Weird characters appear after write to a file

Question

Here is my story:
I open a file using RandomAccessFile to replace some characters inside it. Let's say I have in the file string 4000, I replace it by string 400. It works fine.
Now if I replace this number by string 40000, when I check the text file, some weird characters like ^M appear everywhere at the end of lines.
What's the reason and how to solve it? I believe there is some invisible characters at the end of string 4000 that causes the problem.
Note that I use function writeBytes to write strings into the file.

Answer 1

public static void changeFileRAF(String sFile, String paramName, String paramValue) {
try {
RandomAccessFile rf = new RandomAccessFile(sFile, "rws");
String currentLine;
long pos;
while (true) {
pos = rf.getFilePointer();
currentLine = rf.readLine();
if (currentLine == null)
break;
if (currentLine.trim().startsWith(paramName)) {
rf.seek(pos);
char[] replaceLine = currentLine.toCharArray();
char[] value = paramValue.toCharArray();
int i = 0;
//pass all white space characters
while (i < replaceLine.length && Character.isWhitespace(replaceLine)) {
i++;
//pass all characters representing tag name
while (i < replaceLine.length && !Character.isWhitespace(replaceLine[i])) {
i++;
//again, pass all white space characters
while (i < replaceLine.length && Character.isWhitespace(replaceLine[i])) {
i++;
if (i == replaceLine.length) {//no value, do nothing                                                           
return;
int oldValueLength = replaceLine.length - i;
String newLine = System.getProperty("line.separator");
if (oldValueLength >= value.length) {
System.out.println("different: " + (oldValueLength - value.length));
for (int j = 0; j < value.length; j++)
replaceLine[j + i] = value[j];
for (int j = value.length; j < oldValueLength; j++)
replaceLine[j + i] = ' ';
rf.writeBytes(String.copyValueOf(replaceLine));
else {
System.out.println("old value length: " + oldValueLength);
System.out.println("new value length: " + value.length);
char[] compensation = new char[value.length - oldValueLength];
for (int j = 0; j < oldValueLength; j++)
replaceLine[j + i] = value[j];
for (int j = oldValueLength; j < value.length; j++)
compensation[j - oldValueLength] = value[j];
System.out.println("it is " + String.copyValueOf(replaceLine) + String.copyValueOf(compensation));
rf.writeBytes(String.copyValueOf(replaceLine) + String.copyValueOf(compensation));
break;
rf.close();
} catch (IOException e) {
System.out.println(e.getMessage());
I hope it's not too long.