 
     this.failed = true; 
     doNotify(); 
    } 

    public void waitForResponse(int cmdTimeout) throws IOException, InterruptedException { 
     // ... 
     if (failed) 
      throw new IOException(cause); 
    } 
} 

Answer 2

由於存在的String和Throwable沒有共同的父類，這將是作爲參數傳遞給IOException接受，你必須將它轉換爲一個或另一個，並以此來確定哪些丟給，你有使用instanceof。

Answer 3

Class.cast和Class.isAssignableFrom方法可分別用作cast和instanceof運算符的動態對應方法。