Java Files and I/O: Understanding Streams and Data Handling

Explore the java.io package in Java, which encompasses essential classes for efficient input and output (I/O) operations. Learn about data streams, including their types and functionalities, enabling you to manage various data forms—primitives, objects, and localized characters—in your Java applications.

Java - Files and I/O

The java.io package contains almost all the classes required for input and output (I/O) in Java. These classes handle streams, which represent an input source and an output destination. Streams in Java support various types of data such as primitives, objects, and localized characters.

Stream

A stream can be defined as a sequence of data. There are two main types of streams:

InputStream - Used to read data from a source.
OutputStream - Used to write data to a destination.

Byte Streams

Byte streams perform input and output of 8-bit bytes. The most commonly used classes for byte streams are FileInputStream and FileOutputStream. Here’s an example of copying data from one file to another using these classes:

Example


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

public class CopyFile {
public static void main(String args[]) throws IOException {
FileInputStream in = null;
FileOutputStream out = null;

try {
    in = new FileInputStream("input.txt");
    out = new FileOutputStream("output.txt");

    int c;
    while ((c = in.read()) != -1) {
        out.write(c);
    }
} finally {
    if (in != null) in.close();
    if (out != null) out.close();
}
}
}

Output


Contents of input.txt are copied to output.txt

Character Streams

While byte streams handle 8-bit bytes, character streams manage 16-bit Unicode characters. Common classes are FileReader and FileWriter. Here’s an example using character streams:

Example


import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class CopyFile {
public static void main(String args[]) throws IOException {
FileReader in = null;
FileWriter out = null;

try {
    in = new FileReader("input.txt");
    out = new FileWriter("output.txt");

    int c;
    while ((c = in.read()) != -1) {
        out.write(c);
    }
} finally {
    if (in != null) in.close();
    if (out != null) out.close();
}
}
}

Output


Contents of input.txt are copied to output.txt

Standard Streams

Java provides three standard streams:

Standard Input (System.in) - Usually from the keyboard.
Standard Output (System.out) - Usually to the display screen.
Standard Error (System.err) - For error messages.

The following program reads characters from the console until the user types 'q':

Example


import java.io.InputStreamReader;
import java.io.IOException;

public class ReadConsole {
public static void main(String args[]) throws IOException {
InputStreamReader cin = null;

try {
    cin = new InputStreamReader(System.in);
    System.out.println("Enter characters, 'q' to quit.");

    char c;
    do {
        c = (char) cin.read();
        System.out.print(c);
    } while (c != 'q');
} finally {
    if (cin != null) cin.close();
}
}
}

Output


Enter characters, 'q' to quit.
1
1
a
a
q
q

File Navigation and I/O

FileInputStream

The FileInputStream class is used to read data from a file:

Example


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

public class FileStreamExample {
public static void main(String args[]) {
try (FileInputStream f = new FileInputStream("input.txt")) {
    int i;
    while ((i = f.read()) != -1) {
        System.out.print((char) i);
    }
} catch (IOException e) {
    e.printStackTrace();
}
}
}

Output


This is the content of the input file.

FileOutputStream

The FileOutputStream class is used to write data to a file:

Example


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

public class WriteFileExample {
public static void main(String args[]) {
try (FileOutputStream fos = new FileOutputStream("output.txt")) {
    String data = "Hello, World!";
    fos.write(data.getBytes());
} catch (IOException e) {
    e.printStackTrace();
}
}
}

Output


"Hello, World!" is written to output.txt

Directories in Java

Creating Directories

You can use the mkdir() or mkdirs() methods to create directories in Java. mkdir() creates a single directory, while mkdirs() creates both the specified directory and all its parents.

Example


import java.io.File;

public class CreateDirectoryExample {
public static void main(String args[]) {
String dirname = "/tmp/user/java/bin";
File d = new File(dirname);

// Create directory now
if (d.mkdirs()) {
    System.out.println("Directory created successfully!");
} else {
    System.out.println("Failed to create directory.");
}
}
}

Output


Directory created successfully!

Listing Directories

The list() method of the File class can be used to list files and directories in a directory:

Example


import java.io.File;

public class ListDirectoryExample {
public static void main(String args[]) {
File dir = new File("/tmp");
String[] files = dir.list();

if (files != null && files.length > 0) {
    for (String file : files) {
        System.out.println(file);
    }
} else {
    System.out.println("Directory is empty or does not exist.");
}
}
}

Output


bin
tmp
docs

File Permissions

Checking File Permissions

The canRead(), canWrite(), and canExecute() methods can be used to check a file’s read, write, and execute permissions, respectively.

Example


import java.io.File;

public class FilePermissionExample {
public static void main(String args[]) {
File file = new File("input.txt");

System.out.println("Can read: " + file.canRead());
System.out.println("Can write: " + file.canWrite());
System.out.println("Can execute: " + file.canExecute());
}
}

Output


Can read: true
Can write: true
Can execute: false