File

In Java we can read lines, as strings, from a file with BufferedReader. We use this to wrap around other types, such as FileReader.

On BufferedReader, we can call readLine() to get a line. These methods are optimized, and will perform well for most Java programs.

Read lines

We put these objects together into a program. We specify a file on the disk—you will want to change this before running this program. Make a text file that has some lines.

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

public class Program {
    public static void main(String[] args) throws IOException {

        // Create a BufferedReader from a FileReader.
        BufferedReader reader = new BufferedReader(new FileReader(
                "C:\\file.txt"));

        // Loop over lines in the file and print them.
        while (true) {
            String line = reader.readLine();
            if (line == null) {
                break;
            }
            System.out.println(line);
        }

        // Close the BufferedReader.
        reader.close();
    }
}
First line.
Second line.

Read lines into ArrayList

We often use an ArrayList to store many strings. We can add lines from a text file to an ArrayList. We call add() when BufferedReader returns a line in our loop.

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;

public class Program {
    public static void main(String[] args) throws IOException {

        ArrayList<String> list = new ArrayList<>();

        // New BufferedReader.
        BufferedReader reader = new BufferedReader(new FileReader(
                "C:\\file.txt"));

        // Add all lines from file to ArrayList.
        while (true) {
            String line = reader.readLine();
            if (line == null) {
                break;
            }
            list.add(line);
        }

        // Close it.
        reader.close();

        // Print size of ArrayList.
        System.out.println("Lines: " + list.size());

        // Print each line.
        for (String line : list) {
            System.out.println(line);
        }
    }
}
Lines: 2
First line.
Second line.

Count lines

It is possible to count the lines in a file with BufferedReader and its lines() Stream. Lines() returns a Stream of Strings. It is lazily evaluated.

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

public class Program {
    public static void main(String[] args) throws IOException {

        BufferedReader reader = new BufferedReader(new FileReader(
                "C:\\programs\\file.txt"));

        // Count lines in the file.
        // ... Call count() on the lines Stream.
        long lines = reader.lines().count();
        System.out.println("Lines: " + lines);

        // Close it.
        reader.close();
    }
}
Lines: 9

BufferedWriter

This type writes Strings to a file. It also handles chars, ranges of characters (like substrings) and ints. We write a String array to lines in a file.

Copy files

With the clearly-named Files.copy method, we copy a file's contents from one location to another. We use a CopyOption to improve our programs.

Many types are available in the java.io packages. But BufferedStream, with its useful readLine method, is one of the easiest and most efficient types, at least for text files.