Develop Reference

Read and edit the file from Java

I'm trying to introduce a line break at every 100th character of the line from the existing file.But it doesn't write anything to it. below is the code written in java to read the existing file and write to it with a temporary file.
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
public class ReplaceFileContents {
public static void main(String[] args) {
new ReplaceFileContents().replace();
}
public void replace() {
String oldFileName = "Changed1.ldif";
String tmpFileName = "Changed2.ldif";
BufferedReader br = null;
BufferedWriter bw = null;
try {
br = new BufferedReader(new FileReader(oldFileName));
bw = new BufferedWriter(new FileWriter(tmpFileName));
String line;
while ((line = br.readLine()) != null) {
line.replaceAll("(.{100})", "$1\n");
}
} catch (Exception e) {
return;
} finally {
try {
if(br != null)
br.close();
} catch (IOException e) {
//
}
try {
if(bw != null)
bw.close();
} catch (IOException e) {
//
}
}
// Once everything is complete, delete old file..
File oldFile = new File(oldFileName);
oldFile.delete();
// And rename tmp file's name to old file name
File newFile = new File(tmpFileName);
newFile.renameTo(oldFile);
}
}

while ((line = br.readLine()) != null) {
line.replaceAll("(.{100})", "$1\n");
}
First off, line.replaceAll does not replace your line variable with the result. Because Strings are immutable, this method returns the new string, so your line should be line = line.replaceAll(....
Second, you're never writing the new String back into the file. Using replaceAll doesn't change the file itself in any way. Instead, try using your bw object to write the new String to the same line.

From what you've published here, you never try to write line back to bw. Try this:
package hello;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
public class Test {
public static void main(String[] args) {
new Test().replace();
}
public void replace() {
String oldFileName = "d:\\1.txt";
String tmpFileName = "d:\\2.txt";
BufferedReader br = null;
BufferedWriter bw = null;
try {
br = new BufferedReader(new FileReader(oldFileName));
bw = new BufferedWriter(new FileWriter(tmpFileName));
String line;
while ((line = br.readLine()) != null) {
line = line.replaceAll("(.{100})", "$1\n");
bw.write(line);
}
} catch (Exception e) {
return;
} finally {
try {
if(br != null)
br.close();
} catch (IOException e) {
//
}
try {
if(bw != null)
bw.close();
} catch (IOException e) {
//
}
}
// Once everything is complete, delete old file..
File oldFile = new File(oldFileName);
oldFile.delete();
// And rename tmp file's name to old file name
File newFile = new File(tmpFileName);
newFile.renameTo(oldFile);
}
}
You never try to write line back to bw;
String#replaceAll will return the copy of the source not the original String;

Code deletes the content of the file rather than replacing a text

In my below code I wanted to replace the text "DEMO" with "Demographics" but instead of replacing the text it deletes the entire content of the text file.
Contents inside the file:
DEMO
data
morning
PS: I'm a beginner in java
package com.replace.main;
import java.io.*;
public class FileEdit {
public static void main(String[] args) {
BufferedReader br = null;
BufferedWriter bw = null;
String readLine, replacedData;
try {
bw = new BufferedWriter(
new FileWriter(
"Demg.ctl"));
br = new BufferedReader(
new FileReader(
"Demg.ctl"));
System.out.println(br.readLine()); //I Get Null Printed Here
while ((readLine = br.readLine())!= null) {
System.out.println("Inside While Loop");
System.out.println(readLine);
if (readLine.equals("DEMO")) {
System.out.println("Inside if loop");
replacedData = readLine.replaceAll("DEMO","Demographics");
}
}
System.out.println("After While");
br.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}

You open a Writer to your file, but you don't write anything. This means that your file is replaced with an empty file.
Besides this you also need to close your writer, not just the reader.
And last but not least, your if condition is wrong.
if (readLine.equals("DEMO")) {
should read
if (readLine.contains("DEMO")) {
Otherwise it would only return true if your line contained "DEMO" but nothing else.

