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Java how to read a file line by line and change the specific part of the line [duplicate]

This question already has answers here:
Modify the content of a file using Java
(6 answers)
Closed 3 years ago.
I am new at java and teacher gave us a project homework. I have to implement read the file line by line, slice the lines at the comma and store the parts at a multidimensional array, change the specific part of the line (I want to change the amount).
The given file:
product1,type,amount
product2,type,amount
product3,type,amount
product4,type,amount
product5,type,amount
I tried this code but I couldn't change the specific part.
BufferedReader reader;
int j=0;
int i=0;
try {
reader = new BufferedReader(new FileReader("file.txt"));
String line = reader.readLine();
while (line != null) {
j++;
}
reader.close();
} catch (IOException e) {
e.printStackTrace();
}
String total_length[][]=new String[j][3];
try {
reader = new BufferedReader(new FileReader("file.txt"));
String line = reader.readLine();
while (line != null) {
line = reader.readLine();
String[] item = line.split(",");
total_length[i][0]=item[0];
total_length[i][1]=item[0];
total_length[i][2]=item[0];
i++;
}
reader.close();
} catch (IOException e) {
e.printStackTrace();
}
Thanks a lot!

First, you need to read the file. There are plenty of way to do it, one of them is:
BufferedReader s = new BufferedReader(new FileReader("filename"));
Which allows you to do s.readLine() to read it line by line.
You can use a while loop to read it until the end. Note that readLine will return null if you reach the end of the file.
Then, for each line, you want to split them with the coma. You can use the split method of Strings:
line.split(",");
Putting it all together, and using a try-catch for IOException, you get:
List<String[]> result = new ArrayList<>();
try (BufferedReader s = new BufferedReader(new FileReader("filename"))) {
String line;
while ((line = s.readLine()) != null) {
result.add(line.split(","));
}
} catch (IOException e) {
// Handle IOExceptions here
}
If you really need a two dimensional array at the end, you can do:
String[][] array = new String[0][0];
array = result.toArray(array);
You then have read the file in the format you wanted, you can now modify the data that you parsed.

Reading a specific set of lines in a file [duplicate]

In Java, is there any method to read a particular line from a file? For example, read line 32 or any other line number.

For small files:
String line32 = Files.readAllLines(Paths.get("file.txt")).get(32)
For large files:
try (Stream<String> lines = Files.lines(Paths.get("file.txt"))) {
line32 = lines.skip(31).findFirst().get();
}

Unless you have previous knowledge about the lines in the file, there's no way to directly access the 32nd line without reading the 31 previous lines.
That's true for all languages and all modern file systems.
So effectively you'll simply read lines until you've found the 32nd one.

Not that I know of, but what you could do is loop through the first 31 lines doing nothing using the readline() function of BufferedReader
FileInputStream fs= new FileInputStream("someFile.txt");
BufferedReader br = new BufferedReader(new InputStreamReader(fs));
for(int i = 0; i < 31; ++i)
br.readLine();
String lineIWant = br.readLine();

Joachim is right on, of course, and an alternate implementation to Chris' (for small files only because it loads the entire file) might be to use commons-io from Apache (though arguably you might not want to introduce a new dependency just for this, if you find it useful for other stuff too though, it could make sense).
For example:
String line32 = (String) FileUtils.readLines(file).get(31);
http://commons.apache.org/io/api-release/org/apache/commons/io/FileUtils.html#readLines(java.io.File, java.lang.String)

You may try indexed-file-reader (Apache License 2.0). The class IndexedFileReader has a method called readLines(int from, int to) which returns a SortedMap whose key is the line number and the value is the line that was read.
Example:
File file = new File("src/test/resources/file.txt");
reader = new IndexedFileReader(file);
lines = reader.readLines(6, 10);
assertNotNull("Null result.", lines);
assertEquals("Incorrect length.", 5, lines.size());
assertTrue("Incorrect value.", lines.get(6).startsWith("[6]"));
assertTrue("Incorrect value.", lines.get(7).startsWith("[7]"));
assertTrue("Incorrect value.", lines.get(8).startsWith("[8]"));
assertTrue("Incorrect value.", lines.get(9).startsWith("[9]"));
assertTrue("Incorrect value.", lines.get(10).startsWith("[10]"));
The above example reads a text file composed of 50 lines in the following format:
[1] The quick brown fox jumped over the lazy dog ODD
[2] The quick brown fox jumped over the lazy dog EVEN
Disclamer: I wrote this library

