Develop Reference

How can i remove a particular text from a file in java?

I have tried to implement a simple program to delete a particular text from a file, some how it is not able to delete it. I am reading entire file content into a temp file , delete the user input string from it and update the content to the original file.
Any help would be highly appreciated.
public class TextEraser{
public static void main(String[] args) throws IOException {
System.out.print("Enter a string to remove : ");
Scanner scanner = new Scanner(System. in);
String inputString = scanner. nextLine();
// Locate the file
File file = new File("/Users/lobsang/documents/input.txt");
//create temporary file
File temp = File.createTempFile("file", ".txt", file.getParentFile());
String charset = "UTF-8";
try {
// Create a buffered reader
// to read each line from a file.
BufferedReader in = new BufferedReader(new FileReader(file));
PrintWriter writer = new PrintWriter(new OutputStreamWriter(new FileOutputStream(temp), charset));
String s = in.readLine();
// Read each line from the file and echo it to the screen.
while (s !=null) {
s=s.replace(inputString,"");
s = in.readLine();
}
writer.println(s);
// Close the buffered reader
in.close();
writer.close();
file.delete();
temp.renameTo(file);
} catch (FileNotFoundException e1) {
// If this file does not exist
System.err.println("File not found: " + file);
}
}

After replace with input string, write string immediate in file.
while (s != null) {
s = s.replace(inputString, "");
writer.write(s);
// writer.newLine();
s = in.readLine();
}
For new line , use BufferedWriter in place of PrintWriter, it contains method newLine()
writer.newLine();
Remove this
writer.println(s);

set encoding as UTF-8 for a FileWriter [duplicate]

This question already has answers here:
Write a file in UTF-8 using FileWriter (Java)?
(9 answers)
Closed 5 years ago.
Below is my code, it is intended to take two .ckl files, compare the two, add the new items and create a new merged file. The program executes correctly when run in Netbeans however, when executing the .jar the program doesn't appear to be encoding the file in UTF-8. I am rather new to programming and would like to know where or how I might need to be enforcing this encoding to take place?
** I have removed the Swing code and other lines so that only my method is shown, the method that does all of the comparing and merging.
public void mergeFiles(File[] files, File mergedFile) {
ArrayList<String> list = new ArrayList<String>();
FileWriter fstream = null;
BufferedWriter out = null;
try {
fstream = new FileWriter(mergedFile, false);
out = new BufferedWriter(fstream);
} catch (IOException e1) {
e1.printStackTrace();
}
// Going in a different direction. We are using a couple booleans to tell us when we want to copy or not. So at the beginning since we start
// with our source file we set copy to true, we want to copy everything and insert vuln names into our list as we go. After that first file
// we set the boolean to false so that we dont start copying anything from the second file until it is a vuln. We set to true when we see vuln
// and set it to false if we already have that in our list.
// We have a tmpCopy to store away the value of copy when we see a vuln, and reset it to that value when we see an </VULN>
Boolean copy = true;
Boolean tmpCopy = true;
for (File f : files) {
textArea1.append("merging files into: " + mergedFilePathway + "\n");
FileInputStream fis;
try {
fis = new FileInputStream(f);
// BufferedReader in = new BufferedReader(new InputStreamReader(new FileInputStream(mergedFile), "UTF-8"));
BufferedReader in = new BufferedReader(new InputStreamReader(fis));
String aLine;
while ((aLine = in.readLine()) != null) {
// Skip the close checklist and we can write it in at the end
if (aLine.trim().equals("</iSTIG>")) {
continue;
}
if (aLine.trim().equals("</STIGS>")) {
continue;
}
if (aLine.trim().equals("</CHECKLIST>")) {
continue;
}
if (aLine.trim().equals("<VULN>")) {
// Store our current value of copy
tmpCopy = copy;
copy = true;
String aLine2 = in.readLine();
String aLine3 = in.readLine();
String nameLine = in.readLine();
if (list.contains(nameLine.trim())) {
textArea1.append("Skipping: " + nameLine + "\n");
copy = false;
while (!(aLine.trim().equals("</VULN>"))) {
aLine = in.readLine();
}
continue; // this would skip the writing out to file part
} else {
list.add(nameLine.trim());
textArea1.append("::: List is now :::");
textArea1.append(list.toString() + "\n");
}
if (copy) {
out.write(aLine);
out.newLine();
out.write(aLine2);
out.newLine();
out.write(aLine3);
out.newLine();
out.write(nameLine);
out.newLine();
}
} else if (copy) {
out.write(aLine);
out.newLine();
}
// after we have written to file, if the line was a close vuln, switch copy back to original value
if (aLine.trim().equals("</VULN>")) {
copy = tmpCopy;
}
}
in.close();
} catch (IOException e) {
e.printStackTrace();
}
copy = false;
}
// Now lets add the close checklist tag we omitted before
try {
out.write("</iSTIG>");
out.write("</STIGS>");
out.write("</CHECKLIST>");
} catch (IOException e) {
e.printStackTrace();
}
try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}

