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PrintWriter Class

layout: default title: PrintStream and PrintWriter

You've been using PrintStream since your first "Hello, World" program: System.out and System.err are PrintStream objects.

PrintStream provides print, println, and printf, as well as methods for writing bytes and byte arrays.

PrintWriter has all the same methods as PrintStream.

  • A PrintWriter can also write char arrays instead of byte arrays.

You can construct a PrintStream using a File, an OutputStream, or a filename.

try {
  FileOutputStream fs = new FileOutputStream("test.txt");
  PrintStream ps = new PrintStream(fs);
}
catch (FileNotFoundException e) {
  e.printStackTrace();
}
* You can construct a PrintWriter using a File, an OutputStream, a filename, or another Writer object.

FileWriter fw = new FileWriter("test.txt");
PrintWriter pw = new PrintWriter(fw);

Practice Exercise

PrintStream was one of the first I/O classes in Java. When writers were later added to java.io to handle character output, PrintWriter was created as a replacement for PrintStream. However, System.out and System.err were already in wide use so they remain PrintStreams. You will usually use PrintWriter in your own code.

The various output methods don't throw IOException, though constructors might.

  String[] data = { "Cat", "Dog", "Frog", "Giraffe" };
  try {
    FileWriter fw = new FileWriter("test.txt");
    PrintWriter pw = new PrintWriter(fw);
    for (String string : data) {
      pw.println(string);
    }
    pw.close();
  }
  catch (IOException e) {
    e.printStackTrace();
  }
}

Practice Exercise

Output does not have to go to a file. The java.io package has classes allowing you to use these I/O methods to read and write data in a String buffer in memory, or as data streamed between two Java objects in different threads of an application, among other possibilities. We'll stick with text files here though.

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