A switch statement without a default case may allow execution to 'fall through' silently, if no cases are matched.

In a switch statement that is based on a variable of a non-enumerated type, include a default case to prevent execution from falling through silently when no cases are matched. If the default case is intended to be unreachable code, it is advisable that it throws a RuntimeException to alert the user of an internal error.

In the following example, the switch statement outputs the menu choice that the user has made. However, if the user does not choose 1, 2, or 3, execution falls through silently.

In the following modified example, the switch statement includes a default case, to allow for the user making an invalid menu choice.

Java SE Documentation: 7.8 switch Statements.