How do i append an existing line in a text file? What if the line to be edited is in the middle of the file? Please kindly offer a suggestion, given the following code.
Have went through & tried the following:
How to add a new line of text to an existing file in Java?
How to append existing line within a java text file
My code:
filePath = new File("").getAbsolutePath();
BufferedReader reader = new BufferedReader(new FileReader(filePath + "/src/DBTextFiles/Customer.txt"));
try
{
String line = null;
while ((line = reader.readLine()) != null)
{
if (!(line.startsWith("*")))
{
//System.out.println(line);
//check if target customer exists, via 2 fields - customer name, contact number
if ((line.equals(customername)) && (reader.readLine().equals(String.valueOf(customermobilenumber))))
{
System.out.println ("\nWelcome (Existing User) " + line + "!");
//w target customer, alter total number of bookings # 5th line of 'Customer.txt', by reading lines sequentially
reader.readLine();
reader.readLine();
int total_no_of_bookings = Integer.valueOf(reader.readLine());
System.out.println (total_no_of_bookings);
reader.close();
valid = true;
//append total number of bookings (5th line) of target customer # 'Customer.txt'
try {
BufferedWriter writer = new BufferedWriter(new FileWriter(new File(filePath + "/src/DBTextFiles/Customer.txt")));
writer.write(total_no_of_bookings + 1);
//writer.write("\n");
writer.close();
}
catch (IOException ex)
{
ex.printStackTrace();
}
//finally
// {
//writer.close();
//}
}
}
}
To be able to append content to an existing file you need to open it in append mode. For example using FileWriter(String fileName, boolean append) and passing true as second parameter.
If the line is in the middle then you need to read the entire file into memory and then write it back when all editing was done.
This might be workable for small files but if your files are too big, then I would suggest to write the actual content and the edited content into a temp file, when done delete the old one an rename the temp file to be the same name as the old one.
The reader.readLine() method increments a line each time it is called. I am not sure if this is intended in your program, but you may want to store the reader.readline() as a String so it is only called once.
To append a line in the middle of the text file I believe you will have to re-write the text file up to the point at which you wish to append the line, then proceed to write the rest of the file. This could possibly be achieved by storing the whole file in a String array, then writing up to a certain point.
Example of writing:
BufferedWriter writer = new BufferedWriter(new FileWriter(new File(path)));
writer.write(someStuff);
writer.write("\n");
writer.close();
You should probably be following the advice in the answer to the second link you posted. You can access the middle of a file using a random access file, but if you start appending at an arbitrary position in the middle of a file without recording what's there when you start writing, you'll be overwriting its current contents, as noted in this answer. Your best bet, unless the files in question are intractably large, is to assemble a new file using the existing file and your new data, as others have previously suggested.
AFAIK you cannot do that. I mean, appending a line is possible but not inserting in the middle. That has nothing to do with java or another language...a file is a sequence of written bytes...if you insert something in an arbitrary point that sequence is no longer valid and needs to be re-written.
So basically you have to create a function to do that read-insert-slice-rewrite
Related
Looking for best practice to read a file line by line which has >10 records and storing it in ArrayList.
My program was able to read to 3.5k records and ignoring rest of the records.
URL cityurl = ClassLoader.getSystemResource(citypath);
citybr = new BufferedReader(new FileReader(cityurl.getFile()));
for (String city = citybr.readLine(); city != null; city = citybr.readLine()) {
citycountryairport.add(citybr.readLine());
}
Thanks in advance!!
BufferedReader is a good choice for reading large files because it buffers the file and thus avoids loading the whole file into memory, see BufferedReader Doc.
Each time you call
readLine();
The next line of the file is read, in your code change :
citycountryairport.add(citybr.readLine());
to :
citycountryairport.add(city);
otherwise the lines read by the line
city = citybr.readLine()
will not be added to your list because you never add the String city to your list.
I have written a method in Java to delete a caret at the end of each line of a file. The method is as follows:
//Creates a new file, and deletes the temp file
public void createFinalFile() throws FileNotFoundException, IOException{
// read file data into a String
String data1 = new Scanner(new File(fileDirectoryString + "tempFile.txt")).useDelimiter("\\Z").next();
// replace all ^ from end of line using (?m) - MULTILINE switch
data1 = data1.replaceAll("(?m)\\^$", "");
PrintWriter docketFile3 = new PrintWriter(new FileWriter(fileDirectoryString + "Forclosure-Docket-"+startingYear+startingMonth+startingDay+"-"+endingYear+endingMonth+endingDay+".txt", true));
docketFile3.write(data1);
}
The issue is that sometimes the temp file will have all the information, but after the method is run the newly created file is blank and I am not sure why. An example of the temp file is:
04/02/2014^BR-12-005193^09/12/2012^P1^SF^DEPOSIT AMOUNT PAID CUYAHOGA COUNTY SHERIFF^
04/02/2014^BR-12-005193^09/12/2012^P1^CS^COST PAYMENT $860.90 CUYAHOGA COUNTY SHERIFF^
While it should just delete the caret at the end of each line, it seems to be deleting every line.
