Java Programming Lesson 8 Part - 2 | Java Methods + Exception Handling + Advanced Input Parsing

Java Programming Lesson 8

✅ We will treat Lesson-8 as a 3-part strong foundation for OOP:

✅ Part 1 — Methods (Functions) – Very important
✅ Part 2 — Exception Handling (try–catch)
✅ Part 3 — User Input Advanced (Parsing)

This makes you ready for OOP (Classes & Objects) next 🚀

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📘 LESSON-8: Java Methods + Exception Handling + Advanced Input Parsing

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✅ PART-1: Methods (Functions) — Core of OOP

✅ Method Syntax

returnType methodName(parameters) {
    // statements
    return value;  // optional
}

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✅ Types of Methods in OOP

Type	Example	Notes
No return + No parameters	void show()	Simple action
Return + Parameters	int sum(int a, int b)	Most used
Method Overloading	Same name, different parameters	Very important
Static Methods	static	Call without object
Non-Static Methods	Needs object	OOP behavior

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✅ Good Example — Strong OOP Concept

public class Calculator {

    int add(int a, int b){
        return a + b;
    }

    int sub(int a, int b){
        return a - b;
    }

    public static void main(String[] args){
        Calculator c = new Calculator();
        System.out.println(c.add(5, 3));
        System.out.println(c.sub(10, 4));
    }
}

✔ Object created → Methods used
✔ This is how OOP works ✅

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✅ PART-2: Exception Handling — (try, catch, finally)

Exceptions = Errors at Runtime ⚠
We use try-catch to handle problems without crashing program ✅

✅ Syntax

try {
    // risky code
} catch(Exception e) {
    System.out.println("Error: " + e.getMessage());
} finally {
    System.out.println("Always runs!");
}

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✅ Example: Divide by Zero Error Handling

try {
    int a = 10/0;
} catch(ArithmeticException e) {
    System.out.println("Cannot divide by zero!");
}

✅ Safe program
✅ No crash

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✅ Multiple Exception Handling

try {
    int[] arr = new int[3];
    System.out.println(arr[5]);
} catch(ArrayIndexOutOfBoundsException e){
    System.out.println("Index Error!");
} catch(Exception e){
    System.out.println("Other Error!");
}

✔ Most specific exception first
✔ Exception at bottom

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✅ PART-3: Advanced Scanner Input Parsing

Parse = convert String → Number ✅
Useful in real apps 🔥

✅ Example: String → Integer

Scanner sc = new Scanner(System.in);

System.out.print("Enter a number: ");
String numStr = sc.nextLine();

try {
    int number = Integer.parseInt(numStr);
    System.out.println("You entered: " + number);
} catch(NumberFormatException e){
    System.out.println("Invalid Number!");
}

✔ If user enters text → program still safe ✅

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✅ Example: Convert String to Double

String price = "99.99";
double value = Double.parseDouble(price);
System.out.println(value);

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✅ Combined Example Program (Best Practice)

import java.util.*;

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

        try {
            System.out.print("Enter age: ");
            int age = Integer.parseInt(sc.nextLine());

            System.out.print("Enter score: ");
            double score = Double.parseDouble(sc.nextLine());

            System.out.println("Age: " + age + ", Score: " + score);
        }
        catch(Exception e) {
            System.out.println("Invalid Input!");
        }
        finally {
            sc.close();
            System.out.println("Scanner Closed.");
        }
    }
}

✔ Prevents crashes
✔ Proper parsing
✔ Resource closed ✅

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🧠 Lesson-8 Summary (Very Important)

Topic	You Learned ✅
Methods	Reusable code, static & non-static
Overloading	Same method name, different parameters
try-catch-finally	Safe programs
Parsing Input	String → Number conversions

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✅ Practice Tasks (Very Useful)

1️⃣ Create ATM Program

• deposit(), withdraw(), checkBalance() methods
• Try-catch for wrong inputs

2️⃣ Take user age as String
→ convert to integer → if invalid show error

3️⃣ Student Marks Calculator

• 3 subjects
• total marks + average
• try-catch for wrong input

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📌 You are now ready for OOP (Classes & Objects)

🔥 Next lesson will be 100% Practical OOP Implementation

✅ Let’s dive into more exercises from Lesson-8 (Methods, Exception Handling, Advanced Input Parsing) so you can master the concepts before moving to OOP.

