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How to read every second line from a file in java

Can someone tell me how to read every second line from a file in java?
BufferedReader br = new BufferedReader(new FileReader(file));
String line = br.readLine();
while(line != null){
//Do something ..
line = br.readLine()
}
br.close

One simple way would be to just maintain a counter of number of lines read:
int count = 0;
String line;
while ((line = br.readLine()) != null) {
if (count % 2 == 0) {
// do something with this line
}
++count;
}
But this still technically reads every line in the file, only choosing to process every other line. If you really only want to read every second line, then something like RandomAccessFile might be necessary.

You can do it in Java 8 fashion with very few lines :
static final int FIRST_LINE = 1;
Stream<String> lines = Files.lines(path);
String secondLine = lines.limit(2).skip(FIST_LINE).collect(Collectors.joining("\n"));
First you stream your file lines
You keep only the two first lines
Skip the first line
Note : In java 8, when using Files.lines(), you are supposed to close the stream afterwards or use it in a try-with-resource block.

This is similar to #Tim Biegeleisen's approach, but I thought I would show an alternative to get every other line using a boolean instead of a counter:
boolean skipOddLine = true;
String line;
while ((line = br.readLine()) != null) {
if (skipOddLine = !skipOddLine) {
//Use the String line here
}
}
This will toggle the boolean value every loop iteration, skipping every odd line. If you want to skip every even line instead you just need to change the initial condition to boolean skipOddLine = false;.
Note: This approach only works if you do not need to extend functionality to skip every 3rd line for example, where an approach like Tim's would be easier to modify. It also has the downside of being harder to read than the modulo approach.

This will help you to do it very well
You can use try with resource
You can use stream api java 8
You can use stream api supplier to use stream object again and again
I already hane added comment area to understand you
try (BufferedReader reader =
new BufferedReader(
new InputStreamReader(
new ByteArrayInputStream(x.getBytes()),
"UTF-8"))) { //this will help to you for various languages reading files
Supplier<Stream<String>> fileContentStream = reader::lines; // this will help you to use stream object again and again
if (FilenameUtils.getExtension(x.getOriginalFilename()).equals("txt")) { this will help you to various files extension filter
String secondLine = lines.limit(2).skip(FIST_LINE).collect(Collectors.joining("\n"));
String secondLine =
fileContentStream
.get()
.limit(2)
.skip(1)// you can skip any line with this action
.collect(Collectors.joining("\n"));
}
else if (FilenameUtils.getExtension(x.getOriginalFilename()).equals("pdf")) {
} catch (Exception ex) {
}

write multiple string line in a file

I want to write a file in multiple line in a file. for example:
first line: a b c d e f g h i
second line: j k l m n o p q r
third line: s t u v w x y z 1
but the code I made cannot do so. it only prints on one line every time I try to write in it. Here's my code:
FileOutputStream write = new FileOutputStream ("file.txt");
PrintStream print = new PrintStream (write);
BufferedReader in = new BufferedReader(new FileReader(data));
String read;
while ((read = in.readLine()) != null) {
String[] splited = read.split("\n");
for (int z = 0; z<splited.length; z++){
print.print(splited[z]+" ");
}
}
print.println();
how can i fix this?

You need to move print.println() inside the while loop.
The correct way is as follows :
while ((read = in.readLine()) != null) {
String[] splited = read.split("\n");
for (int z = 0; z<splited.length; z++){
print.print(splited[z]+" ");
}
print.println(); // correct place for println
}

So first of all, readLine reads one line, up to a newline, so there is no need to call split("\n") because it will always return the string itself.
Second, you never write a newline to the output file, so it ends up having all the lines unrolled in one. You can just modify the line print.print(splited[z] + " "); to print.print(splited[z] + "\n");.

I have altered your code in a few ways to improve readability and performance.
I am now using a BufferedWriter instead of a PrintStream, which has a nice method for writing newlines.
I am using try-with-resources. This is a Java 7+ feature that automatically closes connections, streams, and buffers for you. In your current code I don't see you closing either the reader or the writer. For such a small program this is not a big deal, but it is good practice to close them.
I used a for loop instead of a while loop. This is a nice trick I picked up that saves me from defining read in a higher scope than I need it (scope-creep).
-
try (BufferedWriter writer = new BufferedWriter(new FileWriter("file.txt"));
BufferedReader reader = new BufferedReader(new FileReader(data)))
{
for (String read = reader.readLine(); read != null; read = reader.readLine()) {
writer.append(read);
writer.newLine();
}
}

NullPointerException when trying to read a file line by line (Java)?

