Java I/O Class Hierarchy

All the stream classes in java.io grow from four abstract base classes: InputStream and OutputStream for bytes, and Reader and Writer for characters. Each base class defines the basic methods, such as read, write and close, and each subclass connects them to a particular source or adds a feature, such as buffering. Once you know the four base classes, every other stream class is easy to place. In this lesson, you will learn the hierarchy and the methods of each base class.

The Hierarchy

Methods of InputStream

Methods of OutputStream

Methods of Reader and Writer

Reader has the same kind of methods as InputStream, but with characters: read() returns one char (as an int), read(char[] buf) fills a char array, and transferTo(Writer) copies everything. Writer mirrors OutputStream and adds write(String s) and append(CharSequence).

Example: Programming to the Base Class

Because all streams share these methods, a method can accept any InputStream without caring where the bytes come from:

import java.io.ByteArrayInputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
 
public class AnySource {
    static int countBytes(InputStream in) throws IOException {
        return in.readAllBytes().length;
    }
 
    public static void main(String[] args) throws IOException {
        try (FileOutputStream out = new FileOutputStream("notes.txt")) {
            out.write("Navika Anamaneni".getBytes());
        }
 
        try (InputStream file = new FileInputStream("notes.txt");
             InputStream memory = new ByteArrayInputStream(new byte[100])) {
            System.out.println("File:   " + countBytes(file) + " bytes");
            System.out.println("Memory: " + countBytes(memory) + " bytes");
        }
    }
}

File:   16 bytes
Memory: 100 bytes

Explanation: countBytes only knows about InputStream. It works the same with a file stream and with an in-memory stream, because both extend InputStream. Two resources in one try-with-resources statement are closed in reverse order.

Common Mistakes

Calling read() in a loop on a large file without buffering. Each call may go to the disk; wrap the stream in a buffered stream.

Relying on available() for the file size. It's only an estimate; read to the end or use Files.size.

Forgetting flush() when a stream must be sent before closing. Buffered data stays in memory until flushed or closed.

Practice

1. Easy: Write a method copy(InputStream in, OutputStream out) with transferTo, and use it to copy one file to another.
2. Harder: Write a method that reads any Reader and returns the number of lines, and test it with a FileReader and a StringReader.

Key Takeaways