Java™ How To Program (Early Objects), Tenth Edition

Throwing Exceptions When Preconditions or Postconditions Are Not Met

When their preconditions or postconditions are not met, methods typically throw exceptions. As an example, examine String method charAt, which has one int parameter—an index in the String. For a precondition, method charAt assumes that index is greater than or equal to zero and less than the length of the String. If the precondition is met, the postcondition states that the method will return the character at the position in the String specified by the parameter index. Otherwise, the method throws an IndexOutOfBoundsException. We trust that method charAt satisfies its postcondition, provided that we meet the precondition. We need not be concerned with the details of how the ...