I'm trying to read a file line by line, but every time I run my program I get a NullPointerException at the line spaceIndex = line.indexOf(" "); which obviously means that line is null. HOWEVER. I know for a fact that the file I'm using has exactly 7 lines (even if I print the value of numLines, I get the value 7. And yet I still get a nullpointerexception when I try to read a line into my string.
// File file = some File I take in after clicking a JButton
Charset charset = Charset.forName("US-ASCII");
try (BufferedReader reader = Files.newBufferedReader(file.toPath(), charset)) {
String line = "";
int spaceIndex;
int numLines = 0;
while(reader.readLine()!=null) numLines++;
for(int i = 0; i<numLines; i++) {
line = reader.readLine();
spaceIndex = line.indexOf(" ");
System.out.println(spaceIndex);
}
PS: (I'm not actually using this code to print the index of the space, I replaced the code in my loop since there's a lot of it and it would make it longer to read)
If i'm going about reading the lines the wrong way, it would be great if someone could suggest another way, since so far every way I've tried gives me the same exception. Thanks.

By the time you start your for loop, the reader is already at the end of the file
(from the while loop).
Therefore, readLine() will always return null.
You should get rid of the for loop and do all of your work in the while loop as you first read the file.

You have two options.
First, you could read number of lines from a file this way:
LineNumberReader lnr = new LineNumberReader(new FileReader(new File("File1")));
lnr.skip(Long.MAX_VALUE);
System.out.println(lnr.getLineNumber());
Then read the file right after:
while((line = reader.readLine())!=null)
{
spaceIndex = line.indexOf(" ");
System.out.println(spaceIndex);
}
This first option is an alternative (and in my my opinion, cooler) way of doing this.
Second option (and probably the more sensible) is to do it all at once in the while loop:
while((line = reader.readLine())!=null)
{
numLines++;
spaceIndex = line.indexOf(" ");
System.out.println(spaceIndex);
}

Java StringBuffer: replace the content from a starting index to the end of a line

I have content of a file in a StringBuffer. The content of the file includes many lines (not on a single line). I want to edit the content of a line from index 4 (just for example) to the end of that line. I use replace() to edit the content of the StringBuffer.
The point is that the replace method has parameters such as starting index and ending index. But I don't know what is the ending index since each line have different number of characters
I think of using str.indexOf("\n") to find the ending index of the line, but then the file have many lines, so it will return incorrect results.
this is the readFile() if u need to read the code
Thank you
public StringBuffer readFile(){ //read file line by line
File f = getFilePath(fileName);
StringBuffer sb = new StringBuffer();
String textinLine;
try {
FileInputStream fs = new FileInputStream(f);
InputStreamReader in = new InputStreamReader(fs);
BufferedReader br = new BufferedReader(in);
while (true){
textinLine = br.readLine();
if (textinLine == null) break;
sb.append(textinLine+ "\n");
}
fs.close();
in.close();
br.close();
} ... // just some catch statements heres
}

Use String.indexOf() as you indicated, but pass in the starting position, e.g. indexOf('\n', 4);

I agree with Jim's idea, why not process string before appending it to StringBuffer.
By the way, I think you can use indexOf(String str, int fromIndex) function to parse StringBuffer, and each time when you get '\n', you can set an offset value, then next time when you get the next \n, you can just let index value plus the offset.

Question about Java File Reader

I'm having some problems with the FileReader class.
How do I specify an offset in the lines it goes through, and how do I tell it when to stop?
Let's say I want it to go through each line in a .txt file, but only lines 100-200 and then stop?
How would I do this? Right now I'm using ReadLine() but I don't think there's a way to specify offset with that.
Any fast help is VERY appreciated. Thanks.

You can't. FileReader reads a character at a time or a line at a time. Obviously you can write your own code extending or wrapping it to skip to the unneeded lines.
An aside: Be CAREFUL using FileReader or FileWriter - they use the default LOCALE character set. If you want to force a character set use OutputStreamWriter or InputStreamReader. Example
Writer w = new FileWriter(file) can be replaced by
Writer w = new OutputStreamWriter(new FileOutputStream(file),"UTF-8"); <=== see how I can set the character set.
An alternative: If you have FIXED-WIDTH text, then look at RandomAccessFile which lets you seek to any position. This doesn't help you much unless you have fixed width text or an index to skip to a line. But it is handy :)

Read all the lines but use another variable to count which line you are on. Call continue if you are on a line that you don't want to process (say, before the 100th line) and break when you will not want to process any more lines (after the 200th line).

There is not a way to tell the reader to only read certain lines, you can just use a counter to do it.
try {
BufferedReader in = new BufferedReader(new FileReader("infilename"));
String str;
int lineNumber = 0;
while ((str = in.readLine()) != null) {
lineNumber++;
if (lineNumber >= 100 && lineNumber <= 200) {
System.out.println("Line " + lineNumber + ": " + str);
}
}
in.close();
} catch (IOException e) { }

BufferedReader in = new BufferedReader(new FileReader("foo.in"));
for(int i=0;i<100;i++,in.readLine()){}
String line101 = in.readLine();