Develop Reference

How do I use BufferedReader to read lines from a txt file into an array

I know how to read in lines with Scanner, but how do I use a BufferedReader? I want to be able to read lines into an array. I am able to use the hasNext() function with a Scanner but not a BufferedReader, that is the only thing I don't know how to do. How do I check when the end of the file text has been reached?
BufferedReader reader = new BufferedReader(new FileReader("weblog.txt"));
String[] fileRead = new String[2990];
int count = 0;
while (fileRead[count] != null) {
fileRead[count] = reader.readLine();
count++;
}

readLine() returns null after reaching EOF.
Just
do {
fileRead[count] = reader.readLine();
count++;
} while (fileRead[count-1]) != null);
Of course this piece of code is not the recommended way of reading the file, but shows how it might be done if you want to do it exactly the way you attempted to ( some predefined size array, counter etc. )

The documentation states that readLine() returns null if the end of the stream is reached.
The usual idiom is to update the variable that holds the current line in the while condition and check if it's not null:
String currentLine;
while((currentLine = reader.readLine()) != null) {
//do something with line
}
As an aside, you might not know in advance the number of lines you will read, so I suggest you use a list instead of an array.
If you plan to read all the file's content, you can use Files.readAllLines instead:
//or whatever the file is encoded with
List<String> list = Files.readAllLines(Paths.get("weblog.txt"), StandardCharsets.UTF_8);

using readLine(), try-with-resources and Vector
try (BufferedReader bufferedReader = new BufferedReader(new FileReader("C:\\weblog.txt")))
{
String line;
Vector<String> fileRead = new Vector<String>();
while ((line = bufferedReader.readLine()) != null) {
fileRead.add(line);
}
} catch (IOException exception) {
exception.printStackTrace();
}

Looping every single lines with readLine();

I am currently stuck with my current code of printing my file out with only 1 line of code in it. I am trying to loop through every single line with readLine() but uanble to achieve it. Stuck with looping either the first row or last row of line in the file.
The purpose of this is to print out exactly the same file with this program but printing it out as a folder with other different files.
try
{
String sCurrentLine;
br = new BufferedReader(new FileReader("C:\\testsample.csv"));
if((sCurrentLine = br.readLine()) != null)
{
String info = br.readLine();
resultString += sCurrentLine.toString();
}
this.WriteToFile(resultString);
}

String info = br.readLine();
is not helping you. And you need while, not if
while((sCurrentLine = br.readLine()) != null)
{
resultString += sCurrentLine.toString();
}

You are reading only two lines. To read all lines you need while loop.
while((sCurrentLine = br.readLine()) != null)
{
//String info = br.readLine();- > Remove this line.
resultString += sCurrentLine.toString();
}

Java BufferedReader to String Array

I was looking through a lot of diffrent subjects here on stackoverflow but couldn't find anything helpful so far :/
So this is my problem. I am writing a filecopier. The problem occurs already at reading the file. My test docoument got 3 lines of random text. All those 3 lines should get written in a string array. The problem is that only the 2nd line of the textdocument gets written in the array and I can't figure out why. Already debugged it, but didn't get me any further.
I know there are diffrent solutions for a filecopier with diffrent classes etc. But I would really like to get it running with the classes I used here.
String[] array = new String[5];
String datei = "test.txt";
public String[] readfile() throws FileNotFoundException {
FileReader fr = new FileReader(datei);
BufferedReader bf = new BufferedReader(fr);
try {
int i=0;
//String Zeile = bf.readLine();
while(bf.readLine() != null){
array[i] = bf.readLine();
// System.out.println(array[i]); This line is for testing
i++;
}
bf.close();
} catch (IOException e) {
e.printStackTrace();
}
return array;

You're calling readLine() twice for each iteration of the loop, thereby discarding every other line. You need to capture the value returned by every call to readLine(), because each readLine() call advances the reader's position in the file.
Here's the idiomatic solution:
String line;
while((line = bf.readLine()) != null){
array[i] = line;
i++;
}

Here you read 2 lines:
while(bf.readLine() != null){
array[i] = bf.readLine();
// System.out.println(array[i]); This line is for testing
i++;
}
You have to change your Code to:
String line = null;
while((line =bf.readLine()) != null){
array[i] = line;
// System.out.println(array[i]); This line is for testing
i++;
}

The problem is here :
while(bf.readLine() != null)
readLine() reads a line and returns the same at the same time it moves to the next line.
So instead of just checking if the returned value was null also store it.
String txt = null;
while((txt = bf.readLine()) != null)
array[i++] = txt;

I think its because you are calling readLine() twice. First time in the loop, and then second time when you put it in the array. So, it reads a line at the beginning of the loop (line 1), then first line of code inside the loop (line 2 that you see)

I am use Stream.
Not a. This form only applies to reading text files.
BufferedReader bf = new BufferedReader(fr);
// ...
List<String> lines = bf.lines().collect(Collectors.toList());

