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I'm taking some files from different folders in Java as below:

    File d=new File(path); // Path here is surely not null
    String s[]=d.list(); 
    System.out.println("Directory: "+path+s[0]+"/");

Above code works in Eclipse and my OS is Ubuntu 14.04. When I compile and run the project from terminal, I get the following error: 

Exception in thread "main" java.lang.NullPointerException
at io.ReadFile.read2Dir(ReadFile.java:97)
at io.ReadFile.readDir(ReadFile.java:134)
at gui.Run.readFile(Run.java:35)
at gui.Run.main(Run.java:304)

Even if s[0] corresponds to a folder, I get NullPointerException in terminal.

This is how I compile from terminal:

javac -sourcepath src -cp /home/myUsername/workspace/tezoz_my/ -encoding ISO-8859-1 src/gui/Run.java

By the way, I had to change encoding because project is created with ISO 8859-1.

Then I run the project: java gui.Run

This is where the error occurs: System.out.println("Directory: "+path+s[0]+"/");

Thanks.

Kerem
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  • have u debug the application ?? – Vikrant Kashyap Mar 19 '16 at 05:45
  • Can you show us, how you start it from terminal? – Nikolay Tomitov Mar 19 '16 at 05:54
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    Which line is `ReadFile.java:97` ? – Stephen C Mar 19 '16 at 06:08
  • @StephenC its a `java.io.ReadFile` man. there must be a problem with `path` variable. okk – Vikrant Kashyap Mar 19 '16 at 06:14
  • @kerem I think Your path variable doesn't contains valid URL Location. this is the only reason behind this.. – Vikrant Kashyap Mar 19 '16 at 06:16
  • @VikrantKashyap - You misread my comment. What **>>LINE<<** ? And no, your diagnosis is not correct. If path contained a URL then the code would not work at all. `new File(String)` requires a pathname not a URL. – Stephen C Mar 19 '16 at 06:16
  • [Use JSR 203 instead](http://java7fs.wikia.com/wiki/Why_File_sucks#Listing_files_in_a_directory.2C_oh_my) – fge Mar 19 '16 at 06:20
  • There is no way for the line `File d=new File(path)` to throw a NullPointerException. Even if `path` is null, the exception would be thrown by a line inside the `File` constructor, not by the line given. Ergo, the NPE is not in the first line shown, which is why @StephenC was asking what's in line 97 of file `ReadFile.java`. The NPE may be from line 3, if the `path` is invalid, because that makes `list()` return null, and `s[0]` in line 3 will throw NPE. – Andreas Mar 19 '16 at 06:21
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    @fge So which part exactly am I wrong about? – Andreas Mar 19 '16 at 07:21

1 Answers1

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If the exception occurs on one of the three lines shown, then I suspect that the problem occurs on this line:

   System.out.println("Directory: "+path+s[0]+"/");

because s is null.

The javadoc for File.list() says:

Returns null if this abstract pathname does not denote a directory, or if an I/O error occurs.

So, I surmise that this is happening because path contains a pathname1 that either resolves to a file (not a directory), to a directory that cannot be read, or to nothing at all. (There are other, more obscure possibilities too)

1 - If you did mistakenly try to use a URL, the code would attempt to resolve it as if it was a pathname. For example "http://example.com/index.html" would refer to something in a directory whose name is "http:" which is highly unlikely to exist. (Colon is a legal character in a filename on most modern operating systems ... though apparently not on Mac OSX.)

Stephen C
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  • I finally found what's the problem. In order to get the current directory, I was using `System.getProperty("user.dir")`. The silly mistake is I run the project in `/src` directory from the terminal. But my file is in the root directory of the project. Took the file in src folder and it worked. – Kerem Mar 20 '16 at 10:53