Difference Between System.out.println() and System.err.println() in Java

If we are using a simple Java console application, both outputs will be the same but we can reconfigure the streams so that for example, prints to the console but System.err writes to a file. In this section, we will discuss the differences between System.out.println() and System.err.println() in Java.

1. Purpose

System.out.println(): Used for printing general output, like program results or messages to the user.
System.err.println(): Used for printing error messages or diagnostic information.

2. Output Stream

System.out.println(): Prints to the standard output stream (stdout).
System.err.println(): Prints to the standard error stream (stderr).

3. Redirection

System.out.println(): Output can be redirected to a file or another process using standard redirection techniques.
System.err.println(): Output can also be redirected, but it's often handled separately to ensure error messages are visible even if standard output is redirected.

4. Use Case

System.out.println(): It is used for displaying information that is part of the normal program flow.
System.err.println(): It is used for reporting issues, exceptions, or warnings that need attention.

System.out.println() Vs. System.err.println()

Feature	System.out.println()	System.err.println()
Purpose	General output	Error and debugging output
Stream Type	Standard output stream	Standard error stream
Usage	Used for regular program output	Used for printing error messages and diagnostics
Default Destination	Console	Console
Redirection	Can be redirected separately from System.err	Can be redirected separately from System.out
Buffering	Buffered by default	Unbuffered by default
Output Distinction	Less suitable for distinguishing error messages	Suitable for distinguishing error messages
Typical Use Cases	Displaying normal program output like results, logs	Printing stack traces, error messages, warnings
Code Example	System.out.println("Hello, World!");	System.err.println("An error occurred!");
Performance Impact	Minor impact due to buffering	May have a slight performance impact

Example

File Name: SystemOutput.java

 
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
public class SystemOutput {
    public static void main(String[] args) {
        try {
            // Create PrintStream objects for file output
            PrintStream fileOut = new PrintStream(new File("output.txt"));
            PrintStream fileErr = new PrintStream(new File("error.txt"));
            // Save the original System.out and System.err PrintStream
            PrintStream consoleOut = System.out;
            PrintStream consoleErr = System.err;
            // Set System.out to the file PrintStream
            System.setOut(fileOut);
            // Set System.err to the error PrintStream
            System.setErr(fileErr);
            // Normal program output redirected to output.txt
            System.out.println("This is a regular message to the file.");
            // Error message output redirected to error.txt
            System.err.println("This is an error message to the file.");
            // Reset System.out and System.err to the console PrintStream
            System.setOut(consoleOut);
            System.setErr(consoleErr);
            // Verify by printing messages to the console
            System.out.println("This message is back to the console.");
            System.err.println("This error message is back to the console.");
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}   

Output:

 
This message is back to the console.
This error message is back to the console.

error.txt

Output.txt

 
This is a regular message to the file.

Conclusion

It is easier to write more reliable and maintainable Java programs when we know the differences between System.out.println() and System.err.println(). To ensure that error messages and diagnostics are easily distinguished from regular program output-a crucial feature for debugging and logging-System.err.println() should be used.

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