Although as said in other answers, it is not possible to get to the exact line without knowing the offset (pointer) before. So, I've achieved this by creating an temporary index file which would store the offset values of every line. If the file is small enough, you could just store the indexes (offset) in memory without needing a separate file for it.
The offsets can be calculated by using the RandomAccessFile
RandomAccessFile raf = new RandomAccessFile("myFile.txt","r");
//above 'r' means open in read only mode
ArrayList<Integer> arrayList = new ArrayList<Integer>();
String cur_line = "";
while((cur_line=raf.readLine())!=null)
{
arrayList.add(raf.getFilePointer());
}
//Print the 32 line
//Seeks the file to the particular location from where our '32' line starts
raf.seek(raf.seek(arrayList.get(31));
System.out.println(raf.readLine());
raf.close();
Also visit the Java docs on RandomAccessFile for more information:
Complexity: This is O(n) as it reads the entire file once. Please be aware for the memory requirements. If it's too big to be in memory, then make a temporary file that stores the offsets instead of ArrayList as shown above.
Note: If all you want in '32' line, you just have to call the readLine() also available through other classes '32' times. The above approach is useful if you want to get the a specific line (based on line number of course) multiple times.

Another way.
try (BufferedReader reader = Files.newBufferedReader(
Paths.get("file.txt"), StandardCharsets.UTF_8)) {
List<String> line = reader.lines()
.skip(31)
.limit(1)
.collect(Collectors.toList());
line.stream().forEach(System.out::println);
}

No, unless in that file format the line lengths are pre-determined (e.g. all lines with a fixed length), you'll have to iterate line by line to count them.

In Java 8,
For small files:
String line = Files.readAllLines(Paths.get("file.txt")).get(n);
For large files:
String line;
try (Stream<String> lines = Files.lines(Paths.get("file.txt"))) {
line = lines.skip(n).findFirst().get();
}
In Java 7
String line;
try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
for (int i = 0; i < n; i++)
br.readLine();
line = br.readLine();
}
Source: Reading nth line from file

If you are talking about a text file, then there is really no way to do this without reading all the lines that precede it - After all, lines are determined by the presence of a newline, so it has to be read.
Use a stream that supports readline, and just read the first X-1 lines and dump the results, then process the next one.

It works for me:
I have combined the answer of
Reading a simple text file
But instead of return a String I am returning a LinkedList of Strings. Then I can select the line that I want.
public static LinkedList<String> readFromAssets(Context context, String filename) throws IOException {
BufferedReader reader = new BufferedReader(new InputStreamReader(context.getAssets().open(filename)));
LinkedList<String>linkedList = new LinkedList<>();
// do reading, usually loop until end of file reading
StringBuilder sb = new StringBuilder();
String mLine = reader.readLine();
while (mLine != null) {
linkedList.add(mLine);
sb.append(mLine); // process line
mLine = reader.readLine();
}
reader.close();
return linkedList;
}

Use this code:
import java.nio.file.Files;
import java.nio.file.Paths;
public class FileWork
{
public static void main(String[] args) throws IOException {
String line = Files.readAllLines(Paths.get("D:/abc.txt")).get(1);
System.out.println(line);
}
}

You can use LineNumberReader instead of BufferedReader. Go through the api. You can find setLineNumber and getLineNumber methods.

