Java 26-Day Course - Day 4: Conditional Statements

Day 4: Conditional Statements

Conditional statements are essential syntax for controlling program flow. They allow different code to be executed depending on specific conditions. Java provides two types of conditional statements: if-else and switch.

if-else Statement

The most basic conditional branch. The block executes when the condition evaluates to true.

public class IfElseExample {
    public static void main(String[] args) {
        int score = 85;

        // Simple if
        if (score >= 60) {
            System.out.println("You passed!");
        }

        // if-else
        if (score >= 90) {
            System.out.println("Grade A");
        } else {
            System.out.println("Not grade A");
        }

        // if-else if-else (multiple branches)
        if (score >= 90) {
            System.out.println("Grade: A");
        } else if (score >= 80) {
            System.out.println("Grade: B");
        } else if (score >= 70) {
            System.out.println("Grade: C");
        } else if (score >= 60) {
            System.out.println("Grade: D");
        } else {
            System.out.println("Grade: F");
        }

        // Nested if (prefer early return when possible)
        boolean isMember = true;
        int purchaseAmount = 50000;

        if (isMember) {
            if (purchaseAmount >= 30000) {
                System.out.println("Member discount + free shipping");
            } else {
                System.out.println("Member discount only");
            }
        }
    }
}

Combining Conditions with Logical Operators

Complex conditions can be combined using &&, ||, and ! operators.

public class CombinedCondition {
    public static void main(String[] args) {
        int age = 25;
        boolean hasLicense = true;
        boolean isDrunk = false;

        // AND condition: all conditions must be met
        if (age >= 18 && hasLicense && !isDrunk) {
            System.out.println("You can drive.");
        }

        // OR condition: at least one must be met
        String day = "Saturday";
        if (day.equals("Saturday") || day.equals("Sunday")) {
            System.out.println("It's the weekend! Take a rest.");
        }

        // Range check
        int temperature = 24;
        if (temperature >= 20 && temperature <= 28) {
            System.out.println("The temperature is comfortable.");
        }

        // Concise with the ternary operator
        String result = (age >= 18) ? "adult" : "minor";
        System.out.println(result);
    }
}

switch Statement

When branching based on a single variable’s value, switch is cleaner than if-else if.

public class SwitchExample {
    public static void main(String[] args) {
        // Traditional switch
        int month = 4;
        String monthName;

        switch (month) {
            case 1:
                monthName = "January";
                break;
            case 2:
                monthName = "February";
                break;
            case 3:
                monthName = "March";
                break;
            case 4:
                monthName = "April";
                break;
            default:
                monthName = "Other";
                break;
        }
        System.out.println("Current month: " + monthName);

        // Grouping multiple values in a single case
        switch (month) {
            case 3: case 4: case 5:
                System.out.println("It's spring");
                break;
            case 6: case 7: case 8:
                System.out.println("It's summer");
                break;
            case 9: case 10: case 11:
                System.out.println("It's autumn");
                break;
            case 12: case 1: case 2:
                System.out.println("It's winter");
                break;
        }
    }
}

Enhanced switch Expressions (Java 14+)

The arrow syntax introduced in Java 14 allows concise code without break.

public class EnhancedSwitch {
    public static void main(String[] args) {
        String grade = "B";

        // Arrow syntax switch expression
        String comment = switch (grade) {
            case "A" -> "Excellent!";
            case "B" -> "Well done!";
            case "C" -> "Average.";
            case "D" -> "Try harder.";
            case "F" -> "You need to retake the course.";
            default -> "Invalid grade.";
        };
        System.out.println(comment);

        // Multiple value matching
        int day = 3;
        String type = switch (day) {
            case 1, 2, 3, 4, 5 -> "weekday";
            case 6, 7 -> "weekend";
            default -> "invalid day";
        };
        System.out.println("Today is a " + type + ".");

        // Using yield for complex logic
        int score = 85;
        String result = switch (score / 10) {
            case 10, 9 -> "A";
            case 8 -> "B";
            case 7 -> "C";
            default -> {
                if (score >= 60) {
                    yield "D";
                } else {
                    yield "F";
                }
            }
        };
        System.out.println("Grade: " + result);
    }
}

Today’s Exercises

1. BMI Calculator: Store height (cm) and weight (kg) in variables, calculate the BMI, and classify it as “underweight” if below 18.5, “normal” if 18.5-24.9, “overweight” if 25-29.9, and “obese” if 30 or above. (BMI = weight / (height_m * height_m))

2. Day of Week Converter: Convert an integer (1-7) to a day name using a switch expression, and also print whether it is a weekday or weekend.

3. Amusement Park Pricing: Calculate the admission fee based on age and discount eligibility (boolean). Age 3 and under: free, age 4-12: 15,000 KRW, age 13-64: 30,000 KRW, age 65 and over: free. Apply a 50% discount for eligible guests.