Introduction

Welcome to your Decision Control Structure Lab Activity! The best way to learn programming is by writing and running actual code. In this lab, you'll code 10 different programs covering all types of decision statements.

By the end of this activity, you'll be comfortable with if, if-else, nested if, and else-if ladder — the building blocks of almost every program!

Objective

To understand and implement decision control statements (if, if-else, nested if, else-if ladder) using Java in BlueJ IDE.

Requirements

Learning Outcomes

10 Lab Tasks (Complete All Programs)

Task 1: Check Positive or Negative Number

import java.util.Scanner;

public class CheckNumber {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = sc.nextInt();

        if (num > 0) {
            System.out.println("Positive");
        } else if (num < 0) {
            System.out.println("Negative");
        } else {
            System.out.println("Zero");
        }
    }
}

Task 2: Even or Odd Number

import java.util.Scanner;

public class EvenOdd {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = sc.nextInt();

        if (num % 2 == 0) {
            System.out.println("Even");
        } else {
            System.out.println("Odd");
        }
    }
}

Task 3: Find Largest of Two Numbers

import java.util.Scanner;

public class LargestOfTwo {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter first number: ");
        int a = sc.nextInt();
        System.out.print("Enter second number: ");
        int b = sc.nextInt();

        if (a > b) {
            System.out.println(a + " is larger");
        } else if (b > a) {
            System.out.println(b + " is larger");
        } else {
            System.out.println("Both are equal");
        }
    }
}

Task 4: Find Largest of Three Numbers

import java.util.Scanner;

public class LargestOfThree {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter three numbers: ");
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();

        if (a >= b && a >= c) {
            System.out.println(a + " is largest");
        } else if (b >= c) {
            System.out.println(b + " is largest");
        } else {
            System.out.println(c + " is largest");
        }
    }
}

Task 5: Voting Eligibility Check

import java.util.Scanner;

public class VotingEligibility {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter your age: ");
        int age = sc.nextInt();

        if (age >= 18) {
            System.out.println("Can Vote");
        } else {
            System.out.println("Cannot Vote");
        }
    }
}

Task 6: Grade Calculator

import java.util.Scanner;

public class GradeCalculator {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter marks: ");
        int marks = sc.nextInt();

        if (marks >= 90) {
            System.out.println("Grade: A");
        } else if (marks >= 75) {
            System.out.println("Grade: B");
        } else if (marks >= 60) {
            System.out.println("Grade: C");
        } else if (marks >= 40) {
            System.out.println("Grade: D");
        } else {
            System.out.println("Grade: F");
        }
    }
}

Task 7: Leap Year Checker

import java.util.Scanner;

public class LeapYear {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter year: ");
        int year = sc.nextInt();

        if (year % 4 == 0) {
            System.out.println("Leap Year");
        } else {
            System.out.println("Not Leap Year");
        }
    }
}

Task 8: Positive/Negative and Even/Odd

import java.util.Scanner;

public class CombinedCheck {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = sc.nextInt();

        if (num > 0) {
            if (num % 2 == 0) {
                System.out.println("Positive and Even");
            } else {
                System.out.println("Positive and Odd");
            }
        } else if (num < 0) {
            if (num % 2 == 0) {
                System.out.println("Negative and Even");
            } else {
                System.out.println("Negative and Odd");
            }
        } else {
            System.out.println("Zero");
        }
    }
}

Task 9: Simple Calculator

import java.util.Scanner;

public class Calculator {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter first number: ");
        double a = sc.nextDouble();
        System.out.print("Enter operator (+, -, *, /): ");
        char op = sc.next().charAt(0);
        System.out.print("Enter second number: ");
        double b = sc.nextDouble();
        double result = 0;

        if (op == '+') {
            result = a + b;
        } else if (op == '-') {
            result = a - b;
        } else if (op == '*') {
            result = a * b;
        } else if (op == '/') {
            if (b != 0) {
                result = a / b;
            } else {
                System.out.println("Cannot divide by zero!");
                return;
            }
        } else {
            System.out.println("Invalid operator!");
            return;
        }
        System.out.println("Result: " + result);
    }
}

Task 10: Traffic Light Signal

import java.util.Scanner;

public class TrafficLight {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter light color (RED/YELLOW/GREEN): ");
        String color = sc.next().toUpperCase();

        if (color.equals("RED")) {
            System.out.println("Stop");
        } else if (color.equals("YELLOW")) {
            System.out.println("Ready");
        } else if (color.equals("GREEN")) {
            System.out.println("Go");
        } else {
            System.out.println("Invalid color!");
        }
    }
}

Submission Format

For each task, include:

Evaluation Criteria (Total: 100 Marks)

Tips for Success

Common Errors to Avoid

How to Run Your Programs in BlueJ

Suggested Timeline

Did You Know?

Bonus Challenge Tasks (Optional)

Frequently Asked Questions

Q1. What if my program doesn't compile?

Read the error message carefully. Common issues: missing semicolon, missing braces, misspelled keywords. Fix and try again!

Q2. Should I use Scanner or command-line arguments?

For beginners, use Scanner. It's easier to test interactively. Command-line arguments are for advanced programs.

Q3. What if I get wrong output?

Check your logic. Print variable values to see what's happening. Use test cases to verify.

Q4. Can I skip a task?

You need to complete all 10 tasks for full marks. Ask for help if you're stuck.

Q5. Should I add comments?

Yes! Comments help you and others understand the code. They also earn marks.

Q6. What's the best way to test?

Test with normal cases, edge cases (0, negative), maximum values, and invalid inputs.

Q7. Can I work with a friend?

You can discuss ideas, but write your own code. Copying is not learning!

Q8. What if I don't have BlueJ at home?

BlueJ is free! Download from bluej.org. It works on Windows, Mac, and Linux.