嘗試與資源時調用超級構造函數

Question

當在構造函數中打開InputStream，然後將其傳遞給超級構造函數時，是否有任何使用try-with-resources的好方法？

基本上就是我想要做的是這樣的：

public class A { 
    public A(InputStream stream) { 
     // Do something with the stream but don't close it since we didn't open it 
    } 
} 

public class B { 
    public B(File file) { 
     // We open the stream so we need to ensure it's properly closed 
     try (FileInputStream stream = new FileInputStream(file)) { 
      super(new FileInputStream(file)); 
     } 
    } 
} 

但是，當然，因爲super必須在構造函數中的第一條語句，這是不允許的。有沒有什麼好的方法來實現這一目標？

Answer 1

考慮使用靜態工廠方法，而不是直接使用構造函數。使至少B的構造私人，並創建一個方法，如

private B(InputStream is) { 
    super(is); 
    // Whatever else is needed 
} 

public static B newInstance(File file) { 
    B result; 
    try (FileInputStream stream = new FileInputStream(file)) { 
     result = new B(stream); 
    } 
    // Further processing 
    return result; 
} 

Answer 2

另一種方式去：

public class A { 
    protected A(){ 
     // so he can't be called from the outside, subclass ensure that init is done properly. 
    } 

    public A(InputStream stream) { 
     init(stream); 
    } 
    // not be able to call it from outside 
    protected final init(InputStream is){ 
     //here goes the code 
    } 
} 

public class B { 
    public B(File file) { 
     // We open the stream so we need to ensure it's properly closed 
     try (FileInputStream stream = new FileInputStream(file)) { 
      init(stream); 
     } 
    } 
} 

我在這裏張貼這是一個可能的答案，但是我在這裏consdering：

1. 可以更新的代碼
2. 你正在移動的構造函數的代碼到init方法，由於受保護的空參數構造函數，只有子類必須正確處理對init的調用。有些人可能會認爲設計不太好。我的觀點是儘快對子類進行子類化，只有當你使用它時，你必須更多地瞭解它。

Answer 3

很遺憾，我手邊沒有編譯器來測試，但是你可以不這樣做，如下所示。

public class B { 
    private static InputStream file2stream(File f){ 
     // We open the stream so we need to ensure it's properly closed 
     try (FileInputStream stream = new FileInputStream(file)) { 
      return stream; 
     }catch(/*what you need to catch*/){ 
      //cleanup 
      // possibly throw runtime exception 
     } 
    } 
    public B(File file) { 
     super(file2stream(file)) 
    } 
}