question: public TimeCounter start ( ) { switch ( state ) { case UNSTARTED : startTimeMillis = System . currentTimeMillis ( ) ; break ; case RUNNING : throw new IllegalStateException ( "Already started" ) ; case STOPPED : startTimeMillis += System . currentTimeMillis ( ) - stopTimeMillis ; break ; default : throw new IllegalStateException ( "Unrecognized state " + state ) ; } state = State . RUNNING ; return this ; }
target:  Starts or resumes the time count.