Ch 2.6 Console Input and Output (I/O)

So far, we have printed information by directly assigning values to variables. In this chapter, we learn how to print output in a desired format and conversely, how to read values typed by the user on the keyboard and store them in variables.

1. Comparing Three Output Methods

Java provides three ways to print to the console.

Method	Line Break	Use Case
System.out.println()	Automatic newline	General output
System.out.print()	No newline	Continue printing on the same line
System.out.printf()	None (\n must be added manually)	Formatted output

public class OutputMethods {
    public static void main(String[] args) {
        // println: prints and automatically adds a newline
        System.out.println("First line");
        System.out.println("Second line");
        System.out.println(); // Print blank line

        // print: prints without a newline
        System.out.print("A");
        System.out.print("B");
        System.out.print("C");
        System.out.println(); // Add newline

        // printf: formatted output
        System.out.printf("Name: %s, Age: %d%n", "Alice", 20);
        // %n is a platform-independent newline (same as \n but more portable)
    }
}

Output:

First line
Second line

ABC
Name: Alice, Age: 20

2. Complete Guide to printf and Format Specifiers

printf is a formatted output method originating from the C language. It is used by placing a type character after the % symbol.

Basic Format Specifiers

Specifier	Type	Description	Example
%d	Integer	Decimal integer	%d -> 42
%o	Integer	Octal integer	%o -> 52
%x	Integer	Hexadecimal (lowercase)	%x -> 2a
%X	Integer	Hexadecimal (uppercase)	%X -> 2A
%f	Float	Fixed-point notation	%f -> 3.140000
%e	Float	Scientific notation (lowercase)	%e -> 3.140000e+00
%E	Float	Scientific notation (uppercase)	%E -> 3.140000E+00
%g	Float	Automatically selects shorter form	-
%s	String	String	%s -> "hello"
%S	String	Uppercase String	%S -> "HELLO"
%c	Character	char	%c -> A
%b	Boolean	boolean	%b -> true
%n	-	Newline (platform-independent)	-
%%	-	The % character itself	%% -> %

public class PrintfBasic {
    public static void main(String[] args) {
        int intVal = 42;
        double dblVal = 3.14;
        String strVal = "Java";
        char charVal = 'J';
        boolean boolVal = true;

        System.out.printf("Integer: %d%n", intVal);
        System.out.printf("Octal: %o%n", intVal);
        System.out.printf("Hex (lower): %x%n", intVal);
        System.out.printf("Hex (upper): %X%n", intVal);
        System.out.printf("Float: %f%n", dblVal);
        System.out.printf("Scientific: %e%n", dblVal);
        System.out.printf("String: %s%n", strVal);
        System.out.printf("Uppercase: %S%n", strVal);
        System.out.printf("Char: %c%n", charVal);
        System.out.printf("Boolean: %b%n", boolVal);
        System.out.printf("Percent: 50%%%n");
    }
}

Output:

Integer: 42
Octal: 52
Hex (lower): 2a
Hex (upper): 2A
Float: 3.140000
Scientific: 3.140000e+00
String: Java
Uppercase: JAVA
Char: J
Boolean: true
Percent: 50%

3. printf Flags, Width, and Precision

You can fine-tune the output format by adding extra options to format specifiers.

Syntax: %[flags][width][.precision]type

Flag/Option	Meaning	Example
number	Specify output width	%10d -> 10-char right-aligned
-	Left-align	%-10d -> 10-char left-aligned
0	Pad empty spaces with zeros	%010d -> 0000000042
+	Always show sign	%+d -> +42 or -42
	Space before positive numbers	% d -> 42
,	Thousands separator	%,d -> 1,000,000
.number	Decimal places	%.2f -> 3.14

public class PrintfFlags {
    public static void main(String[] args) {
        int num = 42;
        double d = 3.14159;
        long big = 1_000_000;

        // Width (right-aligned by default)
        System.out.printf("[%10d]%n", num);   // [        42]
        System.out.printf("[%-10d]%n", num);  // [42        ]

