Input in Java

A. The Two Methods of Supplying Data:

• 1. Compile-Time Initialization: Values are assigned directly to variables within the source code:

  int length = 10;
  int breadth = 5;
  int area = length * breadth;

  Limitation: Rigid and non-interactive. To calculate the area of a different rectangle, the source code must be modified and recompiled!
• 2. Run-Time Input: Data is supplied dynamically by the end-user while the program is running, either via keyboard streams (Scanner, BufferedReader) or command-line arguments.
  Advantage: Highly flexible and interactive. The same compiled program can process infinite data variations without source code changes.

A. The Setup Protocol:

The Scanner class resides in the java.util package and must be explicitly imported:

import java.util.Scanner; // Step 1: Import package

public class InputDemo {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in); // Step 2: Instantiate object
        // System.in represents the standard input stream (keyboard)
    }
}

B. Complete Scanner Input Methods Catalog:

A. The Classic Buffer Skip Bug:

Methods like nextInt(), nextDouble(), and next() scan only the target tokens, leaving the trailing newline character (\n generated by the ENTER key) sitting unread in the keyboard stream.

System.out.print("Enter Roll No: ");
int roll = in.nextInt(); // User types 105 and presses ENTER (
)

System.out.print("Enter Full Name: ");
String name = in.nextLine(); // BUG! Immediately consumes the leftover '
' and skips!

B. The Fail-Safe Solution (Buffer Clearing):

Always insert an extra dummy in.nextLine(); immediately after reading numeric or single-word inputs before attempting to read a full line:

System.out.print("Enter Roll No: ");
int roll = in.nextInt();

in.nextLine(); // BUFFER FLUSH! Consumes and discards the orphan '
'

System.out.print("Enter Full Name: ");
String name = in.nextLine(); // Works flawlessly! Pauses for actual user input.

A. The BufferedReader Pipeline:

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

public class ReaderDemo {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        System.out.print("Enter age: ");
        int age = Integer.parseInt(br.readLine());
    }
}

The Three Categories of Software Defects:

1. Syntax Errors (Compile-Time Errors): Violations of Java grammatical rules detected by javac before execution. (e.g., missing semicolon, mismatched braces, misspelled keywords).
2. Logical Errors (Bugs): The program compiles and runs without crashing, but produces incorrect outputs due to flawed algorithmic design. (e.g., writing area = length + breadth instead of length * breadth).
3. Run-Time Errors (Exceptions): The code is syntactically valid, but an illegal operation causes the program to crash during execution. Common input exceptions:
   • InputMismatchException: User enters letters when nextInt() expects digits.
   • NumberFormatException: Integer.parseInt("abc") fails to parse non-numeric strings.
   • ArithmeticException: Attempting integer division by zero (10 / 0).

A. Interactive Student Report Card Generator:

import java.util.Scanner;

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

        System.out.print("Enter Student Roll Number: ");
        int rollNo = in.nextInt();

        in.nextLine(); // Buffer flush to clear leftover newline!

        System.out.print("Enter Student Full Name: ");
        String fullName = in.nextLine();

        System.out.print("Enter Marks in English (out of 100): ");
        double english = in.nextDouble();

        System.out.print("Enter Marks in Mathematics (out of 100): ");
        double maths = in.nextDouble();

        System.out.print("Enter Marks in Science (out of 100): ");
        double science = in.nextDouble();

        double totalMarks = english + maths + science;
        double percentage = (totalMarks / 300.0) * 100.0;

        char grade;
        if (percentage >= 90.0) grade = 'A';
        else if (percentage >= 75.0) grade = 'B';
        else if (percentage >= 60.0) grade = 'C';
        else if (percentage >= 40.0) grade = 'D';
        else grade = 'F';

        System.out.println("
========== ACADEMIC REPORT CARD ==========");
        System.out.println("Roll Number : " + rollNo);
        System.out.println("Student Name: " + fullName);
        System.out.println("Total Marks : " + totalMarks + " / 300");
        System.out.printf("Percentage  : %.2f%%
", percentage);
        System.out.println("Final Grade : " + grade);
        System.out.println("==========================================");
    }
}

