Java Tutorial · Advanced Java · Lesson 29 of 37

Java Exceptions

try, catch, finally, try-with-resources and custom exceptions.

try / catch / finally

Example 1

public class Main {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3};
        try {
            System.out.println(numbers[5]);
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("Oops: " + e.getMessage());
        } finally {
            System.out.println("finally always runs");
        }
        try {
            Object s = "text";
            Integer n = (Integer) s;
        } catch (ClassCastException | NullPointerException e) {
            System.out.println("Caught " + e.getClass().getSimpleName());
        }
    }
}

Output

Oops: Index 5 out of bounds for length 3
finally always runs
Caught ClassCastException

Run this on your computer — see “Get Started” for setup.

Checked vs unchecked

Checked exceptions (like IOException) must be caught or declared with throws — the compiler enforces it.
Unchecked exceptions (RuntimeException and subclasses: NullPointerException, IllegalArgumentException…) usually indicate bugs and don't need declaring.

try-with-resources

Resources that implement AutoCloseable (files, database connections) are closed automatically — even when an exception is thrown:

Example 2

public class Main {
    public static void main(String[] args) {
        try (Resource db = new Resource("database"); Resource file = new Resource("file")) {
            System.out.println("using both");
            throw new IllegalStateException("something failed");
        } catch (IllegalStateException e) {
            System.out.println("handled: " + e.getMessage());
        }
    }
}

class Resource implements AutoCloseable {
    private final String name;

    Resource(String name) {
        this.name = name;
        System.out.println("open " + name);
    }

    @Override
    public void close() {
        System.out.println("close " + name);
    }
}

Output

open database
open file
using both
close file
close database
handled: something failed

Run this on your computer — see “Get Started” for setup.

Custom exceptions

Example 3

public class Main {
    public static void main(String[] args) {
        try {
            withdraw(50, 80);
        } catch (InsufficientFundsException e) {
            System.out.println(e.getMessage() + " (short by " + e.shortBy + ")");
        }
    }

    static double withdraw(double balance, double amount) throws InsufficientFundsException {
        if (amount > balance) throw new InsufficientFundsException(amount - balance);
        return balance - amount;
    }
}

class InsufficientFundsException extends Exception {
    final double shortBy;

    InsufficientFundsException(double shortBy) {
        super("Insufficient funds");
        this.shortBy = shortBy;
    }
}

Output

Insufficient funds (short by 30.0)

Run this on your computer — see “Get Started” for setup.

Test yourself

Which kind of exception must be caught or declared with throws?

Checked exceptions
Unchecked exceptions
Errors
RuntimeExceptions

Show answer

Answer: Checked exceptions