Need help writing to input/output file in java

Question

New to java and need some help. I am working with 2 files. One defines a class and all of it's methods and another constructs an instance of this class. My project requires that we write the output from the class methods to a file. My question is how do I get the methods in my "class definition" file to write to the same file as that defined in my "main method"? I am currently using System.out.print which I know is not correct. //File #1

public class JessiahP3 {
    boolean  isPlaying =  false;
    int strings  = 1;
    boolean isTuned = false;
    public String instrumentName;


    //is tuned
    public void tune() {
        if(isTuned == false)
            isTuned = true;
        else
            isTuned = false;
    }

    //instrument name
    public void setInstrumentName(String insName){
        instrumentName = insName;
    }

    //get instrument name
    public String getInstrumentName(){
        return instrumentName;
    }

    private Boolean getTuned(){
        return isTuned;
    }

    public void playInstrument(){
        isPlaying = true;
        System.out.println("The instrument is now playing.");
    }

    public void stopInstrument( ){
        isPlaying = false;
        System.out.print("The instrument has stopped playing.");
    }

    public void setString(int newString){
        if (newString >=  1 && newString <= 6)    
            strings = newString;
        else
            System.out.print("You have exeeded the maximum number of strings.");
    }

    public void stringUp(){
        if (strings < 10)
            strings++;
    }

    public void stringdown(){
        if (strings > 1)
            strings--;
    }

    public String[] getStringNames(String[] stringNames) {
        for (String i:stringNames){
            System.out.println(i);
        }
        return stringNames;
    }
}

File #2

import java.util.*;
import java.io.*;

public class JessiahP3Test {

    public static void main(String[] args)throws Exception {
        //declare new input/ output file
        java.io.File newFile = new java.io.File("Jessiahp4.txt");
        //java.io.File newFile = new java.io.File(args[0]);

        //check to see if file exist
        if (newFile.exists()) {
            //delete existing file
            newFile.delete();
        }

        //create a file
        java.io.PrintWriter output = new java.io.PrintWriter(newFile);

        //instance 1 of Instrument = Guitar    
        JessiahP3 guitar = new JessiahP3();
        guitar.setInstrumentName("Guitar");
        guitar.setString(3);
        output.println("Your current string number is " + guitar.strings);
        String[] stringNames = {"Abe", "Blue", "Dream"};
        guitar.getStringNames(stringNames);
        output.print("Tuning " + guitar.getInstrumentName());
        guitar.tune();
        guitar.playInstrument();
        //output.print("The" + guitar.getInstrumentName() + "is playing");
        guitar.stopInstrument();


        //close i/o file
        output.close();
    }
}

Kaivosukeltaja · Accepted Answer · 2011-10-12 05:24:44Z

1

Add a private PrintWriter property to the first class and define a setPrintWriter() method which you call from the second class after you have both instantiated the first class and created the PrintWriter. After that you can change playInstrument() to use it instead of System.out.

NB: All .java files (with the exception of interfaces) can be categorized as "class definition files", because no code can exist outside a class in Java.

answered Oct 12, 2011 at 5:24

Kaivosukeltaja

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2 Comments

Jessiah Campbell Over a year ago

hi there, can you help me define the setPrintWriter method? 'private void setPrintWriter() { java.io.PrintWriter = new java.io.PrintWriter(newFile); }'

Kaivosukeltaja Over a year ago

@Jessiah: First add the property in the start of your class: private java.io.PrintWriter printWriter; and define the method as public void setPrintWriter(java.io.PrintWriter pw) { this.printWriter = pw; }. It can't be private or it can't be called from your main class.

Eric Siebeneich · Accepted Answer · 2011-10-12 07:49:20Z

1

Because most of the things you are outputting look like debugging statements, there is a second way to accomplish it. You can use System.setOut(PrintStream printStream) in the first file, and pass it a PrintStream (change your PrintWriter to a PrintStream, its the same thing for what you are using it for). Then, in your other file, System.out.println() will print to the file instead of standard output.

Using this approach you won't have to change the first file. Essentially this allows you to redirect all output to a file rather than the command line, which is useful for GUI applications.

So in the second file, change:

java.io.PrintWriter output = new java.io.PrintWriter(newFile);

to

System.setOut(new PrintStream(newFile));

and change:

output.println("Your current string number is " + guitar.strings);
...
output.print("Tuning " + guitar.getInstrumentName());

to

System.out.println("Your current string number is " + guitar.strings);
...
System.out.print("Tuning " + guitar.getInstrumentName());

answered Oct 12, 2011 at 7:49

Eric Siebeneich

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1 Comment

Jessiah Campbell Over a year ago

This works perfect. Thanks. Im only 8 weeks into this course so we haven't gotten that far yet.

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