Conditions

Learn how to make decisions in your code using conditional statements.

What are Conditions?

Conditions allow your program to make decisions and execute different code based on whether something is true or false. They're the foundation of program logic.

Think of it like a flowchart: "If it's raining, take an umbrella. Otherwise, wear sunglasses."

The `if` Statement

The most basic conditional statement:

if (condition) {
    // Code runs only if condition is true
}

Example

int age = 18;

if (age >= 18) {
    System.out.println("You can vote");
}

The `if-else` Statement

Provide an alternative when the condition is false:

if (condition) {
    // Runs if condition is true
} else {
    // Runs if condition is false
}

Example

int temperature = 15;

if (temperature > 20) {
    System.out.println("It's warm outside");
} else {
    System.out.println("It's cold outside");
}

The `if-else if-else` Statement

Check multiple conditions:

if (condition1) {
    // Runs if condition1 is true
} else if (condition2) {
    // Runs if condition1 is false and condition2 is true
} else if (condition3) {
    // Runs if condition1 and condition2 are false and condition3 is true
} else {
    // Runs if all conditions are false
}

Example

int score = 75;

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` Statements

You can put if statements inside other if statements:

int age = 25;
boolean hasLicence = true;

if (age >= 18) {
    if (hasLicence) {
        System.out.println("You can drive");
    } else {
        System.out.println("You need a licence");
    }
} else {
    System.out.println("You're too young to drive");
}

tip

Try to limit nesting to 2-3 levels. Too much nesting makes code hard to read!

The Ternary Operator

A shorthand for simple if-else statements:

variable = (condition) ? valueIfTrue : valueIfFalse;

Example

int age = 20;
String status = (age >= 18) ? "Adult" : "Minor";
System.out.println(status);  // "Adult"

// Equivalent to:
String status;
if (age >= 18) {
    status = "Adult";
} else {
    status = "Minor";
}

More Examples

int a = 5;
int b = 10;
int max = (a > b) ? a : b;  // max = 10

String message = (score >= 50) ? "Pass" : "Fail";

int absoluteValue = (number >= 0) ? number : -number;

The `switch` Statement

Used when you have multiple specific values to check:

switch (variable) {
    case value1:
        // Code for value1
        break;
    case value2:
        // Code for value2
        break;
    default:
        // Code if no cases match
}

Example

int day = 3;
String dayName;

switch (day) {
    case 1:
        dayName = "Monday";
        break;
    case 2:
        dayName = "Tuesday";
        break;
    case 3:
        dayName = "Wednesday";
        break;
    case 4:
        dayName = "Thursday";
        break;
    case 5:
        dayName = "Friday";
        break;
    case 6:
        dayName = "Saturday";
        break;
    case 7:
        dayName = "Sunday";
        break;
    default:
        dayName = "Invalid day";
}

System.out.println(dayName);  // "Wednesday"

Multiple Cases

You can group cases together:

int month = 12;

switch (month) {
    case 12:
    case 1:
    case 2:
        System.out.println("Winter");
        break;
    case 3:
    case 4:
    case 5:
        System.out.println("Spring");
        break;
    case 6:
    case 7:
    case 8:
        System.out.println("Summer");
        break;
    case 9:
    case 10:
    case 11:
        System.out.println("Autumn");
        break;
    default:
        System.out.println("Invalid month");
}

warning

Don't forget the break statement! Without it, execution "falls through" to the next case.

Comparing Strings

Use .equals() to compare strings, not ==:

String name = "Alice";

// Wrong
if (name == "Alice") {  // Don't use == for strings!
    System.out.println("Hello Alice");
}

// Correct
if (name.equals("Alice")) {
    System.out.println("Hello Alice");
}

// Case-insensitive comparison
if (name.equalsIgnoreCase("alice")) {
    System.out.println("Hello Alice");
}

Complex Conditions

Combine multiple conditions using logical operators:

int age = 25;
double salary = 30000;
boolean hasExperience = true;

// AND - all must be true
if (age >= 18 && salary > 25000 && hasExperience) {
    System.out.println("Eligible for loan");
}

// OR - at least one must be true
if (age < 18 || age > 65) {
    System.out.println("Special ticket discount");
}

// Combining AND and OR with parentheses
if ((age >= 18 && age <= 65) && (hasExperience || salary > 40000)) {
    System.out.println("Eligible for programme");
}

