Master Java I/O Streams: Byte, Character, Buffered, and More

I/O Streams Overview

IO streams are the solution for storing and reading data, analogous to water flowing.

Two main categories: byte streams (operate on all file types) and character streams (operate on text files only).

Byte Streams

FileInputStream reads bytes from a local file.

Typical steps:

Create a FileInputStream object.

Read data using read() or read(byte[]).

Close the stream.

FileInputStream fis = new FileInputStream("example.txt");
int data;
while ((data = fis.read()) != -1) {
    System.out.print((char) data);
}
fis.close();

FileOutputStream writes bytes to a file.

Create a FileOutputStream object.

Write data using write(int) or write(byte[]).

Close the stream.

FileOutputStream fos = new FileOutputStream("out.txt");
fos.write(97); // writes 'a'
fos.close();

Character Streams

FileReader and FileWriter operate on characters, built on top of byte streams with charset conversion.

FileReader fr = new FileReader("text.txt");
int ch;
while ((ch = fr.read()) != -1) {
    System.out.print((char) ch);
}
fr.close();

FileWriter writes characters to a file.

FileWriter fw = new FileWriter("out.txt");
fw.write('H');
fw.write("Hello");
fw.close();

Buffered Streams

Buffered streams add an internal buffer to improve I/O efficiency.

BufferedInputStream bis = new BufferedInputStream(new FileInputStream("data.bin"), 8192);
int b = bis.read();
bis.close();

BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream("gbk.txt"), "GBK"));
String line;
while ((line = br.readLine()) != null) {
    System.out.println(line);
}
br.close();

Conversion Streams

Conversion streams bridge byte streams and character streams.

InputStreamReader isr = new InputStreamReader(new FileInputStream("gbk.txt"), Charset.forName("GBK"));
BufferedReader br = new BufferedReader(isr);
String line;
while ((line = br.readLine()) != null) {
    System.out.println(line);
}
br.close();

Serialization Streams

ObjectOutputStream writes objects to a file; the class must implement Serializable.

Student stu = new Student("zhangsan", 23);
ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("student_object.txt"));
oos.writeObject(stu);
oos.close();

ObjectInputStream reads objects back.

ObjectInputStream ois = new ObjectInputStream(new FileInputStream("student_object.txt"));
Student stu = (Student) ois.readObject();
System.out.println(stu);
ois.close();

Print Streams

PrintStream and PrintWriter provide convenient methods for formatted output.

PrintStream ps = new PrintStream("output.txt");
ps.printf("Name: %s%n", "Alice");
ps.println("---");
ps.close();

System.out is a default PrintStream.

PrintStream ps = System.out;
ps.println("123");
// ps.close(); // should not close System.out
ps.println("456");

Compression Streams

ZipInputStream and ZipOutputStream handle zip compression and decompression.

public static void unzip(File src, File dest) throws IOException {
    ZipInputStream zip = new ZipInputStream(new FileInputStream(src));
    ZipEntry entry;
    while ((entry = zip.getNextEntry()) != null) {
        if (entry.isDirectory()) {
            new File(dest, entry.getName()).mkdirs();
        } else {
            FileOutputStream fos = new FileOutputStream(new File(dest, entry.getName()));
            int b;
            while ((b = zip.read()) != -1) {
                fos.write(b);
            }
            fos.close();
            zip.closeEntry();
        }
    }
    zip.close();
}

public static void myZip(File src, File dest) throws IOException {
    ZipOutputStream zipos = new ZipOutputStream(new FileOutputStream(new File(dest, "a.zip")));
    ZipEntry entry = new ZipEntry("a.txt");
    zipos.putNextEntry(entry);
    FileInputStream fis = new FileInputStream(src);
    int b;
    while ((b = fis.read()) != -1) {
        zipos.write(b);
    }
    fis.close();
    zipos.closeEntry();
    zipos.close();
}