Preconditions

• Precondition: Requirement that the caller of a method must meet

• Publish preconditions so the caller won't call methods with bad parameters

/**
   Deposits money into this account.
   @param amount the amount of money to deposit  
   (Precondition: amount >= 0)
*/

• Typical use:

• To restrict the parameters of a method

• To require that a method is only called when the object is in an appropriate state

• If precondition is violated, method is not responsible for computing the correct result. It is free to do anything

  • Method may throw exception if precondition violated – more in Chapter 11

    if (amount < 0) throw new IllegalArgumentException();
    balance = balance + amount;

  • Method doesn't have to test for precondition. (Test may be costly)

    // if this makes the balance negative, it's the caller's fault
    balance = balance + amount;

  • Method can do an assertion check (click here for example)
    assert amount >= 0;
    balance = balance + amount;

  • To enable assertion checking:
    java -enableassertions MyProg

  • You can turn assertions off after you have tested your program, so that it runs at maximum speed

  • Many beginning programmers silently return to the caller  
    if (amount < 0)
       return; // Not recommended; hard to debug
    balance = balance + amount;