When to Use Each

Use `if-else` when:

Checking ranges or complex conditions
Conditions involve comparisons (>, <, >=, <=)

if (temperature > 30) {
    System.out.println("Hot");
} else if (temperature > 20) {
    System.out.println("Warm");
} else {
    System.out.println("Cold");
}

Use `switch` when:

Checking a single variable against specific values
You have many exact matches to check

switch (userChoice) {
    case 1:
        startGame();
        break;
    case 2:
        viewScores();
        break;
    case 3:
        exitGame();
        break;
}

Use ternary operator when:

Simple condition with short expressions
Assigning one of two values

String message = (isLoggedIn) ? "Welcome back" : "Please log in";

Common Mistakes

1. Using `=` Instead of `==`

// Wrong
if (age = 18) {  // This assigns 18 to age!
    System.out.println("Adult");
}

// Correct
if (age == 18) {
    System.out.println("Adult");
}

2. Comparing Strings with `==`

// Wrong
if (name == "Alice") {  // Compares references, not values
    System.out.println("Hello");
}

// Correct
if (name.equals("Alice")) {
    System.out.println("Hello");
}

3. Forgetting `break` in Switch

// Wrong - will print multiple cases
switch (day) {
    case 1:
        System.out.println("Monday");
        // Missing break!
    case 2:
        System.out.println("Tuesday");
        break;
}

// Correct
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
}

4. Unreachable Code

// Wrong - second condition never checked
if (score >= 60) {
    System.out.println("Pass");
} else if (score >= 80) {  // Never reached!
    System.out.println("Distinction");
}

// Correct - order matters!
if (score >= 80) {
    System.out.println("Distinction");
} else if (score >= 60) {
    System.out.println("Pass");
}

Practice Exercise

Create a program that determines ticket prices based on age:

public class TicketPrice {
    public static void main(String[] args) {
        int age = 25;
        double price;

        // Children (0-12): £5
        // Teenagers (13-17): £8
        // Adults (18-64): £12
        // Seniors (65+): £7

        // Write your if-else statements here

        System.out.println("Ticket price: £" + price);
    }
}

Challenge Problems

Grade Calculator: Write a program that takes a score (0-100) and prints the letter grade (A, B, C, D, F)
Leap Year Checker: Determine if a year is a leap year:
- Divisible by 4 → leap year
- BUT if divisible by 100 → not a leap year
- UNLESS also divisible by 400 → leap year
Number Classifier: Check if a number is:
- Positive, negative, or zero
- Even or odd
- Greater than, less than, or equal to 100
Menu System: Create a simple menu with switch statement:
```
1. Start Game
2. Load Game
3. Options
4. Exit
```

Key Takeaways

Use if-else for making decisions based on conditions
Use else if to check multiple conditions
Use switch for checking a variable against specific values
Use the ternary operator for simple conditions
Always use .equals() to compare strings
Order matters in if-else if chains - put most specific conditions first
Don't forget break statements in switch

Next Steps

Now that you can make decisions in your code, let's learn about loops - how to repeat actions multiple times.

What are Conditions?​

The if Statement​

Example​

The if-else Statement​

Example​

The if-else if-else Statement​

Example​

Nested if Statements​

The Ternary Operator​

Example​

More Examples​

The switch Statement​

Example​

Multiple Cases​

Comparing Strings​

Complex Conditions​

When to Use Each​

Use if-else when:​

Use switch when:​

Use ternary operator when:​

Common Mistakes​

1. Using = Instead of ==​

2. Comparing Strings with ==​

3. Forgetting break in Switch​

4. Unreachable Code​

Practice Exercise​

Challenge Problems​

Key Takeaways​

Next Steps​

What are Conditions?

The `if` Statement

Example

The `if-else` Statement

Example

The `if-else if-else` Statement

Example

Nested `if` Statements

The Ternary Operator

Example

More Examples

The `switch` Statement

Example

Multiple Cases

Comparing Strings

Complex Conditions

When to Use Each

Use `if-else` when:

Use `switch` when:

Use ternary operator when:

Common Mistakes

1. Using `=` Instead of `==`

2. Comparing Strings with `==`

3. Forgetting `break` in Switch

4. Unreachable Code

Practice Exercise

Challenge Problems

Key Takeaways

Next Steps