I'm updating the answer to my own question.
package com.replace.main;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
public class FileEdit
{
public static void main(String args[])
{
try
{
BufferedReader reader = new BufferedReader(new FileReader("Demg.ctl"));
String readLine = "";
String oldtext = "";
while((readLine = reader.readLine()) != null)
{
oldtext += readLine + "\r\n";
}
reader.close();
// To replace the text
String newtext = oldtext.replaceAll("DEMO", "Demographics");
FileWriter writer = new FileWriter("Demg.ctl");
writer.write(newtext);
writer.close();
}
catch (IOException e)
{
e.printStackTrace();
}
}
}

compiling java file from another java class

import java.io.BufferedReader;
import java.io.InputStreamReader;
public class ExecuteShellComand {
public static void main(String[] args) {
ExecuteShellComand obj = new ExecuteShellComand();
String className = "str.java";
String command = "javac " + className;
String output = obj.executeCommand(command);
System.out.println(output);// prints the output of the executed command
}
private String executeCommand(String command) {
StringBuffer output = new StringBuffer();
Process p;
try {
p = Runtime.getRuntime().exec(command);
p.waitFor();
BufferedReader reader = new BufferedReader(new InputStreamReader(p.getInputStream()));
String line = "";
while ((line = reader.readLine()) != null) {
output.append(line + "\n");
}
} catch (Exception e) {
e.printStackTrace();
}
return output.toString();
}
}
I am trying to compile a Java file (str.java) from another Java class(ExecuteShellComand.java). What I am trying to do is if "str.java" compiles successfully then I want to execute "java str" command, but if the compilation fails then proper stacktrace or errors should be printed. I am storing the stacktrace or the errors in output variable.
But when I execute this code although "str.java" has somes errors in it System.out.println(output) is not printing the errors.

If you want to capture the errors from a command then you shall capture error stream instead of Input stream
So replace
BufferedReader reader = new BufferedReader(new InputStreamReader(p.getInputStream()));
with
BufferedReader reader = new BufferedReader(new InputStreamReader(p.getErrorStream()));

The Process class tries to mimetize OS process. It means, process keep different output stream for error and normal messages and one stream for input. In UNIX, should be:
wc < file > wc.count 2> wc.error
In Java...
abstract InputStream getErrorStream()
Gets the error stream of the subprocess.
abstract InputStream getInputStream()
Gets the input stream of the subprocess.
abstract OutputStream getOutputStream()
So, you should use getErrorStream() to get errors..
Refactoring your code:
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
public class ExecuteShellComand {
public static void main(String[] args) {
ExecuteShellComand obj = new ExecuteShellComand();
String className = "str.java";
String command = "javac " + className;
obj.executeCommand(command);
System.out.println(obj.output);
System.out.println(obj.errors);
}
private String errors;
private String output;
private void executeCommand(String command) {
Process p;
try {
p = Runtime.getRuntime().exec(command);
p.waitFor();
errors = readStream(p.getErrorStream());
output = readStream(p.getInputStream());
} catch (Exception e) {
e.printStackTrace();
}
}
private String readStream(InputStream inputStream) throws IOException {
StringBuffer output = new StringBuffer();
BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
String line = "";
while ((line = reader.readLine()) != null) {
output.append(line + "\n");
}
return output.toString();
}
}

I cant make the text file as read only mode in java

package com.studytrails.tutorials.springremotingrmiserver;
import java.lang.Object;
import java.awt.Desktop;
import java.io.*;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.core.io.Resource;
public class GreetingServiceImpl implements GreetingService
{
#Override
public String getGreeting(String name) {
return "Hello " + name + "!";
}
public String getText() {
ApplicationContext appContext = new ClassPathXmlApplicationContext(new String[]{"spring-config-server.xml"});
Resource resource = appContext.getResource("file:D:\\text\\test.txt");
StringBuilder builder = new StringBuilder();
try {
InputStream is = resource.getInputStream();
BufferedReader br = new BufferedReader(new InputStreamReader(is));
File temp=File.createTempFile("output", ".tmp");
String filePath=temp.getAbsolutePath();
System.out.println(""+filePath);
String tem=temp.getName();
String line;
PrintWriter out = new PrintWriter(new FileWriter(tem));
//System.out.println(""+filePath);
while ((line = br.readLine()) != null) {
out.println(line);
}
temp.setReadOnly();
String[] cmd = {"notepad",tem};
Runtime runtime = Runtime.getRuntime();
Process proc = runtime.exec(cmd);
// proc.getInputStream();
out.close();
br.close();
//temp.deleteOnExit();
}
catch(IOException e) {
e.printStackTrace();
}
return builder.toString();
}
}
In above code I am not able to setReadonly(); command to temp file. File displayed with all options can you suggest how to make the temp file as not modified and even it is not able to save to another location. I need this file only displayed at run time of the program. During that time user does not change any content and it could not be save as another location.

