DrJava Interactions Pane Output Redirection

We got a question today about how the output of a Java program running in DrJava‘s Interactions pane could be redirected into a file, just like it could be done in a Unix or DOS shell:

java Motion > motion.dat

The Interactions pane supports a simulated java command, but this is merely for convenience. java Motion actually translates to Motion.main(String[] {}), which is a bit unsightly. The Interactions pane is not a Unix or DOS shell, and therefore doesn’t support input/output redirection, nor does the java command support any of the command line options.

However, there is still a way to do input/output redirection in the Interactions pane. Here are instructions on how to redirect the System.out stream:

  1. Compile your program (e.g. Motion.java).
  2. In the Interactions pane, import the classes from the java.io package:
    import java.io.*
  3. Store the old System.out stream in a variable:
    PrintStream oldOut = System.out
  4. Create a new PrintStream that writes to a file:
    PrintStream newOut = new PrintStream(new FileStream("motion.dat"))
  5. Set the new stream as System.out:
  6. Now run your program (but not using the “Run” toolbar button or menu item):
    java Motion
  7. Your program will run, but all output from System.out is written to
    the motion.dat file.
  8. To get the original behavior of System.out back, set the old stream again:

Perhaps we could support > and < to do this kind of setup automatically (although only when the java or applet commands are used, to avoid ambiguity). There is a feature request for that already: 1506832: Interactions commands.


About Mathias

Software development engineer. Principal developer of DrJava. Recent Ph.D. graduate from the Department of Computer Science at Rice University.
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