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 🚀

---

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

---

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

✅ Method Syntax

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

---

✅ 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

---

✅ 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 ✅

---

✅ 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!");
}

---

✅ 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

---

✅ 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

---

✅ 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 ✅

---

✅ Example: Convert String to Double

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

---

✅ 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 ✅

---

🧠 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

---

✅ 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

---

📌 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.

---

📘 Lesson-8: Practice Exercises (Advanced)

---

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

---

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

---

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

---

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

---

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

---

🔹 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

---

💡 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)

---

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();

}

}

}

---

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();

}

}

}

---

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();

}

}

}

---

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();

}

}

---

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