Preconditions

• Method can do an assertion check

  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;