        // Zero padding
        System.out.printf("[%010d]%n", num);  // [0000000042]

        // Sign display
        System.out.printf("[%+d]%n", num);    // [+42]
        System.out.printf("[%+d]%n", -num);   // [-42]

        // Thousands separator
        System.out.printf("[%,d]%n", big);    // [1,000,000]

        // Decimal places
        System.out.printf("[%.2f]%n", d);     // [3.14]
        System.out.printf("[%.5f]%n", d);     // [3.14159]
        System.out.printf("[%10.2f]%n", d);   // [      3.14]
        System.out.printf("[%-10.2f]%n", d);  // [3.14      ]

        // Table format example
        System.out.printf("%-15s %5s %8s%n", "Name", "Age", "Score");
        System.out.printf("%-15s %5d %8.2f%n", "Alice Java", 20, 95.5);
        System.out.printf("%-15s %5d %8.2f%n", "Bob Python", 22, 88.3);
        System.out.printf("%-15s %5d %8.2f%n", "Carol Sharp", 21, 92.1);
    }
}

Output:

[        42]
[42        ]
[0000000042]
[+42]
[-42]
[1,000,000]
[3.14]
[3.14159]
[      3.14]
[3.14      ]
Name              Age      Score
Alice Java         20    95.50
Bob Python         22    88.30
Carol Sharp        21    92.10

4. Scanner Class in Detail

Scanner is the most basic way to read keyboard input. You must declare import java.util.Scanner; before using it.

Main Scanner Methods

Method	Return Type	Description
next()	String	Read one word up to whitespace
nextLine()	String	Read a full line (until Enter)
nextInt()	int	Read an integer
nextLong()	long	Read a long integer
nextDouble()	double	Read a floating-point number
nextFloat()	float	Read a float number
nextBoolean()	boolean	Read true/false
hasNext()	boolean	Check if there is a next token
hasNextInt()	boolean	Check if the next token is an int

import java.util.Scanner;

public class ScannerMethods {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        // Reading various types of input
        System.out.print("Enter name: ");
        String name = scanner.nextLine();   // Reads the full line

        System.out.print("Enter age: ");
        int age = scanner.nextInt();         // Reads only the integer

        System.out.print("Enter height: ");
        double height = scanner.nextDouble(); // Reads a floating-point number

        System.out.printf("Name: %s, Age: %d, Height: %.1f cm%n",
            name, age, height);

        scanner.close();
    }
}

5. Scanner Pitfall: nextLine After nextInt

nextInt(), nextDouble(), etc. read only the number and leave the newline character (\n) in the buffer. When nextLine() is called afterward, it reads the remaining \n and returns an empty string.

import java.util.Scanner;

public class ScannerPitfall {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter age: ");
        int age = scanner.nextInt(); // Input "25\n" -> reads 25, \n remains in buffer

        // Problem occurs!
        // System.out.print("Enter name: ");
        // String name = scanner.nextLine(); // Reads \n and returns empty string!

        // Solution 1: Consume remaining \n with an empty nextLine() call
        scanner.nextLine(); // Consume the \n left in buffer

        System.out.print("Enter name: ");
        String name = scanner.nextLine(); // Now works correctly

        System.out.printf("Name: %s, Age: %d%n", name, age);

        scanner.close();
    }
}

warning

Scanner pitfalls:

1. After nextInt(), must flush the buffer before calling nextLine()
2. Flush buffer with: call scanner.nextLine(); one more time
3. Alternatively, use only nextLine() and convert with Integer.parseInt()

Safer Approach: nextLine + parseInt Combination

import java.util.Scanner;

public class ScannerSafeMethod {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        // Read all input with nextLine(), convert when needed
        System.out.print("Enter name: ");
        String name = scanner.nextLine();

        System.out.print("Enter age: ");
        int age = Integer.parseInt(scanner.nextLine().trim());

        System.out.print("Enter height: ");
        double height = Double.parseDouble(scanner.nextLine().trim());