Your regular expression is not what's doing this. Though the function overall reminds me more of this than anything else, it should work. The one thing though that could be going wrong is that you aren't closing your output file. Add docketFile3.close(); as a line after you write the data out.
I have a text file in the following format:
Details.txt
The file is a .txt file. I want to read course title from this file and print corresponding textbook and instructor information. But i am not sure about what process to follow ? storing the information in an array won't be efficient! How should i proceed ? NOTE: I can't change the info in the file, it should not be changed!! obviously the file will be read by the following code:
File newFile=new File("C:/details");
but how should i extract the data from this file according to the labels course title, textbook and instructor!?
First read the file correctly line by line, and search for your entered course title, lets consider "Java"
Now you hit your title and you know you need 3 consecutive lines from your file as all information related to that title are there.
if(str.startsWith(title)); { // for title = "Java"
line1 = 1st line // contains ISBN and First Name
line2 = 2nd line // Title and Last Name
line3 = 3rd line // Author and Department
line4 = 4th line // Email
break; // this will take you out of while loop
}
Now on those four lines do string operations and extract your data as you need and use it.
I am home so I can't give you exact code. But if you follow this it will solve your issue. Let me know if any problem you got while doing this.
Follow this to get some info on String operations
Use String Tokenizer and separate each string and then store them in a Linked List or Array List. Have Separate List for each title like course title, instructor etc. and then print them
//Find the directory for the SD Card using the API
//*Don't* hardcode "/sdcard"
File sdcard = Environment.getExternalStorageDirectory();
//Get the text file
File file = new File(sdcard,"file.txt");
//Read text from file
StringBuilder text = new StringBuilder();
try {
BufferedReader br = new BufferedReader(new FileReader(file));
String line;
while ((line = br.readLine()) != null) {
text.append(line);
text.append('\n');
}
}
catch (IOException e) {
//You'll need to add proper error handling here
}
//Find the view by its id
TextView tv = (TextView)findViewById(R.id.text_view);
//Set the text
tv.setText(text);
you can use FileUtils.readFileToString(new File(""C:/details.txt");
Now you can extract the required data based on your wish
Use Scanner class
Scanner s=new Scanner(new File("C:/Details.txt"));
while(s.hasNext())
{
System.out.println(s.nextLine());
}
if you want in work by word then use String Tokenizer
see this article
I have a program that loads lines from a user file, then selects the last part of the String (which would be an int)
Here's the style it's saved in:
nameOfValue = 0
nameOfValue2 = 0
and so on. I have selected the value for sure - I debugged it by printing. I just can't seem to save it back in.
if(nameOfValue.equals(type)) {
System.out.println(nameOfValue+" equals "+type);
value.replace(value, Integer.toString(Integer.parseInt(value)+1));
}
How would I resave it? I've tried bufferedwriter but it just erases everything in the file.
My suggestion is, save all the contents of the original file (either in memory or in a temporary file; I'll do it in memory) and then write it again, including the modifications. I believe this would work:
public static void replaceSelected(File file, String type) throws IOException {
// we need to store all the lines
List<String> lines = new ArrayList<String>();
// first, read the file and store the changes
BufferedReader in = new BufferedReader(new FileReader(file));
String line = in.readLine();
while (line != null) {
if (line.startsWith(type)) {
String sValue = line.substring(line.indexOf('=')+1).trim();
int nValue = Integer.parseInt(sValue);
line = type + " = " + (nValue+1);
}
lines.add(line);
line = in.readLine();
}
in.close();
// now, write the file again with the changes
PrintWriter out = new PrintWriter(file);
for (String l : lines)
out.println(l);
out.close();
}
And you'd call the method like this, providing the File you want to modify and the name of the value you want to select:
replaceSelected(new File("test.txt"), "nameOfValue2");
I think most convenient way is:
Read text file line by line using BufferedReader
For each line find the int part using regular expression and replace
it with your new value.
Create a new file with the newly created text lines.
Delete source file and rename your new created file.
Please let me know if you need the Java program implemented above algorithm.
Hard to answer without the complete code...
Is value a string ? If so the replace will create a new string but you are not saving this string anywhere. Remember Strings in Java are immutable.
You say you use a BufferedWriter, did you flush and close it ? This is often a cause of values mysteriously disappearing when they should be there. This exactly why Java has a finally keyword.
Also difficult to answer without more details on your problem, what exactly are you trying to acheive ? There may be simpler ways to do this that are already there.
I would like to create a simple program (in Java) which edits text files - particularly one which performs inserting arbitrary pieces of text at random positions in a text file. This feature is part of a larger program I am currently writing.
