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How to replace a specific character in a text file by giving the index value?

Came across this code which replaces all the characters of the given value.
File temp = File.createTempFile("newfile", ".txt");
FileWriter fw = new FileWriter(temp);
Reader reader = new FileReader(file);
BufferedReader br = new BufferedReader(reader);
while (br.ready()) {
fw.write(br.readLine().replaceAll("n", "j") + "\n");
}
fw.close();
br.close();
reader.close();
temp.renameTo(file);
}
instead of replacing all 'n's with 'j's isn't there a way to specify the index I want to change only?

You can use replaceFirst (that accepts a regex), using the following pattern:
(?<=.{N-1}).
Where "N" is the index you want to replace.
Of course there are many alternatives, look at the String API to fuel your creative fire.

The line within the while statement as follows:
fw.write(br.readLine().replaceAll("n", "j") + "\n");
This can be expanded to:
String str = br.readLine(); //get text
String replace = str.replaceAll("n", "j"); //replace content
replace = replace + "\n"; //add new line
fw.write(replace); //write to file
For your instance of replacing a certain index, you would want to do something similar:
StringBuilder str = new StringBuilder(br.readLine()); //Read line into StringBuilder
if(str.length() > 3) //Check if string is long enough
str.setChar(4, 'x'); //Replace character in line at index 4 to 'x'
fw.write(str); //write to file

Java String Matching in a Sorted File and grouping similar data

i have sorted file and i need to do the following pattern match. I read the row and then compare or do patern match with the row just after it , if it matches then insert the string i used to match after a comma in that row and move on to the next row. I am new to Java and overwhelmed with options from Open CSV to BufferedReader. I intend to iterate through the file till it reaches the end. I may always have blanks and have a dated in quotes. The file size would be around 100 MBs.
My file has data like
ABCD
ABCD123
ABCD456, 123
XYZ
XYZ890
XYZ123, 890
and output is expected as
ABCD, ABCD
ABCD123, ABCD
ABCD456, 123, ABCD
XYZ, XYZ
XYZ890, XYZ
XYZ123, 890, XYZ
Not sure about the best method. Can you please help me.

To open a file, you can use File and FileReader classes:
File csvFile = new File("file.csv");
FileReader fileReader = null;
try {
fileReader = new FileReader(csvFile);
} catch (FileNotFoundException e) {
e.printStackTrace();
}
You can get a line of the file using Scanner:
Scanner reader = new Scanner(fileReader);
while(reader.hasNext()){
String line = reader.nextLine();
parseLine(line);
}
You want to parse this line. For it, you have to study Regex for using Pattern and Matcher classes:
private void parseLine(String line) {
Matcher matcher = Pattern.compile("(ABCD)").matcher(line);
if(matcher.find()){
System.out.println("find: " + matcher.group());
}
}
To find the next pattern of the same row, you can reuse matcher.find(). If some result was found, it will return true and you can get this result with matcher.groud();

Read line by line and use regex to replace it as per your need using String.replaceAll()
^([A-Z]+)([0-9]*)(, [0-9]+)?$
Replacement : $1$2$3, $1
Here is Online demo
Read more about Java Pattern
Sample code:
String regex = "^([A-Z]+)([0-9]*)(, [0-9]+)?$";
String replacement = "$1$2$3, $1";
String newLine = line.replaceAll(regex,replacement);
For better performance, read 100 or more lines at a time and store in a buffer and finally call String#replaceAll() single time to replace all at a time.
sample code:
String regex = "([A-Z]+)([0-9]*)(, [0-9]+)?(\r?\n|$)";
String replacement = "$1$2$3, $1$4";
StringBuilder builder = new StringBuilder();
int counter = 0;
String line = null;
try (BufferedReader reader = new BufferedReader(new FileReader("abc.csv"))) {
while ((line = reader.readLine()) != null) {
builder.append(line).append(System.lineSeparator());
if (counter++ % 100 == 0) { // 100 lines
String newLine = builder.toString().replaceAll(regex, replacement);
System.out.print(newLine);
builder.setLength(0); // reset the buffer
}
}
}
if (builder.length() > 0) {
String newLine = builder.toString().replaceAll(regex, replacement);
System.out.print(newLine);
}
Read more about Java 7 - The try-with-resources Statement

Buffered Reader find specific line separator char then read that line

My program needs to read from a multi-lined .ini file, I've got it to the point it reads every line that start with a # and prints it. But i only want to to record the value after the = sign. here's what the file should look like:
#music=true
#Volume=100
#Full-Screen=false
#Update=true
this is what i want it to print:
true
100
false
true
this is my code i'm currently using:
#SuppressWarnings("resource")
public void getSettings() {
try {
BufferedReader br = new BufferedReader(new FileReader(new File("FileIO Plug-Ins/Game/game.ini")));
String input = "";
String output = "";
while ((input = br.readLine()) != null) {
String temp = input.trim();
temp = temp.replaceAll("#", "");
temp = temp.replaceAll("[*=]", "");
output += temp + "\n";
}
System.out.println(output);
}catch (IOException ex) {}
}
I'm not sure if replaceAll("[*=]", ""); truly means anything at all or if it's just searching for all for of those chars. Any help is appreciated!

