OutputStream is an abstract class for writing raw bytes to a destination (like a file, memory buffer, socket, etc.). Subclasses handle actual implementations, such as FileOutputStream, BufferedOutputStream, etc.
Commonly Used Methods
Simple Program – Write Data to File
LotusJavaPrince wants to write the string “Hello, Mahesh!” to a file using FileOutputStream.
import java.io.FileOutputStream;
import java.io.IOException;
public class SimpleOutputStreamExample {
public static void main(String[] args) {
String message = "Hello, Mahesh!";
try (FileOutputStream fos = new FileOutputStream("greeting.txt")) {
fos.write(message.getBytes()); // Convert string to bytes and write
System.out.println("Message written to file.");
} catch (IOException e) {
System.err.println("Error writing to file: " + e.getMessage());
}
}
}Output:
A file named greeting.txt will contain:
Hello, Mahesh!Problem Statement:
LotusJavaPrince is building a logging system. Every log message (INFO, WARNING, ERROR) must be written to a file using OutputStream. The log should append messages for every run.
import java.io.FileOutputStream;
import java.io.IOException;
import java.time.LocalDateTime;
public class Logger {
private static final String LOG_FILE = "system.log";
public static void log(String level, String message) {
String timestamp = LocalDateTime.now().toString();
String logEntry = timestamp + " [" + level.toUpperCase() + "] " + message + "\n";
try (FileOutputStream fos = new FileOutputStream(LOG_FILE, true)) {
fos.write(logEntry.getBytes());
fos.flush();
} catch (IOException e) {
System.err.println("Logging failed: " + e.getMessage());
}
}
public static void main(String[] args) {
log("INFO", "System started by LotusJavaPrince");
log("WARNING", "Mahesh noticed high CPU usage");
log("ERROR", "Database connection lost");
}
}Output in system.log:
2025-05-30T12:45:32.789 [INFO] System started by LotusJavaPrince
2025-05-30T12:45:32.790 [WARNING] Mahesh noticed high CPU usage
2025-05-30T12:45:32.791 [ERROR] Database connection lostCode language: CSS (css)In the above output each execution appends new logs.
OutputStream is the superclass for writing byte-based output.
- It cannot be instantiated directly – use subclasses like
FileOutputStream. - Useful for writing to files, sockets, buffers, etc.
- Works at low-level byte granularity, suitable for binary data.