調用帶有空參數的重載構造函數

Question

由於編譯器不知道要調用哪個構造函數，因此no參數構造函數將引發錯誤。解決辦法是什麼？

private Test() throws Exception { 
    this(null);//THIS WILL THROW ERROR, I WAN'T TO CALL A SPECIFIC CONSTRUCTOR FROM THE TWO BELOW. HOW TO DO?? 
} 
private Test(InputStream stream) throws Exception { 

} 



private Test(String fileName) throws Exception { 

} 

Answer 1

類型轉換null：

private Test() throws Exception { 
    this((String)null); // Or of course, this((InputStream)null); 
} 

但似乎有點奇怪，你會想打電話Test(String)或Test(InputStream)有null參數...

Answer 2

我不明白爲什麼所有那些精心打造的建築師都是私人的。

我會做這種方式：

private Test() throws Exception { 
    this(new PrintStream(System.in); 
} 

private Test(InputStream stream) throws Exception { 
    if (stream == null) { 
     throw new IllegalArgumentException("input stream cannot be null"); 
    } 
    // other stuff here. 
}  

private Test(String fileName) throws Exception { 
    this(new FileInputStream(fileName)); 
}