Introduction#

Readers and Writers and their respective subclasses provide simple I/O for text / character-based data.

BufferedReader

Introduction

The BufferedReader class is a wrapper for other Reader classes that serves two main purposes:

1. A BufferedReader provides buffering for the wrapped Reader. This allows an application to read characters one at a time without undue I/O overheads.

2. A BufferedReader provides functionality for reading text a line at a time.

Basics of using a BufferedReader

The normal pattern for using a BufferedReader is as follows. First, you obtain the Reader that you want to read characters from. Next you instantiate a BufferedReader that wraps the Reader. Then you read character data. Finally you close the BufferedReader which close the wrapped `Reader. For example:

File someFile = new File(...);
int aCount = 0;
try (FileReader fr = new FileReader(someFile);
     BufferedReader br = new BufferedReader(fr)) {
    // Count the number of 'a' characters.
    int ch;
    while ((ch = br.read()) != -1) {
        if (ch == 'a') {
            aCount++;
        }
    }
    System.out.println("There are " + aCount + " 'a' characters in " + someFile);
}

You can apply this pattern to any Reader

Notes:

1. We have used Java 7 (or later) try-with-resources to ensure that the underlying reader is always closed. This avoids a potential resource leak. In earlier versions of Java, you would explicitly close the BufferedReader in a finally block.

2. The code inside the try block is virtually identical to what we would use if we read directly from the FileReader. In fact, a BufferedReader functions exactly like the Reader that it wraps would behave. The difference is that this version is a lot more efficient.

The BufferedReader buffer size

The BufferedReader.readLine() method

Example: reading all lines of a File into a List

This is done by getting each line in a file, and adding it into a List<String>. The list is then returned:

public List<String> getAllLines(String filename) throws IOException {
    List<String> lines = new ArrayList<String>();
    try (BufferedReader br = new BufferedReader(new FileReader(filename))) {
        String line = null;
        while ((line = reader.readLine) != null) {
            lines.add(line);
        }
    }
    return lines;
}

Java 8 provides a more concise way to do this using the lines() method:

public List<String> getAllLines(String filename) throws IOException {
    try (BufferedReader br = new BufferedReader(new FileReader(filename))) {
        return br.lines().collect(Collectors.toList());
    }
    return Collections.empty();
}

StringWriter Example