java - JavaFX - how to wait for selection? -

i'm programing tableview. , after user presses button, program should wait until has selected 2 columns , continue...

how can wait 2 selects?

pseudocode:

button(){   if(selected2column != true){      wait   } else{   stuff..   } } 

when 2 columns selected, should skip if

my problem wait. how can wait 2 selects?
, don't want use multithreads if there other possibility...