Try following:
if (temp.startsWith("#")){
String[] splitted = temp.split("=");
output += splitted[1] + "\n";
}
Explanation:
To process lines only starting with desired character use String#startsWith method. When you have string to extract values from, String#split will split given text with character you give as method argument. So in your case, text before = character will be in array at position 0, text you want to print will be at position 1.
Also note, that if your file contains many lines starting with #, it should be wise not to concatenate strings together, but use StringBuilder / StringBuffer to add strings together.
Hope it helps.

Better use a StringBuffer instead of using += with a String as shown below. Also, avoid declaring variables inside loop. Please see how I've done it outside the loop. It's the best practice as far as I know.
StringBuffer outputBuffer = new StringBuffer();
String[] fields;
String temp;
while((input = br.readLine()) != null)
{
temp = input.trim();
if(temp.startsWith("#"))
{
fields = temp.split("=");
outputBuffer.append(fields[1] + "\n");
}
}

How to split a string by tabs and newlines?

I got a tab-delimited file that I want to split by tabs and by newlines where a tab represents the delimiter between fields and a newline represents a new object that should be created. The file can look like this:
Peter\tpeter#example.com\tpeterpassword\nBob\tbob#bobby.com\tbobbypassword\n...
where \t is a tab and \n is a newline.
I want to enable uploading this file to my program that creates a new user for every line in the file with the fields on the line. But how can I use two tokens - both tab and newline? My code would look something like the following:
String everything = "";
BufferedReader br = null;
try {
br = new BufferedReader(new InputStreamReader(file.getInputStream()));
StringBuilder sb = new StringBuilder();
String line = br.readLine();
while (line != null) {
//now create object according to the string
StringTokenizer st = new StringTokenizer(line , "\t");
String name = st.nextToken();
String email = st.nextToken();
String password = st.nextToken();
User.createNewUser(name, email, password);
sb.append(line);
sb.append('\n');
line = br.readLine();
}
everything = sb.toString();
br.close();
} catch (IOException e) {
e.printStackTrace();
}
System.out.println("Everything: " + everything);
Would code like the above work?

I would do a String.split("\\n") for each line. Then you have all the information you need for each user. Do another String.split("\\t") and construct your object using the resulting array.
From the Java Doc:
StringTokenizer is a legacy class that is retained for compatibility reasons although its use is discouraged in new code. It is recommended that anyone seeking this functionality use the split method of String or the java.util.regex package instead.
http://docs.oracle.com/javase/7/docs/api/java/util/StringTokenizer.html

java read properties and xml file using stringbuilder

I need to read a set of xml and property files and parse the data. Currently I am using inputstream ans string builder to do this. But this does not create the file in the same way as input file is. I donot want to remove the white spaces and new lines. How do i achieve this.
is = test.getInputStream();
br = new BufferedReader(new InputStreamReader(is));
String line5;
StringBuilder sb5 = new StringBuilder();
while ((line5 = br.readLine()) != null) {
sb5.append(line5);
}
String s = sb5.toString();
My output is:
#test 123 #test2 345
Expected output is:
#test
123
#test2
345
Any thoughts ? Thanks

br.readLine() consumes the line breaks, you need to add them to your StringBuilder after appending the line.
is = test.getInputStream();
br = new BufferedReader(new InputStreamReader(is));
String line5;
StringBuilder sb5 = new StringBuilder();
while ((line5 = br.readLine()) != null) {
sb5.append(line5);
sb5.append("\n");
}
If you want an extremely simple solution for reading a file to a String, Apache Commons-IO has a method for performing such a task (org.apache.commons.io.FileUtils).
FileUtils.readFileToString(File file, String encoding);

readLine() method doesn't add the EOL character (\n). So while appending the string to the builder, you need to add the EOL char, like sb5.append(line5+"\n");

The various readLine methods discard the newline from the input.
From the BufferedReader docs:
Returns: A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached
A solution may be as simple as adding back a newline to your StringBuilder for every readLine: sb5.append(line5 + "\n");.
A better alternative is to read into an intermediate buffer first, using the read method, supplying your own char[]. You can still use StringBuilder.append, and get a String will match the file contents.