You can also take a look at LineNumberReader, subclass of BufferedReader. Along with the readline method, it also has setter/getter methods to access line number. Very useful to keep track of the number of lines read, while reading data from file.

public String readLine(int line){
FileReader tempFileReader = null;
BufferedReader tempBufferedReader = null;
try { tempFileReader = new FileReader(textFile);
tempBufferedReader = new BufferedReader(tempFileReader);
} catch (Exception e) { }
String returnStr = "ERROR";
for(int i = 0; i < line - 1; i++){
try { tempBufferedReader.readLine(); } catch (Exception e) { }
}
try { returnStr = tempBufferedReader.readLine(); } catch (Exception e) { }
return returnStr;
}

you can use the skip() function to skip the lines from begining.
public static void readFile(String filePath, long lineNum) {
List<String> list = new ArrayList<>();
long totalLines, startLine = 0;
try (Stream<String> lines = Files.lines(Paths.get(filePath))) {
totalLines = Files.lines(Paths.get(filePath)).count();
startLine = totalLines - lineNum;
// Stream<String> line32 = lines.skip(((startLine)+1));
list = lines.skip(startLine).collect(Collectors.toList());
// lines.forEach(list::add);
} catch (IOException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
list.forEach(System.out::println);
}

EASY WAY - Reading a line using line number.
Let's say Line number starts from 1 till null .
public class TextFileAssignmentOct {
private void readData(int rowNum, BufferedReader br) throws IOException {
int n=1; //Line number starts from 1
String row;
while((row=br.readLine()) != null) { // Reads every line
if (n == rowNum) { // When Line number matches with which you want to read
System.out.println(row);
}
n++; //This increments Line number
}
}
public static void main(String[] args) throws IOException {
File f = new File("../JavaPractice/FileRead.txt");
FileReader fr = new FileReader(f);
BufferedReader br = new BufferedReader(fr);
TextFileAssignmentOct txf = new TextFileAssignmentOct();
txf.readData(4, br); //Read a Specific Line using Line number and Passing buffered reader
}
}

for a text file you can use an integer with a loop to help you get the number of the line, don't forget to import the classes we are using in this example
File myObj = new File("C:\\Users\\LENOVO\\Desktop\\test.txt");//path of the file
FileReader fr = new FileReader(myObj);
fr.read();
BufferedReader bf = new BufferedReader(fr); //BufferedReader of the FileReader fr
String line = bf.readLine();
int lineNumber = 0;
while (line != null) {
lineNumber = lineNumber + 1;
if(lineNumber == 7)
{
//show line
System.out.println("line: " + lineNumber + " has :" + line);
break;
}
//lecture de la prochaine ligne, reading next
line = bf.readLine();
}

They are all wrong I just wrote this in about 10 seconds.
With this I managed to just call the object.getQuestion("linenumber") in the main method to return whatever line I want.
public class Questions {
File file = new File("Question2Files/triviagame1.txt");
public Questions() {
}
public String getQuestion(int numLine) throws IOException {
BufferedReader br = new BufferedReader(new FileReader(file));
String line = "";
for(int i = 0; i < numLine; i++) {
line = br.readLine();
}
return line; }}

Read one line of a csv file in Java

I have a csv file that currently has 20 lines of data.
The data contains employee info and is in the following format:
first name, last name, Employee ID
So one line would like this: Emma, Nolan, 2
I know how to write to the file in java and have all 20 lines print to the console, but what I'm not sure how to do is how to get Java to print one specific line to the console.
I also want to take the last employee id number in the last entry and have java add 1 to it one I add new employees. I thinking this needs to be done with a counter just not sure how.

You can do something like this:
BufferedReader reader = new BufferedReader(new FileReader(<<your file>>));
List<String> lines = new ArrayList<>();
String line = null;
while ((line = reader.readLine()) != null) {
lines.add(line);
}
System.out.println(lines.get(0));
With BufferedReader you are able to read lines directly. This example reads the file line by line and stores the lines in an array list. You can access the lines after that by using lines.get(lineNumber).

You can read text from a file one line at a time and then do whatever you want to with that line, print it, compare it, etc...
// Construct a BufferedReader object from the input file
BufferedReader r = new BufferedReader(new FileReader("employeeData.txt"));
int i = 1;
try {
// "Prime" the while loop
String line = r.readLine();
while (line != null) {
// Print a single line of input file to console
System.out.print("Line "+i+": "+line);
// Prepare for next loop iteration
line = r.readLine();
i++;
}
} finally {
// Free up file descriptor resources
r.close();
}
// Remember the next available employee number in a one-up scheme
int nextEmployeeId = i;

BufferedReader reader =new BufferedReader(new FileReader("yourfile.csv"));
String line = "";
while((line=reader.readLine())!=null){
String [] employee =line.trim().split(",");
// if you want to check either it contains some name
//index 0 is first name, index 1 is last name, index 2 is ID
}

Alternatively, If you want more control over read CSV files then u can think about CsvBeanReader that will give you more access over files contents..

