@EventHandler
public void onPlayerInteract(PlayerInteractEvent e) {
Block block = e.getBlockClicked();
Player player = e.getPlayer();
if(block.getType() == Material.WALL_SIGN){
BlockState state = block.getState();
if(state instanceof Sign){
Sign sign = (Sign)state;
String ln = sign.getLine(0);
if (ln.equalsIgnoreCase("[firstline]") {
player.sendMessage("This is first line!");
return;
}
}
}
}