how to read and write specific things in java

Question

I come from C so working with files in java is kind of tricky for me.

I want to have the following file:

a = 3 b = 5 c = 1 d = 10

I want to be able to read whenever I want the values for a,b,c,d and also to be able to change the values for some of them.

I have tried a few things but I am not even close. Can anyone help?

Try to divide your problem, tackling it step by step. What is your current solution to read the file? The internet offers plenty of resources for this question. — reto, Aug 11 '15 at 06:46
If you can choose the formatting of your file, I would recommend something with delimiters like `a = 3; b = 5; c = 1; d = 10;`, so it is easier to read the file. — Klaus, Aug 11 '15 at 06:49
@KlausPrinoth I would go even one step further: why having more than one key value pair per line? Makes parsing much harder than it ought to be. — GhostCat, Aug 11 '15 at 06:54
Side note: coming from C is not an excuse to come up with bad questions. This question is very unspecific; it almost reads like "please tell me everything I need and do the work for me". As reto said: focus on smaller parts; and resolve them step by step. And: solving a concrete problem is one way to learn a new language; but not always the best one. Whats wrong with reading some books, tutorials first; to get a glimpse "of the whole thing" - before diving into very specific and narrow coding activities? — GhostCat, Aug 11 '15 at 06:56
I added an answer with an example for the Properties class. If the OP is able to change the style to line by line pairs, then this is the easiest solution i could come up with. — Abbel, Aug 11 '15 at 06:59
sorry for the late response [here](http://stackoverflow.com/a/31934977/4225763) as a link on the word Properties — Abbel, Aug 11 '15 at 08:42

score 1 · Accepted Answer · edited Aug 11 '15 at 07:37

1

If you are able to change the format for a line by line style for the values you could have a look at the Properties in Java. Here you can define Key value pairs, line by line with a separator between key and value that you can specify.

edited Aug 11 '15 at 07:37

Aaron Esau

1,083
3
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answered Aug 11 '15 at 06:48

Abbel

320
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Ravindra babu · Answer 2 · 2015-08-11T08:25:39.290

Sample code on load and Usage of Properties

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

public class ReadLineExample {

public static void main(String args[]) throws FileNotFoundException {
    Properties ps =new Properties();
    // Create the file object
    File fileObj = new File("data.txt");
    try {

        FileInputStream fis = new FileInputStream(fileObj);
        ps.load(fis);
        System.out.println("Properties:"+ps);
        System.out.println("Get A:"+ps.getProperty("a"));

    } catch (Exception err) {

        err.printStackTrace();
    }
}

}

-- Format of data.txt

a = 3

b = 5

c = 1

d = 10

Output:

Properties:{b=5 , a=3 , d=10, c=1 } Get A:3

score 0 · Answer 3 · answered Aug 11 '15 at 07:03

This is a very "NullPointer-ish" way to complete this task, but here's a sample

String[] list = fileContents.split("=");
String[] temp = list[1].split(" ");
a = temp[0]
temp = list[1].split(" ");
b = temp[0]
temp = list[2].split(" ");
c = temp[0]
temp = list[3].split(" ");
d = temp[0]

This way is not very flexible either. If I were you, I'd look at the Properties class. It makes this job way easier.

You could also write a function that parses the text and finds the values between the "=" sign and a " " (But you'd have to remove the " " between the "=" and the value...)

score 0 · Answer 4 · answered Aug 13 '15 at 07:32

Since Properties is not recommended, I have bettered the solution with preferences.

package com.mypack.test;

import java.io.*;
import java.util.*;
import java.util.prefs.Preferences;

public class PreferencesExample {

    public static void main(String args[]) throws FileNotFoundException {
        Preferences ps = Preferences.userNodeForPackage(PreferencesExample.class);
        // Load  file object
        File fileObj = new File("data.xml");
        try {
            FileInputStream fis = new FileInputStream(fileObj);
            ps.importPreferences(fis);
            System.out.println("Prefereces:"+ps);
            System.out.println("Get property1:"+ps.getInt("property1",10));

        } catch (Exception err) {
            err.printStackTrace();
        }
    }
}

data.xml is in below format

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE preferences SYSTEM 'http://java.sun.com/dtd/preferences.dtd'>
<preferences EXTERNAL_XML_VERSION="1.0">
<root type="user">
<map />
<node name="com">
  <map />
  <node name="mypack">
    <map />
    <node name="test">
      <map>
        <entry key="property1" value="80" />
        <entry key="property2" value="Red" />
      </map>
    </node>
  </node>
</node>
</root>
</preferences>

Why isin't Properties recommended? I've used it and it works great — spoke, Aug 13 '15 at 08:16
Properties is effectively deprecated. Visit this question: http://stackoverflow.com/questions/2358651/are-java-properties-effectively-deprecated — Ravindra babu, Aug 13 '15 at 08:30

how to read and write specific things in java

4 Answers4