CODEWITHSUNDEEP
javafileprintwriter
Daniel Boksan
Daniel Boksan

Reputation: 23

how to use the same printwriter in multiple classes (java)

I am working on a small program that requires me to read a set of parameters from a file, do some if else operations on them in 3 classes (all 3 classes inherit the same parent class). After each operation the method is supposed to print a line in an output file like this

output 1 of method 1 output 1 of method 2 output 2 of method 1 output 3 of method 1 etc

The idea is that all methods of all 3 classes should print in the same file, the order is unimportant.

I have used this code at the end of each method/if block

if/method {
.... do something
 text ="output of something";
    try(PrintWriter out = new PrintWriter("outputfile.txt")  ){
        out.println(text);
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    }
  }//end if/method

The code does indeed write something to the file, however it always overwrites the previous line. So instead of having, for example, 12 lines of "something" I only have 1.

How can I fix this? I suspect it's because I make a new PrintWriter each time and have thought about declaring it somewhere else and calling it to each class. Would that work? How would I go about calling it to each class?

This is my first time working with files. Thank you.

Upvotes: 2

Views: 2086

Answers (2)

M. Prokhorov
M. Prokhorov

Reputation: 3993

Constructor you are using to create a PrintWriter uses new instance of FileOutputStream internally.

The FileOutputStream has two general write models:

  • Overwrite (the default)
  • Append

Since you did not specify what mode to use, your writer will use the default. To tell it which mode you want, you will need to create FileOutputStream with correct mode. For example, like this:

try(PrintWriter out = new PrintWriter(new FileOutputStream("outputfile.txt", true))) {
// note the boolean parameter in FileOS constructor above. Its "true" value means "Append"
    out.println(text);
} catch (FileNotFoundException e) {
    e.printStackTrace();
}

There is something to be said about each class creating its own PrintWriter as well:

  • It duplicates the logic
  • It (possibly unnecessarily) ties whatever your class does with operation of output specifically into a file (what if you want to write over http instead?)
  • Operation of opening a file is usually not cheap, so you lose on performance there

I suggest that instead of each class creating its own output facility, it instead should receive one from outside:

class MyClass {
  public void outputTo(PrintWriter w) {
    String text = ...
    w.println(text);
  }
}

and you use it like this:

try (FileOutputStream fos = new FileOutputStream("filename", append);
     PrintWriter w = new PrintWriter(fos)) {
  new MyClass().outputTo(w); // first instance
  new MyClass().outputTo(w); // second instance
  //... etc.
}

Upvotes: 1

Vyshak
Vyshak

Reputation: 136

Try declaring the PrintWriter as a static variable of the parent class and use it in the subclasses as required.

Upvotes: 0

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