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Exceptions

Handle errors with try, catch and finally, and throw your own exceptions.

An exception is an object that signals something went wrong while the program was running, such as dividing by zero or reading past the end of an array. If nothing catches it, the program stops and prints a stack trace. A try / catch block lets you handle it instead.

Java
Open in playground
public class Main {
    public static void main(String[] args) {
        try {
            int[] data = {1, 2, 3};
            System.out.println(data[5]);
            System.out.println("not reached");
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("Caught: " + e.getMessage());
        }
        System.out.println("Program continues");
    }
}
Output
Caught: Index 5 out of bounds for length 3
Program continues

Several catch blocks and finally

StartRun the try blockException thrown?YesNoRun the matching catchRun finallyRun finallyContinue
How try, catch and finally run

finally always runs, whether an exception happened or not.

Java
Open in playground
public class Main {
    static void parseAndDivide(String text) {
        try {
            int n = Integer.parseInt(text);
            System.out.println("100 / " + n + " = " + (100 / n));
        } catch (NumberFormatException e) {
            System.out.println("'" + text + "' is not a number");
        } catch (ArithmeticException e) {
            System.out.println("Cannot divide by zero");
        } finally {
            System.out.println("  done with " + text);
        }
    }

    public static void main(String[] args) {
        parseAndDivide("4");
        parseAndDivide("zero");
        parseAndDivide("0");
    }
}
Output
100 / 4 = 25
  done with 4
'zero' is not a number
  done with zero
Cannot divide by zero
  done with 0

Throwing exceptions

Use throw when a method receives a value it cannot accept.

Java
Open in playground
public class Main {
    static void setAge(int age) {
        if (age < 0) {
            throw new IllegalArgumentException("age must not be negative: " + age);
        }
        System.out.println("Age set to " + age);
    }

    public static void main(String[] args) {
        setAge(21);
        try {
            setAge(-3);
        } catch (IllegalArgumentException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}
Output
Age set to 21
Error: age must not be negative: -3

Checked and unchecked exceptions

ThrowableErrorserious JVM problems, do not catchStackOverflowErrorOutOfMemoryErrorExceptioncheckedIOExceptionRuntimeExceptionuncheckedArithmeticExceptionNullPointerExceptionIllegalArgumentException
The exception class hierarchy

Java has two kinds of exceptions:

  • Unchecked exceptions (subclasses of RuntimeException), such as NullPointerException and ArithmeticException, usually mean a bug. You may catch them, but you do not have to.
  • Checked exceptions, such as IOException, describe problems outside your control, like a missing file. The compiler forces you to either catch them or declare them with throws.
Java
Open in playground
public class Main {
    static double balance = 500;

    static void withdraw(double amount) throws InsufficientFundsException {
        if (amount > balance) {
            throw new InsufficientFundsException("Need " + (amount - balance) + " more");
        }
        balance -= amount;
    }

    public static void main(String[] args) {
        try {
            withdraw(200);
            withdraw(450);
        } catch (InsufficientFundsException e) {
            System.out.println("Failed: " + e.getMessage());
        }
        System.out.println("Balance: " + balance);
    }
}

class InsufficientFundsException extends Exception {
    InsufficientFundsException(String message) {
        super(message);
    }
}
Output
Failed: Need 150.0 more
Balance: 300.0