Java has extensive, highly informative documentation. Keep it bookmarked. Refer to it first, whenever you have difficulty. You'll find it's frequently helpful.
In this case, the documentation for FileWriter says:
The constructors of this class assume that the default character encoding and the default byte-buffer size are acceptable. To specify these values yourself, construct an OutputStreamWriter on a FileOutputStream.
If you want to be sure your file will be written as UTF-8, replace this:
FileWriter fstream = null;
BufferedWriter out = null;
try {
fstream = new FileWriter(mergedFile, false);
with this:
Writer fstream = null;
BufferedWriter out = null;
try {
fstream = new OutputStreamWriter(new FileOutputStream(mergedFile), StandardCharsets.UTF_8);

For those, who use FileWriter in order to append to an existing file, the following will work
try (BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(file, true), StandardCharsets.UTF_8)) {
//code
}

You can just run it with the command java -Dfile.encoding=UTF-8 -jar yourjar.jar.
Follow this for more info.

Writing multiple queries from a test file

public static void main(String[] args) {
ArrayList<String> studentTokens = new ArrayList<String>();
ArrayList<String> studentIds = new ArrayList<String>();
try {
// Open the file that is the first
// command line parameter
FileInputStream fstream = new FileInputStream(new File("file1.txt"));
BufferedReader br = new BufferedReader(new InputStreamReader(fstream, "UTF8"));
String strLine;
// Read File Line By Line
while ((strLine = br.readLine()) != null) {
strLine = strLine.trim();
if ((strLine.length()!=0) && (!strLine.contains("#"))) {
String[] students = strLine.split("\\s+");
studentTokens.add(students[TOKEN_COLUMN]);
studentIds.add(students[STUDENT_ID_COLUMN]);
}
}
for (int i=0; i<studentIds.size();i++) {
File file = new File("query.txt"); // The path of the textfile that will be converted to csv for upload
BufferedReader reader = new BufferedReader(new FileReader(file));
String line = "", oldtext = "";
while ((line = reader.readLine()) != null) {
oldtext += line + "\r\n";
}
reader.close();
String newtext = oldtext.replace("sanid", studentIds.get(i)).replace("salabel",studentTokens.get(i)); // Here the name "sanket" will be replaced by the current time stamp
FileWriter writer = new FileWriter("final.txt",true);
writer.write(newtext);
writer.close();
}
fstream.close();
br.close();
System.out.println("Done!!");
} catch (Exception e) {
e.printStackTrace();
System.err.println("Error: " + e.getMessage());
}
}
The above code of mine reads data from a text file and query is a file that has a query in which 2 places "sanid" and "salabel" are replaced by the content of string array and writes another file final . But when i run the code the the final does not have the queries. but while debugging it shows that all the values are replaced properly.

but while debugging it shows that all the values are replaced properly
If the values are found to be replaced when you debugged the code, but they are missing in the file, I would suggest that you flush the output stream. You are closing the FileWriter without calling flush(). The close() method delegates its call to the underlying StreamEncoder which does not flush the stream either.
public void close() throws IOException {
se.close();
}
Try this
writer.flush();
writer.close();
That should do it.

Modify the content of a file using Java

I want to delete some content of file using java program as below. Is this the write method to replace in the same file or it should be copied to the another file.
But its deleting the all content of the file.
class FileReplace
{
ArrayList<String> lines = new ArrayList<String>();
String line = null;
public void doIt()
{
try
{
File f1 = new File("d:/new folder/t1.htm");
FileReader fr = new FileReader(f1);
BufferedReader br = new BufferedReader(fr);
while (line = br.readLine() != null)
{
if (line.contains("java"))
line = line.replace("java", " ");
lines.add(line);
}
FileWriter fw = new FileWriter(f1);
BufferedWriter out = new BufferedWriter(fw);
out.write(lines.toString());
}
catch (Exception ex)
{
ex.printStackTrace();
}
}
public statc void main(String args[])
{
FileReplace fr = new FileReplace();
fr.doIt();
}
}

I would start with closing reader, and flushing writer:
public class FileReplace {
List<String> lines = new ArrayList<String>();
String line = null;
public void doIt() {
try {
File f1 = new File("d:/new folder/t1.htm");
FileReader fr = new FileReader(f1);
BufferedReader br = new BufferedReader(fr);
while ((line = br.readLine()) != null) {
if (line.contains("java"))
line = line.replace("java", " ");
lines.add(line);
}
fr.close();
br.close();
FileWriter fw = new FileWriter(f1);
BufferedWriter out = new BufferedWriter(fw);
for(String s : lines)
out.write(s);
out.flush();
out.close();
} catch (Exception ex) {
ex.printStackTrace();
}
}
public static void main(String args[]) {
FileReplace fr = new FileReplace();
fr.doIt();
}
}

The accepted answer is great. However, there is an easier way to replace content in a file using Apache's commons-io library (commons-io-2.4.jar - you can use any latest versions)
private void update() throws IOException{
File file = new File("myPath/myFile.txt");
String fileContext = FileUtils.readFileToString(file);
fileContext = fileContext.replaceAll("_PLACEHOLDER_", "VALUE-TO-BE-REPLACED");
FileUtils.write(file, fileContext);
}
Note: Thrown IOException needs to be caught and handled by the application accordingly.

