Eclipse reading stdin (System.in) from a file

Question

Pure Java

You can redirect System.in with a single line of code:

System.setIn(new FileInputStream(filename));

See System.setIn().

Eclipse config

In Eclipse 4.5 or later, the launch configuration dialog can set System.in to read from a file. See the announcement here.

[Update] As it has been pointed out in the comments, this answer was misleading so I have updated it to correct the errors. [/Update]

On bash or command prompt you can do: C:\myprogramm < file.txt (Windows) or ./myprogramm < file.txt (Linux)

Unfortunately in Eclipse it is not possible to achieve the same result, since there is no option to bind the stdin to a file in eclipse. Instead you will have to manually open a file stream in your code, and then read from it instead. One way to do this is by using a program argument to enable it and another one with the file parameter. See Scott's answer on what kind of code you need to add to parse the -d option from the program arguments array.

When you want to point to some file inside your Eclipse Project, you need to be careful to get the location right and use the ${resource_loc:} variable:

-d ${resource_loc:/MyProject/file}

of course you can also put an absolute path:

-d path/to/file or -d C:\path\to\file

The resource_loc parameter refers to your workspace. That is the folder where all you eclipse projects are stored in. From there you still have to reference your project folder and then the file that you want to load.

You may have to tweak the slash direction, depending if you use Linux or Winodws.

2015 update

There is a new Eclipse version, Eclipse 4.5 (Mars) which has fixed the issue!

This is the announcement and description of the feature: http://eclipse.org/mars/noteworthy/#_assigning_stdin_to_a_file

Stdin can now be assigned to a file in the Common tab of launch configuration dialogs.

As expected, this is a new feature of the just released eclipse 4.5 and will therefore not work in an older version.

On the "common" tab of the run dialog, under "Standard Input and Output" there's a checkbox for "file". but it appears to be only for output...

I'd expect to see 2 file fields there, one for standard in, one for standard out with the append options.