Throw

You can create an Exception, or any other Throwable, like you create any object: using the new operator with the Exception constructor.

Exception ex = new Exception("Something went wrong");

• Use both the exception class and its message to provide information about the condition you are responding to.

IllegalArgumentException iae = new IllegalArgumentException("Amount cannot be less than zero.");

The throw keyword takes a Throwable object as its operand, stopping execution immediately and propagating the Throwable to the call stack.

public void withdraw(double amount) {
  if (amount < 0.0) {
    throw new IllegalArgumentException("Amount cannot be less than zero.");
  }
  balance -= amount;
}

Throwing Checked Exceptions

When you throw a checked exception, your method must declare it.

public void withdrawException(double amount) throws Exception {
  if (amount < 0.0) {
    throw new Exception("Amount cannot be less than zero.");
  }
  balance -= amount;
}

• Callers of this method must either handle it with try/catch, or declare they also throw it.

Practice Exercise

A method can declare it throws a runtime exception:

private static void mayCrash2() throws RuntimeException { /*...*/ }

This does not make this a checked exception, or require callers to catch or declare it. While legal, methods don't normally declare RuntimeException or its descendants.

A method can declare multiple exceptions, as a comma-separated list after throws.

private void printWithdrawalReceipt(Account acct, double amount)
  throws IOException, PrintException {
}

* A caller must either catch or declare each declared, checked exception.

try {
  printWithdrawalReceipt(acct, amount);
}
catch (IOException e) {
  e.printStackTrace();
}
catch (PrintException e) {
  e.printStackTrace();
}

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