BufferedReader
- Java course
- Intermediate
- Lesson 72/116
- 5 min
Learn to read lines with BufferedReader.
Reading lines with BufferedReader
BufferedReader wraps FileReader for more efficient reading. Its main method is readLine(): it returns one line as a String, without the line terminator. At end of file it returns null.
Usage: BufferedReader r = new BufferedReader(new FileReader("a.txt"));. Loop with String line = r.readLine(); while (line != null) { ... line = r.readLine(); }. BufferedWriter speeds up writing; newLine() adds a platform-independent line separator.
Using BufferedReader
You create a BufferedReader by wrapping a Reader, not directly: new BufferedReader(new FileReader(path)). This is the decorator pattern: BufferedReader extends FileReader's capabilities. read() still reads a single character, but readLine() gives the whole line at once.
For large files, the buffer reduces disk access; reading is much faster. The default buffer size is 8 KB.
Practice Read a file line by line with BufferedReader
Import java.io.BufferedReader and java.io.FileReader. Add throws IOException to main. First write a file: FileWriter fw = new FileWriter("line.txt"); fw.write("One\nTwo"); fw.close();. Then open with BufferedReader r = new BufferedReader(new FileReader("line.txt"));, declare String line;, loop with while ((line = r.readLine()) != null) printing each line. Close with r.close().
Starter code
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
// First write two lines to a file
// Open with BufferedReader, read line by line, print
}
}Sample solution
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
FileWriter fw = new FileWriter("line.txt");
fw.write("One\nTwo");
fw.close();
BufferedReader r = new BufferedReader(new FileReader("line.txt"));
String line;
while ((line = r.readLine()) != null) {
System.out.println(line);
}
r.close();
}
}Expected output
One Two
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does BufferedReader.readLine() return at end of file?
- An empty string ("")
- null Correct answer
- -1
- A line terminator character
Explanation: readLine() returns null at end of file; the loop ends on null.
2 Complete the line-reading loop
String line;
while ((line = r.____()) != null) {
System.out.println(line);
}String line;
while ((line = r.readLine()) != null) {
System.out.println(line);
}Explanation: readLine returns a line; null ends the loop.
3 Match the BufferedReader concepts
| Term | Meaning |
|---|---|
readLine() |
Returns one line |
null |
End of file |
FileReader |
Class wrapped by BufferedReader |
decorator |
The wrapping pattern |