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📘 Lesson-8: Practice Exercises (Advanced)

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Exercise 1 — Calculator with Exception Handling

Requirements:

• Take two numbers from user input (as String)
• Take operation choice: +, -, *, /
• Use try-catch to handle invalid numbers and division by zero
• Use methods for each operation

// Example Method Structure
static int add(int a, int b) { return a + b; }
static int sub(int a, int b) { return a - b; }
static int mul(int a, int b) { return a * b; }
static double div(int a, int b) { return a / (double)b; }

Goal:

• User inputs safely → operations performed → output printed
• Program never crashes even if input is wrong

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Exercise 2 — Age Validation

Requirements:

• Input age as String
• Convert to integer using Integer.parseInt()
• If invalid input → show "Invalid age"
• If valid → check if age ≥ 18 → "Adult" else "Minor"

Hint: Use try-catch

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Exercise 3 — Student Marks Calculator

Requirements:

• Input marks for 3 subjects (as String)
• Convert to integer using Integer.parseInt()
• Calculate total and average
• Print result

Use methods:

• int total(int a, int b, int c)
• double average(int total)
• Use try-catch to handle wrong input

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Exercise 4 — Password Validator

Requirements:

• Input password as String
  

Check:

• Minimum 6 characters
• Must contain a number
• Use methods for validation
• Handle exceptions if input is empty or null

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Exercise 5 — Advanced Menu Program

Requirements:

• Display Menu:

1. Greet User
2. Add Numbers
3. Exit

• Take choice input safely using Scanner + parseInt()
• Use methods for each choice
• Handle invalid input without crashing program

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🔹 Key Learning Outcomes from Exercises

1. Methods → organize your code, reusable
2. Exception handling → make programs safe
3. Parsing input → convert String → numeric safely
4. Combined skills → prepare for OOP concepts

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💡 Pro Tip:

Try modifying each program to use static and non-static methods, then create objects to call them. This is a bridge to OOP.

I can create a “Lesson-8 Exercise Solutions Guide” with all solutions ready in one file, so you can practice and check instantly.

Lesson8 Exercise Solutions

Lesson-8 Exercise Solutions (Methods, Exception Handling, Advanced Input Parsing)

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Exercise 1: Calculator with Exception Handling

import java.util.Scanner;

public class Calculator {

static int add(int a, int b){ return a + b; }

static int sub(int a, int b){ return a - b; }

static int mul(int a, int b){ return a * b; }

static double div(int a, int b){ return a / (double)b; }

public static void main(String[] args){

Scanner sc = new Scanner(System.in);

try{

System.out.print("Enter first number: ");

int num1 = Integer.parseInt(sc.nextLine());

System.out.print("Enter second number: ");

int num2 = Integer.parseInt(sc.nextLine());

System.out.print("Choose operation (+,-,*,/): ");

char op = sc.nextLine().charAt(0);

switch(op){

case '+': System.out.println("Result: " + add(num1,num2)); break;

case '-': System.out.println("Result: " + sub(num1,num2)); break;

case '*': System.out.println("Result: " + mul(num1,num2)); break;

case '/': System.out.println("Result: " + div(num1,num2)); break;

default: System.out.println("Invalid operation");

}

}catch(NumberFormatException e){

System.out.println("Invalid number input!");

}catch(ArithmeticException e){

System.out.println("Cannot divide by zero!");

}finally{

sc.close();

}

}

}

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Exercise 2: Age Validation

import java.util.Scanner;

public class AgeValidation {

public static void main(String[] args){

Scanner sc = new Scanner(System.in);

System.out.print("Enter your age: ");

try{

int age = Integer.parseInt(sc.nextLine());

if(age >= 18){

System.out.println("Adult");

} else {

System.out.println("Minor");

}

}catch(NumberFormatException e){

System.out.println("Invalid age input!");

}finally{

sc.close();

}

}

}

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Exercise 3: Student Marks Calculator

import java.util.Scanner;

public class StudentMarks {

static int total(int a,int b,int c){ return a+b+c; }

static double average(int total){ return total/3.0; }

public static void main(String[] args){

Scanner sc = new Scanner(System.in);

try{

System.out.print("Enter marks of subject 1: ");

int s1 = Integer.parseInt(sc.nextLine());

System.out.print("Enter marks of subject 2: ");

int s2 = Integer.parseInt(sc.nextLine());

System.out.print("Enter marks of subject 3: ");

int s3 = Integer.parseInt(sc.nextLine());

int t = total(s1,s2,s3);

double avg = average(t);