        System.out.printf("Name: %s, Age: %d, Height: %.1f cm%n",
            name, age, height);

        scanner.close();
    }
}

6. Repeated Input Using hasNext

import java.util.Scanner;

public class ScannerLoop {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int sum = 0;
        int count = 0;

        System.out.println("Enter numbers (stop: blank line):");

        // hasNextInt(): check if the next input is an integer
        while (scanner.hasNextInt()) {
            int num = scanner.nextInt();
            sum += num;
            count++;
            System.out.println("  Input: " + num + " (sum: " + sum + ")");
        }

        if (count > 0) {
            System.out.printf("Total %d numbers, Sum: %d, Average: %.2f%n",
                count, sum, (double) sum / count);
        }

        scanner.close();
    }
}

7. Fast Input Using BufferedReader

In competitive programming or large data processing, BufferedReader is much faster than Scanner.

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

public class BufferedReaderExample {
    public static void main(String[] args) throws IOException {
        // BufferedReader can throw IOException, so exception handling is required
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        System.out.print("Enter name: ");
        String name = br.readLine(); // Read one line

        System.out.print("Enter age: ");
        int age = Integer.parseInt(br.readLine().trim()); // Convert to integer

        System.out.print("Enter height: ");
        double height = Double.parseDouble(br.readLine().trim());

        System.out.printf("Name: %s, Age: %d, Height: %.1f cm%n",
            name, age, height);

        br.close();
    }
}

Scanner vs BufferedReader Comparison

Item	Scanner	BufferedReader
Speed	Slow	Fast
Ease of use	Convenient (type-specific methods)	Cumbersome (all as String)
Exception handling	Not required	IOException must be handled
Best for	General input	Large data, algorithms

8. System.err and System.in

public class SystemStreams {
    public static void main(String[] args) {
        // System.out: standard output stream (console screen)
        System.out.println("Normal output");

        // System.err: standard error stream (usually displayed in red in the console)
        System.err.println("Error message!"); // Used for errors or warnings

        // System.in: standard input stream (keyboard)
        // Used by Scanner and BufferedReader
        System.out.println("System.in type: " + System.in.getClass().getName());

        // Error output usage example
        int divisor = 0;
        if (divisor == 0) {
            System.err.println("[ERROR] Divisor is 0!");
        } else {
            System.out.println("Result: " + (10 / divisor));
        }
    }
}

9. Closing Scanner with try-with-resources

Resources (Scanner, BufferedReader, etc.) must be closed after use. Using the try-with-resources syntax closes them automatically.

import java.util.Scanner;

public class TryWithResources {
    public static void main(String[] args) {
        // try-with-resources: automatically calls scanner.close() when block ends
        try (Scanner scanner = new Scanner(System.in)) {
            System.out.print("Enter name: ");
            String name = scanner.nextLine();

            System.out.print("Enter age: ");
            int age = Integer.parseInt(scanner.nextLine().trim());

            System.out.printf("Hello, %s! (Age: %d)%n", name, age);

        } catch (NumberFormatException e) {
            System.err.println("Age must be a number: " + e.getMessage());
        }
        // At this point, scanner has been automatically closed
    }
}

10. Practical Example: Student Information Input Program

import java.util.Scanner;

public class StudentInfoInput {
    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(System.in)) {
            System.out.println("╔════════════════════════════╗");
            System.out.println("║  Student Information System ║");
            System.out.println("╚════════════════════════════╝");

            // Basic information input
            System.out.print("Name: ");
            String name = scanner.nextLine().trim();

            System.out.print("Student ID: ");
            String studentId = scanner.nextLine().trim();

            System.out.print("Age: ");
            int age = Integer.parseInt(scanner.nextLine().trim());

            System.out.print("Major: ");
            String major = scanner.nextLine().trim();

            // Subject score input
            System.out.println("\nEnter subject scores (0~100):");
            System.out.print("Math: ");
            int math = Integer.parseInt(scanner.nextLine().trim());

