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

Java Programming Lesson-8

📘 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

✅ 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.

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.