Java FileInputStream

FileInputStream

So far, you have used the Scanner class to read text files. Scanner is very convenient for text because it can split input into lines, words, or numbers. However, sometimes you need more control. For example, when reading binary data (like images, audio, or PDFs), or when you need full control of raw bytes. In those cases, you use FileInputStream.

Read a Text File (Basic Example)

This example uses FileInputStream to read a text file, one byte at a time, and print the result as characters:

import java.io.FileInputStream;  // Import FileInputStream
import java.io.IOException;      // Import IOException

public class Main {
  public static void main(String[] args) {
    // try-with-resources: FileInputStream will be closed automatically
    try (FileInputStream input = new FileInputStream("filename.txt")) {

      int i;  // variable to store each byte that is read

      // Read one byte at a time until end of file (-1 means "no more data")
      while ((i = input.read()) != -1) {
        // Convert the byte to a character and print it to the console
        System.out.print((char) i);
      }

    } catch (IOException e) {
      // If an error happens (e.g. file not found), print an error message
      System.out.println("Error reading file.");
    }
  }
}

Explanation: This program opens filename.txt, reads it byte by byte, and prints the result as characters in the console.

Copy a Binary File (Real-World Example)

The real strength of FileInputStream is that it can handle any file type, not just text. Here is an example that copies an image file:

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class CopyFile {
  public static void main(String[] args) {
    // Copy image.jpg into copy.jpg
    try (FileInputStream input = new FileInputStream("image.jpg");
         FileOutputStream output = new FileOutputStream("copy.jpg")) {

      int i;
      while ((i = input.read()) != -1) {
        output.write(i);  // write the raw byte to the new file
      }

      System.out.println("File copied successfully.");

    } catch (IOException e) {
      System.out.println("Error handling file.");
    }
  }
}

Explanation: This program reads image.jpg and writes it into copy.jpg. Since it works with raw bytes, it can copy any kind of file - text, images, audio, or PDFs.

Note: The program also uses FileOutputStream. While FileInputStream is used to read bytes from a file, FileOutputStream is used to write bytes to a file. Together, they make it possible to copy any kind of file.

You will learn more about FileOutputStream in the next chapter.

Choosing the Right Class

Java gives you several ways to read files. Here's when to pick each one:

• Scanner - best for simple text and when you want to parse numbers or words easily.
• BufferedReader - best for large text files, because it is faster and reads line by line.
• FileInputStream - best for binary data (images, audio, PDFs) or when you need full control of raw bytes.