Here is an algorithm which I use for reading csv files. The most effective way is to read all the data in the csv file into a 2D array first. It just makes it a lot more flexible to manipulate the data.
That way you can specify which line of the file to print to the console by specifying it in the index of the array and using a for. I.e: System.out.println(employee_Data[1][y]); for record 1. y is the index variable for fields. You would need to use a For Loop of course, to print every element for each line.
By the way, if you want to use the employee data in a larger program, in which it may for example store the data in a database or write to another file, I'd recommend encapsulating this entire code block into a function named Read_CSV_File(), which will return a 2D String array.
My Code
// The return type of this function is a String.
// The CSVFile_path can be for example "employeeData.csv".
public static String[][] Read_CSV_File(String CSVFile_path){
String employee_Data[][];
int x;
int y;
int noofFields;
try{
String line;
BufferedReader in = new BufferedReader(new FileReader(CSVFile_path));
// reading files in specified directory
// This assigns the data to the 2D array
// The program keeps looping through until the line read in by the console contains no data in it i.e. the end of the file.
while ( (( line = in.readLine()) != null ){
String[] current_Record = line.split(",");
if(x == 0) {
// Counts the number of fields in the csv file.
noofFields = current_Record.length();
}
for (String str : values) {
employee_Data[x][y] = str;
System.out.print(", "+employee_Data[x][y]);
// The field index variable, y is incremented in every loop.
y = y + 1;
}
// The record index variable, x is incremented in every loop.
x = x + 1;
}
// This frees up the BufferedReader file descriptor resources
in.close();
/* If an error occurs, it is caught by the catch statement and an error message
* is generated and displayed to the user.
*/
}catch( IOException ioException ) {
System.out.println("Exception: "+ioException);
}
// This prints to console the specific line of your choice
System.out.println(("Employee 1:);
for(y = 0; y < noofFields ; y++){
// Prints out all fields of record 1
System.out.print(employee_Data[1][y]+", ");
}
return employee_Data;
}

For reading large file,
log.debug("****************Start Reading CSV File*******");
copyFile(inputCSVFile);
StringBuilder stringBuilder = new StringBuilder();
String line= "";
BufferedReader brOldFile = null;
try {
String inputfile = inputCSVFile;
log.info("inputfile:" + inputfile);
brOldFile = new BufferedReader(new FileReader(inputfile));
while ((line = brOldFile.readLine()) != null) {
//line = replaceSpecialChar(line);
/*do your stuff here*/
stringBuilder.append(line);
stringBuilder.append("\n");
}
log.debug("****************End reading CSV File**************");
} catch (Exception e) {
log.error(" exception in readStaffInfoCSVFile ", e);
}finally {
if(null != brOldFile) {
try {
brOldFile.close();
} catch (IOException e) {
}
}
}
return stringBuilder.toString();

InputStreamReader and reading random lines from .txt file

I have a method for my app to read a random line from a text file and return it. Im using the randTxt() to read and return a random line from the txt file.
but it only shows the same line (1st line) everytime.
public String randTxt(){
// Read in the file into a list of strings
InputStreamReader inputStream = new InputStreamReader(getResources().openRawResource(R.raw.randomstuff));
//ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
String theLine="";
int i;
try {
i = inputStream.read();
while (i != -1) {
i = inputStream.read();
}
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
LineNumberReader rdr = new LineNumberReader(inputStream);
int numLines = 30;
Random r = new Random();
rdr.setLineNumber(r.nextInt(numLines));
try {
theLine = rdr.readLine();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return theLine;
}
How can I fix it? and Can someone explain what's wrong in my code?

