System.out.println() / System.err.println() / System.in

// Prints to standard output with a newline.
System.out.println(value);

// Prints to standard output without a newline.
System.out.print(value);

// Prints with format specifiers (similar to printf).
System.out.printf(String format, Object... args);

// Prints to standard error output (displayed in red).
System.err.println(value);
System.err.print(value);

// Reads keyboard input from standard input.
Scanner scanner = new Scanner(System.in);
String line  = scanner.nextLine();   // Reads one line.
int num       = scanner.nextInt();   // Reads an integer.
double d      = scanner.nextDouble();// Reads a decimal number.
scanner.close(); // Close the scanner when done.

// Print values of various types using println().
System.out.println("Hello");     // Prints: Hello
System.out.println(42);          // Prints: 42
System.out.println(3.14);        // Prints: 3.14
System.out.println(true);        // Prints: true
System.out.println(null);        // Prints: null

// Print without newlines using print().
System.out.print("A");
System.out.print("B");
System.out.println("C"); // Prints: ABC on a single line

// Print with formatting using printf().
String name = "Alice";
int age = 30;
System.out.printf("Name: %s, Age: %d%n", name, age); // Prints: Name: Alice, Age: 30
System.out.printf("Pi: %.4f%n", Math.PI);             // Prints: Pi: 3.1416

// Print an error message using err.println().
try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.err.println("An error occurred: " + e.getMessage());
}

// Read input from standard input using Scanner.
import java.util.Scanner;
Scanner sc = new Scanner(System.in);
System.out.print("Enter your name: ");
String input = sc.nextLine();
System.out.println("Hello, " + input + "!");
sc.close();

System.out is a static field of type PrintStream connected to standard output. Similarly, System.err is connected to standard error output, so error messages flow through a separate stream from System.out.

Common format specifiers are %s (string), %d (integer), %f (floating-point), %.2f (two decimal places), and %n (newline). In production applications, it is recommended to use a logging library such as java.util.logging or SLF4J instead of System.out.println().

For timing measurements, see System.currentTimeMillis() / System.nanoTime().