BufferedReader - Search for string inside .txt file

I am trying, using a BufferedReader to count the appearances of a string inside a .txt file. I am using:
File file = new File(path);
try {
BufferedReader br = new BufferedReader(new FileReader(file));
String line;
int appearances = 0;
while ((line = br.readLine()) != null) {
if (line.contains("Hello")) {
appearances++;
}
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
System.out.println("Found " + appearances);
But the problem is that if my .txt file contains for example the string "Hello, world\nHello, Hello, world!" and "Hello" is to be found then the appearances become two instead of three because it searches a line for only one appearance of the string. How could I fix this? Thanks a lot

The simplest solution is to do
while ((line = br.readLine()) != null)
appearances += line.split("Hello", -1).length-1;
Note that, if instead of "Hello", you search for anything with regex-reserved characters, you should escape the string before splitting:
String escaped = Pattern.quote("Hello."); // avoid '.' special meaning in regex
while ((line = br.readLine()) != null)
appearances += line.split(escaped, -1).length-1;

This is an efficent and correct solution:
String line;
int count = 0;
while ((line = br.readLine()) != null)
int index = -1;
while((index = line.indexOf("Hello",index+1)) != -1){
count++;
}
}
return count;
It walks through the line and looks for the next index, starting from the previous index+1.
The problem with Peter's solution is that it is wrong (see my comment). The problem with TheLostMind's solution is that it creates a lot of new strings by replacement which is an unnecessary performance drawback.

A regex-driven version:
String line;
Pattern p = Pattern.compile(Pattern.quote("Hello")); // quotes in case you need 'Hello.'
int count = 0;
while ((line = br.readLine()) != null)
for (Matcher m = p.matcher(line); m.find(); count ++) { }
}
return count;
I am now curious as to performance between this and gexicide's version - will edit when I have results.
EDIT: benchmarked by running 100 times on a ~800k log file, looking for strings that were found once at the start, once around middle-ish, once at the end, and several times throughout. Results:
IndexFinder: 1579ms, 2407200hits. // gexicide's code
RegexFinder: 2907ms, 2407200hits. // this code
SplitFinder: 5198ms, 2407200hits. // Peter Lawrey's code, after quoting regexes
Conclussion: for non-regex strings, the repeated-indexOf approach is fastest by a nice margin.
Essential benchmark code (log file from vanilla Ubuntu 12.04 installation):
public static void main(String ... args) throws Exception {
Finder[] fs = new Finder[] {
new SplitFinder(), new IndexFinder(), new RegexFinder()};
File log = new File("/var/log/dpkg.log.1"); // around 800k in size
Find test = new Find();
for (int i=0; i<100; i++) {
for (Finder f : fs) {
test.test(f, log, "2014"); // start
test.test(f, log, "gnome"); // mid
test.test(f, log, "ubuntu1"); // end
test.test(f, log, ".1"); // multiple; not at start
}
}
test.printResults();
}

while (line.contains("Hello")) { // search until line has "Hello"
appearances++;
line = line.replaceFirst("Hello",""); // replace first occurance of "Hello" with empty String
}

Reading lines of data from text files using java

I have a text file with x amount of lines. each line holds a integer number. When the user clicks a button, an action is performed via actionlistener where it should list all the values as displayed on the text file. However, right now I have linenum set to 10 implying I already told the code that just work with 10 lines of the text file. So, if my text file has only 3 rows/lines of data...it will list those lines and for rest of the other 7 lines, it will spit out "null".
I recall there is a way to use ellipsis to let the program know that you don't know the exact value but at the end it calculates it based on the given information. Where my given information will the number of lines with numbers(data).
Below is part of the code.
private class thehandler implements ActionListener{
public void actionPerformed(ActionEvent event){
BufferedReader inputFile=null;
try {
FileReader freader =new FileReader("Data.txt");
inputFile = new BufferedReader(freader);
String MAP = "";
int linenum=10;
while(linenum > 0)
{
linenum=linenum-1;
MAP = inputFile.readLine();//read the next line until the specfic line is found
System.out.println(MAP);
}
} catch( Exception y ) { y.printStackTrace(); }
}}

just put instead of linenum > 0 the next line (MAP = inputFile.readLine()) != null
and delete the next lines, linenum=linenum-1;
MAP = inputFile.readLine(); and next time a bit of googling might help +)
The null value of the last line would not be printed because it sets the line to be the current line and compares it with the null value so if the last line is null it will not print it and what about the 10 lines limit? you can do it easily you can just add an an index to the for loop and to index and to check with && if the i is lower then 10

Test the value that comes back from BufferedReader.readLine(), if it is null stop looping, like so:
BufferedReader reader = new BufferedReader(new FileReader("Data.txt"));
try {
for (String line; (line = reader.readLine()) != null;) {
System.out.println(line);
}
} finally {
reader.close();
}
EDIT: forgot the requirement to take first 10 lines, you can change above code to put output in a list and return the list, then you can filter it through a function like this:
public List<String> takeFirst(int howMany, List<String> lines) {
return lines.size() <= howMany ? lines : lines.subList(0, howMany);
}
If the file is huge then this will be inefficient, of course, and if that matters you will end up doing something like:
BufferedReader reader = new BufferedReader(new FileReader("Data.txt"));
try {
int linesRead = 0;
for (String line; (line = reader.readLine()) != null && linesRead < 10;) {
System.out.println(line);
linesRead += 1;
}
} finally {
reader.close();
}
which is uglier but reads only the lines you need.

How about you don't print MAP if its value is null?