            System.out.print("English: ");
            int english = Integer.parseInt(scanner.nextLine().trim());

            System.out.print("Science: ");
            int science = Integer.parseInt(scanner.nextLine().trim());

            // Calculations
            int total = math + english + science;
            double average = (double) total / 3;

            // Grade calculation
            char grade;
            if (average >= 90) grade = 'A';
            else if (average >= 80) grade = 'B';
            else if (average >= 70) grade = 'C';
            else if (average >= 60) grade = 'D';
            else grade = 'F';

            // Result output
            System.out.println();
            System.out.println("╔═══════════════════════════════════╗");
            System.out.printf("║  %-33s║%n", "Report Card");
            System.out.println("╠═══════════════════════════════════╣");
            System.out.printf("║  Name:       %-21s║%n", name);
            System.out.printf("║  Student ID: %-21s║%n", studentId);
            System.out.printf("║  Age:        %-21s║%n", age);
            System.out.printf("║  Major:      %-21s║%n", major);
            System.out.println("╠═══════════════════════════════════╣");
            System.out.printf("║  Math:    %3d%22s║%n", math, "");
            System.out.printf("║  English: %3d%22s║%n", english, "");
            System.out.printf("║  Science: %3d%22s║%n", science, "");
            System.out.println("╠═══════════════════════════════════╣");
            System.out.printf("║  Total: %3d  Average: %5.2f  Grade: %c ║%n",
                total, average, grade);
            System.out.println("╚═══════════════════════════════════╝");

        } catch (NumberFormatException e) {
            System.err.println("Scores must be numbers!");
        }
    }
}

Sample Input/Output:

╔════════════════════════════╗
║  Student Information System ║
╚════════════════════════════╝
Name: Alice Java
Student ID: 20240001
Age: 20
Major: Computer Science

Enter subject scores (0~100):
Math: 92
English: 85
Science: 88

╔═══════════════════════════════════╗
║  Report Card                      ║
╠═══════════════════════════════════╣
║  Name:       Alice Java           ║
║  Student ID: 20240001             ║
║  Age:        20                   ║
║  Major:      Computer Science     ║
╠═══════════════════════════════════╣
║  Math:     92                     ║
║  English:  85                     ║
║  Science:  88                     ║
╠═══════════════════════════════════╣
║  Total: 265  Average: 88.33  Grade: B ║
╚═══════════════════════════════════╝

11. I/O Format Quick Reference Cheat Sheet

public class CheatSheet {
    public static void main(String[] args) {
        // Commonly used printf patterns
        int n = 42;
        double d = 3.14159;
        String s = "Java";

        // Integer output
        System.out.printf("%d%n", n);         // 42
        System.out.printf("%5d%n", n);        // [   42]  right-aligned
        System.out.printf("%-5d%n", n);       // [42   ]  left-aligned
        System.out.printf("%05d%n", n);       // [00042]  zero padding
        System.out.printf("%,d%n", 1000000);  // 1,000,000  thousands separator

        // Float output
        System.out.printf("%.2f%n", d);       // 3.14
        System.out.printf("%8.2f%n", d);      // [    3.14]
        System.out.printf("%-8.2f%n", d);     // [3.14    ]
        System.out.printf("%e%n", d);         // 3.141590e+00

        // String output
        System.out.printf("%s%n", s);         // Java
        System.out.printf("%10s%n", s);       // [      Java]
        System.out.printf("%-10s%n", s);      // [Java      ]
        System.out.printf("%.3s%n", s);       // Jav  (max 3 characters)
    }
}

Summary

• println(): print + automatic newline
• print(): print (no newline)
• printf(): formatted output (%d, %s, %f, %.2f, %10s, etc.)
• Scanner: reads keyboard input, next(), nextLine(), nextInt(), nextDouble()
• After nextInt(), be careful with nextLine(): must consume the remaining \n in the buffer
• BufferedReader: fast input, use readLine() + parseInt() combination
• try-with-resources: automatically closes Scanner/BufferedReader
• System.err: stream for printing error messages