Here's the framework for doing what you need using BufferedReader. In this case, you don't need to store the values in a temp array.
InputStreamReader inputStream = new InputStreamReader
(getResources().openRawResource(R.raw.randomstuff));
BufferedReader br = new BufferedReader(inputStream);
int numLines = 30;
Random r = new Random();
int desiredLine = r.nextInt(numLines);
String theLine="";
int lineCtr = 0;
while ((theLine = br.readLine()) != null) {
if (lineCtr == desiredLine) {
break;
}
lineCtr++;
}
...
Log.d(TAG, "Magic line is: " +theLine);

You have gotten an answer of how to fix your code, but no explanation of why our original code did not work.
LineNumberReader.setLineNumber(int) does not go to the actual line, it just changes what number you call the current line.
So, say you read two lines, getLineNumber() will now return 2 (it started at 0 and increased by 1 each time a newline was encountered). if you now setLineNumber(10), getLineNumber() will return 10. Reading yet another line (your third) will cause getLineNumber() to return 11.
This is described in the Java Doc.

inputStream.read does not return a line number. it returns the byte that was read. this isn't how you would read line by line. to read line by line, you should use buffered reader's readLine method. its probably easier at that point to read it all into a local array and use that array to randomly get an entry, rather than using a line number reader.

I think Random() function returns a value between 0 and 1. hence, you may have to multiply it with 100 to get an integer value. May even consider a MOD "your upper limit" operation to guarentee that the index you finally get is between 0 and your upper limit
Use the index you calculated thus, in your setLineNumber() method.
Edit:
As john said, we can get whole number using Random() object.

public String getRandomLine(String fileLoc) throws IOException
{
BufferedReader reader = new BufferedReader(new FileReader(fileLoc));
ArrayList<String> lines = new ArrayList<String>();
String line =null;
while( (line = reader.readLine())!= null )
lines.add(line);
// Choose a random one from the list
return lines.get(new Random().nextInt(lines.size()));
}
public String getRandomLineOpt(String fileLoc)throws IOException
{
File f=new File(fileLoc);
RandomAccessFile rcf=new RandomAccessFile(f, "r");
long rand = (long)(new Random().nextDouble()*f.length());
rcf.seek(rand);
rcf.readLine();
return rcf.readLine();
}

Parsing in Java with C style?

I am new to java text parsing and I'm wondering what is the best way to parse a file when the format of each line is known.
I have a file that has the following format for each line:
Int;String,double;String,double;String,double;String,double;String,double
Note how the String,double act as a pair separated by a comma and each pair is separated by a semicolon.
A few examples:
1;art,0.1;computer,0.5;programming,0.6;java,0.7;unix,0.3
2;291,0.8;database,0.6;computer,0.2;java,0.9;undegraduate,0.7
3;coffee,0.5;colombia,0.2;java,0.1;export,0.4;import,0.5
I'm using the following code to read each line:
public static void main(String args[]) {
try {
// Open the file that is the first
// command line parameter
FileInputStream fstream = new FileInputStream("textfile.txt");
// Get the object of DataInputStream
DataInputStream in = new DataInputStream(fstream);
BufferedReader br = new BufferedReader(new InputStreamReader(in));
String strLine;
// Read File Line By Line
while ((strLine = br.readLine()) != null) {
// Print the content on the console
System.out.println(strLine);
}
// Close the input stream
in.close();
} catch (Exception e) {// Catch exception if any
System.err.println("Error: " + e.getMessage());
}
}
Thanks in advance :)

You could use the Scanner class, for starters:
A simple text scanner which can parse primitive types and strings using regular expressions.

If you are truly trying to do "C" style parsing, where is the buffer which contains the characters being accumulated for the "next" field? Where is the check that sees if the field separator was read, and where is the code that flushes the current field into the correct data structure once the end of line / field separator is read?
A character by character read loop in Java looks like
int readChar = 0;
while ((readChar = in.read()) != -1) {
// do something with the new readChar.
}

You can provide a pattern and use the Scanner
String input = "fish1-1 fish2-2";
java.util.Scanner s = new java.util.Scanner(input);
s.findInLine("(\\d+)");
java.util.regex.MatchResult result = s.match();
for (int i=1; i<=result.groupCount(); i++)
System.out.println(result.group(i));
s.close();