Java Replace Line In Text File

Since Java 7 this is very easy and intuitive to do.

List<String> fileContent = new ArrayList<>(Files.readAllLines(FILE_PATH, StandardCharsets.UTF_8));

for (int i = 0; i < fileContent.size(); i++) {
    if (fileContent.get(i).equals("old line")) {
        fileContent.set(i, "new line");
        break;
    }
}

Files.write(FILE_PATH, fileContent, StandardCharsets.UTF_8);

Basically you read the whole file to a List, edit the list and finally write the list back to file.

FILE_PATH represents the Path of the file.

If replacement is of different length:

1. Read file until you find the string you want to replace.
2. Read into memory the part after text you want to replace, all of it.
3. Truncate the file at start of the part you want to replace.
4. Write replacement.
5. Write rest of the file from step 2.

If replacement is of same length:

1. Read file until you find the string you want to replace.
2. Set file position to start of the part you want to replace.
3. Write replacement, overwriting part of file.

This is the best you can get, with constraints of your question. However, at least the example in question is replacing string of same length, So the second way should work.

Also be aware: Java strings are Unicode text, while text files are bytes with some encoding. If encoding is UTF8, and your text is not Latin1 (or plain 7-bit ASCII), you have to check length of encoded byte array, not length of Java string.

At the bottom, I have a general solution to replace lines in a file. But first, here is the answer to the specific question at hand. Helper function:

public static void replaceSelected(String replaceWith, String type) {
    try {
        // input the file content to the StringBuffer "input"
        BufferedReader file = new BufferedReader(new FileReader("notes.txt"));
        StringBuffer inputBuffer = new StringBuffer();
        String line;

        while ((line = file.readLine()) != null) {
            inputBuffer.append(line);
            inputBuffer.append('\n');
        }
        file.close();
        String inputStr = inputBuffer.toString();

        System.out.println(inputStr); // display the original file for debugging

        // logic to replace lines in the string (could use regex here to be generic)
        if (type.equals("0")) {
            inputStr = inputStr.replace(replaceWith + "1", replaceWith + "0"); 
        } else if (type.equals("1")) {
            inputStr = inputStr.replace(replaceWith + "0", replaceWith + "1");
        }

        // display the new file for debugging
        System.out.println("----------------------------------\n" + inputStr);

        // write the new string with the replaced line OVER the same file
        FileOutputStream fileOut = new FileOutputStream("notes.txt");
        fileOut.write(inputStr.getBytes());
        fileOut.close();

    } catch (Exception e) {
        System.out.println("Problem reading file.");
    }
}

Then call it:

public static void main(String[] args) {
    replaceSelected("Do the dishes", "1");   
}

---

Original Text File Content:

Do the dishes0
Feed the dog0
Cleaned my room1

Output:

Do the dishes0
Feed the dog0
Cleaned my room1
----------------------------------
Do the dishes1
Feed the dog0
Cleaned my room1

New text file content:

Do the dishes1
Feed the dog0
Cleaned my room1

---

And as a note, if the text file was:

Do the dishes1
Feed the dog0
Cleaned my room1

and you used the method replaceSelected("Do the dishes", "1");, it would just not change the file.

---

Since this question is pretty specific, I'll add a more general solution here for future readers (based on the title).

// read file one line at a time
// replace line as you read the file and store updated lines in StringBuffer
// overwrite the file with the new lines
public static void replaceLines() {
    try {
        // input the (modified) file content to the StringBuffer "input"
        BufferedReader file = new BufferedReader(new FileReader("notes.txt"));
        StringBuffer inputBuffer = new StringBuffer();
        String line;

        while ((line = file.readLine()) != null) {
            line = ... // replace the line here
            inputBuffer.append(line);
            inputBuffer.append('\n');
        }
        file.close();

        // write the new string with the replaced line OVER the same file
        FileOutputStream fileOut = new FileOutputStream("notes.txt");
        fileOut.write(inputBuffer.toString().getBytes());
        fileOut.close();

    } catch (Exception e) {
        System.out.println("Problem reading file.");
    }
}