System.out.println("Total Marks: " + t);

System.out.println("Average Marks: " + avg);

}catch(NumberFormatException e){

System.out.println("Invalid input! Please enter numbers only.");

}finally{

sc.close();

}

}

}

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Exercise 4: Password Validator

import java.util.Scanner;

public class PasswordValidator {

static boolean isValid(String pwd){

if(pwd.length() < 6) return false;

for(char c: pwd.toCharArray()){

if(Character.isDigit(c)) return true;

}

return false;

}

public static void main(String[] args){

Scanner sc = new Scanner(System.in);

System.out.print("Enter password: ");

String pwd = sc.nextLine();

if(isValid(pwd)){

System.out.println("Password is valid.");

} else {

System.out.println("Password invalid! Must be 6+ chars and contain a number.");

}

sc.close();

}

}

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Exercise 5: Advanced Menu Program

import java.util.Scanner;

public class MenuProgram {

static void greet(){

System.out.println("Hello User!");

}

static void addNumbers(Scanner sc){

try{

System.out.print("Enter first number: ");

int a = Integer.parseInt(sc.nextLine());

System.out.print("Enter second number: ");

int b = Integer.parseInt(sc.nextLine());

System.out.println("Sum = " + (a+b));

}catch(NumberFormatException e){

System.out.println("Invalid input!");

}

}

public static void main(String[] args){

Scanner sc = new Scanner(System.in);

boolean running = true;

while(running){

System.out.println("1. Greet User");

System.out.println("2. Add Numbers");

System.out.println("3. Exit");

System.out.print("Enter choice: ");

try{

int choice = Integer.parseInt(sc.nextLine());

switch(choice){

case 1: greet(); break;

case 2: addNumbers(sc); break;

case 3: running = false; break;

default: System.out.println("Invalid choice!");

}

}catch(NumberFormatException e){

System.out.println("Invalid input! Enter a number.");

}

}

sc.close();

System.out.println("Program Terminated.");

}

}

✅ Done! I have created the Lesson-8 Exercise Solutions Guide with full working Java programs for:

1. Calculator with Exception Handling
2. Age Validation
3. Student Marks Calculator
4. Password Validator
5. Advanced Menu Program

All solutions include methods, exception handling, and input parsing — perfect for practicing before moving to OOP.

✅ NEXT PART - Lesson-9: Classes, Objects & Constructors

Frequently Asked Questions-(FAQ)

What are methods in Java?

Methods in Java are blocks of reusable code that perform a specific task. They help reduce repetition, improve readability, and make programs modular and easy to maintain.

Why are methods important in Java programming?

Methods allow developers to break complex programs into smaller, manageable pieces. They support code reusability, simplify debugging, and make object-oriented programming more efficient.

What is method overloading in Java?

Method overloading occurs when multiple methods share the same name but differ in parameters. It increases flexibility and allows functions to handle different types of inputs.

What is exception handling in Java?

Exception handling is a mechanism that manages runtime errors using try, catch, throw, throws, and finally blocks. It prevents program crashes and ensures smooth execution.

Why do we use try-catch blocks in Java?

A try-catch block is used to detect and handle exceptions. The try block contains risky code, while the catch block handles specific errors to avoid program failure.

What is the difference between checked and unchecked exceptions?

Checked exceptions must be handled during compilation (e.g., IOException), while unchecked exceptions occur at runtime and do not require explicit handling (e.g., ArithmeticException).

What is input parsing in Java?

Input parsing converts user input (String) to data types like int, double, or boolean using methods such as Integer.parseInt(), Double.parseDouble(), and Boolean.parseBoolean().

What exceptions can occur during input parsing in Java?

Input parsing ensures the data collected from users is correctly converted to the required variable types, making programs more reliable and preventing type errors.

Why is input parsing important for Java programs?

The most common exception during input parsing is NumberFormatException, which occurs when a non-numeric value is parsed into a number type.

How can we handle invalid user input in Java?

You can handle invalid input using try-catch blocks, loops to re-ask for input, and validation methods to ensure the data type is correct before processing.

What is the Scanner class used for?

The Scanner class is used to take input from the user, files, or other input streams. It can read different data types like int, double, and String.

How can we avoid Scanner input issues like skipping lines?

Use nextLine() after nextInt() or nextDouble() to clear the buffer. Scanner often leaves newline characters that cause input skipping.