I suggest to close the file before using it (by notepad):
PrintWriter out = new PrintWriter(new FileWriter(tem));
//System.out.println(""+filePath);
while ((line = br.readLine()) != null) {
out.println(line);
}
out.close();
temp.setReadOnly();
String[] cmd = {"notepad",tem};
Runtime runtime = Runtime.getRuntime();
Process proc = runtime.exec(cmd);
// proc.getInputStream();
To avoid that a user move the file to another location you may additional create the temp file in a temp folder and make the folder readonly too. But if user can read they normally can copy the file to another location. If I remember well, in Windows you can mark for read but prevent from copy a file (thru Windows of course).

How to open a text file?

I can't figure out for the life of me what is wrong with this program:
import java.io.*;
public class EncyptionAssignment
{
public static void main (String[] args) throws IOException
{
String line;
BufferedReader in;
in = new BufferedReader(new FileReader("notepad encypt.me.txt"));
line = in.readLine();
while(line != null)
{
System.out.println(line);
line = in.readLine();
}
System.out.println(line);
}
}
The error message says that the file can't be found, but I know that the file already exists. Do I need to save the file in a special folder?

The error is "notepad encypt.me.txt".
Since your file is named "encypt.me.txt", you can't put a "notepad" in front of its name. Moreover, the file named "notepad encypt.me.txt" probably didn't exist or is not the one that you want to open.
Additionally, you have to provide the path ( absolute or relative ) of your file if it's not located in your project folder.
I will take the hypothesis that your are on a Microsoft Windows system.
If your file has as absolute path of "C:\foo\bar\encypt.me.txt", you will have to pass it as "C:\\foo\\bar\\encypt.me.txt" or as "C:"+File.separatorChar+"foo"+File.separatorChar+"bar"+File.separatorChar+encypt.me.txt".
If it's still not working, you should verify that the file :
1) Exist at the path provided.
You can do it by using the following piece of code:
File encyptFile=new File("C:\\foo\\bar\\encypt.me.txt");
System.out.println(encyptFile.exists());
If the path provided is the right one, it should be at true.
2) Can be read by the application
You can do it by using the following piece of code:
File encyptFile=new File("C:\\foo\\bar\\encypt.me.txt");
System.out.println(encyptFile.canRead());
If you have the permission to read the file, it should be at true.
More informations:
Javadoc of File
Informations about Path in computing

import java.io.*;
public class Test {
public static void main(String [] args) {
// The name of the file to open.
String fileName = "temp.txt";
// This will reference one line at a time
String line = null;
try {
// FileReader reads text files in the default encoding.
FileReader fileReader =
new FileReader(fileName);
// Always wrap FileReader in BufferedReader.
BufferedReader bufferedReader =
new BufferedReader(fileReader);
while((line = bufferedReader.readLine()) != null) {
System.out.println(line);
}
// Always close files.
bufferedReader.close();
}
catch(FileNotFoundException ex) {
System.out.println(
"Unable to open file '" +
fileName + "'");
}
catch(IOException ex) {
System.out.println(
"Error reading file '"
+ fileName + "'");
// Or we could just do this:
// ex.printStackTrace();
}
}
}

package com.mkyong.io;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
public class BufferedReaderExample {
public static void main(String[] args) {
BufferedReader br = null;
try {
String sCurrentLine;
br = new BufferedReader(new FileReader("C:\\testing.txt"));
while ((sCurrentLine = br.readLine()) != null) {
System.out.println(sCurrentLine);
}
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
if (br != null)br.close();
} catch (IOException ex) {
ex.printStackTrace();
}
}
}
}
Reference: http://www.mkyong.com/java/how-to-read-file-from-java-bufferedreader-example/