Reading the description about java.util.RandomAccessFile, it appears that any write operations performed in the middle of a file would actually overwrite the exiting content. This is a side-effect which I would like to avoid (if possible).
Is there a simple way to achieve this?
Thanks in advance.
Okay, this question is pretty old, but FileChannels exist since Java 1.4 and I don't know why they aren't mentioned anywhere when dealing with the problem of replacing or inserting content in files. FileChannels are fast, use them.
Here's an example (ignoring exceptions and some other stuff):
public void insert(String filename, long offset, byte[] content) {
RandomAccessFile r = new RandomAccessFile(new File(filename), "rw");
RandomAccessFile rtemp = new RandomAccessFile(new File(filename + "~"), "rw");
long fileSize = r.length();
FileChannel sourceChannel = r.getChannel();
FileChannel targetChannel = rtemp.getChannel();
sourceChannel.transferTo(offset, (fileSize - offset), targetChannel);
sourceChannel.truncate(offset);
r.seek(offset);
r.write(content);
long newOffset = r.getFilePointer();
targetChannel.position(0L);
sourceChannel.transferFrom(targetChannel, newOffset, (fileSize - offset));
sourceChannel.close();
targetChannel.close();
}
Well, no, I don't believe there is a way to avoid overwriting existing content with a single, standard Java IO API call.
If the files are not too large, just read the entire file into an ArrayList (an entry per line) and either rewrite entries or insert new entries for new lines.
Then overwrite the existing file with new content, or move the existing file to a backup and write a new file.
Depending on how sophisticated the edits need to be, your data structure may need to change.
Another method would be to read characters from the existing file while writing to the edited file and edit the stream as it is read.
If Java has a way to memory map files, then what you can do is extend the file to its new length, map the file, memmove all the bytes down to the end to make a hole and write the new data into the hole.
This works in C. Never tried it in Java.
Another way I just thought of to do the same but with random file access.
Seek to the end - 1 MB
Read 1 MB
Write that to original position + gap size.
Repeat for each previous 1 MB working toward the beginning of the file.
Stop when you reach the desired gap position.
Use a larger buffer size for faster performance.
You can use following code:
BufferedReader reader = null;
BufferedWriter writer = null;
ArrayList list = new ArrayList();
try {
reader = new BufferedReader(new FileReader(fileName));
String tmp;
while ((tmp = reader.readLine()) != null)
list.add(tmp);
OUtil.closeReader(reader);
list.add(0, "Start Text");
list.add("End Text");
writer = new BufferedWriter(new FileWriter(fileName));
for (int i = 0; i < list.size(); i++)
writer.write(list.get(i) + "\r\n");
} catch (Exception e) {
e.printStackTrace();
} finally {
OUtil.closeReader(reader);
OUtil.closeWriter(writer);
}
I don't know if there's a handy way to do it straight otherwise than
read the beginning of the file and write it to target
write your new text to target
read the rest of the file and write it to target.
About the target : You can construct the new contents of the file in memory and then overwrite the old content of the file if the files handled aren't so big. Or you can write the result to a temporary file.
The thing would probably be easiest to do with streams, RandomAccessFile doesn't seem to be meant for inserting in the middle (afaik). Check the tutorial if you need.
I believe the only way to insert text into an existing text file is to read the original file and write the content in a temporary file with the new text inserted. Then erase the original file and rename the temporary file to the original name.
This example is focused on inserted a single line into an existing file, but still maybe of use to you.
If it is a text file,,,,Read the existing file in StringBuffer and append the new content in the same StringBuffer now u can write the SrtingBuffer on file. so now the file contains both the existing and new text.
As #xor_eq answer's edit queue is full, here in a new answer a more documented and slightly improved version of his:
public static void insert(String filename, long offset, byte[] content) throws IOException {
File temp = Files.createTempFile("insertTempFile", ".temp").toFile(); // Create a temporary file to save content to
try (RandomAccessFile r = new RandomAccessFile(new File(filename), "rw"); // Open file for read & write
RandomAccessFile rtemp = new RandomAccessFile(temp, "rw"); // Open temporary file for read & write
FileChannel sourceChannel = r.getChannel(); // Channel of file
FileChannel targetChannel = rtemp.getChannel()) { // Channel of temporary file
long fileSize = r.length();
sourceChannel.transferTo(offset, (fileSize - offset), targetChannel); // Copy content after insert index to
// temporary file
sourceChannel.truncate(offset); // Remove content past insert index from file
r.seek(offset); // Goto back of file (now insert index)
r.write(content); // Write new content
long newOffset = r.getFilePointer(); // The current offset
targetChannel.position(0L); // Goto start of temporary file
sourceChannel.transferFrom(targetChannel, newOffset, (fileSize - offset)); // Copy all content of temporary
// to end of file
}
Files.delete(temp.toPath()); // Delete the temporary file as not needed anymore
}