Slab-Based Commercial Tariff Calculator:

import java.util.Scanner;

public class ElectricityBill {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter Consumer Name: ");
        String name = in.nextLine();
        System.out.print("Enter Units Consumed: ");
        int units = in.nextInt();

        double billAmount = 0.0;
        // Slab-wise calculation:
        // First 100 units: Free (₹0)
        // Next 100 units (101-200): ₹2.50 per unit
        // Next 200 units (201-400): ₹4.00 per unit
        // Above 400 units: ₹6.00 per unit
        if (units <= 100) {
            billAmount = 0.0;
        } else if (units <= 200) {
            billAmount = (units - 100) * 2.50;
        } else if (units <= 400) {
            billAmount = (100 * 2.50) + (units - 200) * 4.00;
        } else {
            billAmount = (100 * 2.50) + (200 * 4.00) + (units - 400) * 6.00;
        }

        double meterRent = 150.0;
        double totalBill = billAmount + meterRent;

        System.out.println("
========== ELECTRICITY INVOICE ==========");
        System.out.println("Consumer Name : " + name);
        System.out.println("Units Consumed: " + units + " kWh");
        System.out.println("Energy Charges: ₹" + billAmount);
        System.out.println("Meter Rent    : ₹" + meterRent);
        System.out.println("Total Payable : ₹" + totalBill);
        System.out.println("=========================================");
    }
}

A. The printf() Method and Format Specifiers:

System.out.printf("format string", arguments) allows precision formatting borrowed from C:

• %d: Formats decimal integers (byte, short, int, long).
• %f: Formats floating-point numbers. Use %.2f to round to 2 decimal places.
• %c: Formats a single character.
• %s: Formats a string of characters.
• %n: Platform-independent newline.

double price = 129.4567;
System.out.printf("Item: %s | Price: ₹%.2f%n", "Scientific Calculator", price);
// Outputs: Item: Scientific Calculator | Price: ₹129.46

A. The String[] args Array in main():

Java permits supplying input values directly from the terminal operating system command line when launching an application:

public class CommandLineDemo {
    public static void main(String[] args) {
        // Values typed after 'java CommandLineDemo' are stored in args[]
        if (args.length >= 2) {
            String name = args[0];
            int age = Integer.parseInt(args[1]);
            System.out.println("Hello " + name + ", in 5 years you will be " + (age + 5));
        }
    }
}

Execution from terminal: java CommandLineDemo Aarav 15 → args[0] = "Aarav" and args[1] = "15".

A. Space-Delimited Multi-Token Input:

By default, Java's Scanner class uses whitespace (spaces, tabs, and newlines) as token delimiters. This enables users to enter multiple data values on a single line separated by spaces:

System.out.print("Enter three integers separated by spaces: ");
int a = in.nextInt();
int b = in.nextInt();
int c = in.nextInt();
int sum = a + b + c;
System.out.println("Sum of three numbers = " + sum);

If the user enters: 10 20 30 followed by ENTER, a receives 10, b receives 20, and c receives 30. The Scanner parses each integer sequentially without requiring separate input lines.

A. Preventing Input Mismatch via hasNext():

To prevent InputMismatchException runtime crashes when accepting dynamic user input, defensive programmers use Scanner's inspection methods: in.hasNextInt() and in.hasNextDouble() before reading:

System.out.print("Enter an integer: ");
if (in.hasNextInt()) {
    int num = in.nextInt();
    System.out.println("You entered: " + num);
} else {
    System.out.println("Invalid input! Not an integer.");
}

In production software, system input streams should be managed with care. Invoking in.close() releases underlying operating system resources. However, when using System.in, closing the scanner also closes the standard input stream for the entire virtual machine session, so close it only upon final termination.

Always verify whether user input consists of single words or complete sentences before deciding between next() and nextLine(). Remember to handle character input with next().charAt(0), and ensure numeric input types match the declared variable storage capacity.