Watch out for Side-effects

If the first method returns false, the second method is not evaluated at all, saving time. The result of evaluating the boolean expression is false whenever methodThatWorksInstantly() is false. Only when it is true is the time consuming method executed.

Danger: This trick works correctly only if the skipped method does nothing permanent. In other words, short-circuit evaluation is safe when the skipped method does nothing but compute true or false. Short-circuit evaluation is not safe when the skipped does more than that.

For example:

boolean methodThatTakesHoursToRun()
{
  // make a permanent change to the state of
  // an object that the rest of the program uses.

  // now return a true or false
}

When a method makes permanent changes to data, it must be called regardless of the true/false value of other parts of the Boolean expression. Otherwise, a computation that might be important to the program might be skipped. When a method makes a permanent change to data the method is said to have a side effect. When methods have side effects, you must be careful when using a short-circuit operator.