New to Java : Constructor error control

Question

I'm new to Java and I'm porting one of my C++ libraries to Java as a learning experiment. This is not homework (as should be obvious from looking at my code). I have a few questions concerning the following code of my constructor for an ESRI shape file reader.

import java.io.*; /** * * @author Bill */ public class ShapeFileReader { public FileInputStream inStream; /* * @param fileName File name string. Must not be null or zero length. * @throws Exception if file specified by fileName fails to open */ public ShapeFileReader(String fileName) throws IOException { if(fileName == null) throw new NullPointerException("fileName was null"); if(fileName.length() == 0) throw new IllegalArgumentException("fileName string length was zero"); File fi = new File(fileName); if(fi.exists() == false) throw new IOException("File does not exist: " + fileName); // Catch-or-specify (this method already throws IOException) inStream = new FileInputStream(fileName); } }

During parameter validation and existence should I be throwing the exceptions as shown? The validation throws unchecked exceptions, and the existence throws checked exceptions. I assume that FileInputStream constructor will also throw an IOException, but I specified that in the method throws clause.

I was considering refactoring the opening of the file to a seperate function, but I figured it would be more useful and simple to do this in the constructor, and also learns me how to control errors here. Besides, any instance of this object will not have a closed/open state. All of these objects are reserved strictly for READING a file only, so they are created on a as-needed basis per file. I will provide a close() method seperately.

Also, from an extensibility point of view, would it be difficult to adapt to reading a file over a network using the FileInputStream with the current constructor? Or should I use a different class and multiple constructors?

Thanks for any and all input.

Tom · Accepted Answer · 2011-11-06 04:41:33Z

I wouldn't bother with the exceptions, FileInputStream will throw an exception for you, you're not adding anything other than clutter to your code.

For it to work with the network rather than just a file you'd modify thus:

public class ShapeFileReader { private final InputStream inStream; public ShapeFileReader(InputStream inStream) { this.inStream = inStream; } /* * @param fileName File name string. Must not be null or zero length. * @throws Exception if file specified by fileName fails to open */ public ShapeFileReader(String fileName) throws IOException { this(new FileInputStream(fileName)); }

Since this has been accepted as the answer I'm editing it as Roland (in the comments) is quite correct and this isn't how I'd have approached the problem.

 public class ShapeReader { public static Shape readShape(InputStream inStream) { ... do the work } /* * @param fileName File name string. Must not be null or zero length. * @throws Exception if file specified by fileName fails to open */ public static Shape readShape(String fileName) throws IOException { FileInputStream fis = new FileInputStream(fileName); try { return readShape(fis); } finally { fis.close(); } } }

The existence of the file is not commonly checked before opening?
The FileInputStream already does the check, so why should you do it twice?
Note that using this style, the ShapeFileReader no longer has the obligation to close the InputStream, since that must be done by the code that created the stream. And note that Java distinguishes a Reader for characters and an InputStream for bytes. Pick the one that suits you best.
Thank you. One more question if you don't mind. When using FileInputStream to read over a network, can the data be read using random access (ie rewind and set file pointer)?
Oops, now I see that the two constructors lead to trouble: which code is responsible for closing the input stream? For the first constructor, it's the code calling the constructor, for the constructor taking the filename it's the ShapeFileReader itself.

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