Read + write to the same file simulatenously is not ok.
EDIT: to rephrase and be more correct and specific - reading and writing to the same file, in the same thread, without properly closing the reader (and flusing the writer) is not ok.

Make sure to:
close any stream when you no longer need them
In particular before reopening it for writing.
truncate the file, to make sure it shrinks if you write less than it had.
then write the output
write individual lines, don't rely on toString.
flush and close when you are finished writing!
If you use buffered IO, you always have to ensure that the buffer is flushed at the end, or you might lose data!

I can see three problems.
First you are writing to out which I assume is System.out, not an output stream to the file.
Second, if you do write to an output stream to the file, you need to close it.
Third, the toString() method on an ArrayList isn't going to write the file like you are expecting. Loop over the list and write each String one at a time. Ask yourself whether you need to write newline characters as well.

The accepted answer is slightly wrong. Here's the correct code.
public class FileReplace {
List<String> lines = new ArrayList<String>();
String line = null;
public void doIt() {
try {
File f1 = new File("d:/new folder/t1.htm");
FileReader fr = new FileReader(f1);
BufferedReader br = new BufferedReader(fr);
while ((line = br.readLine()) != null) {
if (line.contains("java"))
line = line.replace("java", " ");
lines.add(line);
}
fr.close();
br.close();
FileWriter fw = new FileWriter(f1);
BufferedWriter out = new BufferedWriter(fw);
for(String s : lines)
out.write(s);
out.flush();
}
out.close();
catch (Exception ex) {
ex.printStackTrace();
}
}

I am trying to modify some lines in text files and when I write that line back again to the file, I end up with a blank file

This code is reading a bunch of .java files and finding "public [classname]" or "private [classname]" and adding "System.out.println([classname])" to that line.
The problem is When I write that line back in I end up with a blank file
Can anyone see what I am doing wrong?
private static void work(ArrayList<File> fileList) {
for (int i = 0; i < fileList.size(); i++) {
replaceLines(fileList.get(i));
}
}
public static void replaceLines(File file) {
String path = file.getPath();
String fileNameLong = file.getName();
String fileName = null;
if (fileNameLong.contains(".java")) {
fileName = fileNameLong.substring(0, file.getName().indexOf("."));
}
if (fileName != null && fileName != "") {
System.out.println(fileName);
try {
//prepare reading
FileInputStream in = new FileInputStream(path);
BufferedReader br = new BufferedReader(
new InputStreamReader(in));
//prepare writing
FileWriter fw = new FileWriter(file);
PrintWriter out = new PrintWriter(fw);
String strLine;
while ((strLine = br.readLine()) != null) {
// Does it contain a public or private constructor?
boolean containsPrivateCon = strLine.contains("private "
+ fileName);
boolean containsPublicCon = strLine.contains("public "
+ fileName);
if (containsPrivateCon || containsPublicCon) {
int lastIndexOfBrack = strLine.lastIndexOf("{");
while (lastIndexOfBrack == -1) {
strLine = br.readLine();
lastIndexOfBrack = strLine.lastIndexOf("{");
}
if (lastIndexOfBrack != -1) {
String myAddition = "\n System.out.println(\""
+ fileName + ".java\"); \n";
String strLineModified = strLine.substring(0,
lastIndexOfBrack + 1)
+ myAddition
+ strLine.substring(lastIndexOfBrack + 1);
strLine = strLineModified;
}
}
out.write(strLine);
}
} catch (Exception e) {
System.out.println(e);
}
}
}

If you want to write to the same file you're reading from, you should either write to a copy of the file (different filename) and then rename the output file, or use RandomAccessFile interface to edit a file in-place.
Usually, the first solution will be much easier to implement than the second one; unless the files are huge (which is probably not the case with .java files), there is no real reason to use the second.

You forgot to flush and close the file. PrintWriter keeps a buffer and unless you explicitly flush() it, the data will (un)happily sit in the buffer and it will never be written to the output.
So you need to add this before the line catch (Exception e) {
out.flush();
out.close();
Note that this is only necessary for PrintWriter and PrintStream. All other output classes flush when you close them.