Exceptions

Catch problems with try/catch/finally, and throw your own exceptions for bad input.

🎯 What you will learn

Catch exceptions with try/catch
Use finally for clean-up
Throw exceptions for invalid data

👋 Let’s get started!

Users type the wrong thing, files go missing, networks drop 🧯. Exceptions let Java programs handle problems gracefully instead of crashing.

🪜 Step by step

1. 🧯 try / catch

Put risky code in try; handle problems in catch.

try {
    int n = Integer.parseInt("abc");
} catch (NumberFormatException e) {
    System.out.println("That’s not a number!");
}

2. 🧹 finally

finally always runs — perfect for clean-up.

try {
    System.out.println(10 / 0);
} catch (ArithmeticException e) {
    System.out.println("Can’t divide by zero");
} finally {
    System.out.println("Done checking");
}

3. 🚨 throw

Throw an exception when data is invalid.

void setAge(int age) {
    if (age < 0) {
        throw new IllegalArgumentException("Age can’t be negative");
    }
    this.age = age;
}

🧪 Try it!

Read the code, guess what it prints, then press 👀 Show output.

Safe input parser

String[] inputs = { "42", "hello", "7" };
int total = 0;
for (String s : inputs) {
    try {
        total += Integer.parseInt(s);
    } catch (NumberFormatException e) {
        System.out.println("Skipping \"" + s + "\"");
    }
}
System.out.println("Total: " + total);

Catch division by zero

try {
    int r = 8 / 0;
} catch (ArithmeticException e) {
    System.out.println("Oops!");
}

📖 Key words

Tap a card to flip it over!

Exception: An error event Java can handle 🧯
try/catch: Runs risky code and handles errors 🥅
finally: Always runs 🧹
throw: Raises an exception on purpose 🚨

🏆 Challenge

Predict the output.

Challenge

try {
    String s = null;
    System.out.println(s.length());
} catch (NullPointerException e) {
    System.out.println("Nothing there!");
} finally {
    System.out.println("Bye");
}

Knowledge Check Quiz

When does finally run?
Which exception does Integer.parseInt("hi") cause?
Why throw exceptions?