What Exactly Happens When You Call System.out.println() in Java?

What is System.out.println()

System.out.println is a Java statement that prints its argument to the standard console output.

System class

System is a final class in the java.lang package. It provides standard input, output, error streams, access to environment variables, and utility methods.

out field

The out field is a static member of type PrintStream. It is instantiated at JVM startup and mapped to the host’s standard output console.

println method

println

is a method of PrintStream that prints the argument followed by a line separator. The class defines many overloaded println methods, each implemented by calling print and then adding a newline; print ultimately calls write.

Structure diagram:

UML diagram

Key source snippets

public final PrintStream out
public final PrintStream err
public final InputStream in
...
public class System {
    static PrintStream out;
    static PrintStream err;
    static InputStream in;
}
public class PrintStream extends FilterOutputStream {
    public void println() { ... }
}

Redirecting output

The out object can be reassigned at runtime. Using System.setOut you can redirect output to a file:

public class ChangeOut {
    public static void main(String[] args) {
        try {
            System.setOut(new PrintStream(new FileOutputStream("log.txt")));
            System.out.println("Now the output is redirected!");
        } catch (Exception e) {}
    }
}

Performance considerations

Calling System.out.println is relatively slow because the call chain is println → print → write + newLine, and both write and newLine contain synchronized blocks. The overhead becomes noticeable when printing large amounts of data (e.g., >50 characters per line over >50,000 lines).

Why prefer a logging framework

Flexibility : Log4j provides multiple log levels, allowing selective output.

Refactorability : Changing a single configuration can disable all logging.

Maintainability : Scattered System.out.println calls are hard to manage in large codebases.

Granularity : Different classes can have separate loggers.

Practicality : Logging frameworks support output redirection, custom handlers, and richer formatting.

Therefore, System.out.println should be used only for quick experiments or learning, not for production logging or debugging.

Static import shortcut

Static import can shorten the call to out.println, but it reduces readability and is generally discouraged:

import static java.lang.System.out;
public class ShortSOP {
    public static void main(String[] args) {
        out.println("Hello, world");
    }
}

System.err and System.in

System.err

is another PrintStream that writes to the standard error stream. System.in is an InputStream connected to the console keyboard. Example:

import java.io.*;
public class InOutErr {
    public static void main(String[] args) {
        try {
            System.out.println("Enter a line");
            BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
            String filename = reader.readLine();
            InputStream input = new FileInputStream(filename);
            System.out.println("File opened...");
        } catch (IOException e) {
            System.err.println("Where is that file?");
        }
    }
}