Loop through players except certain players.

I am making a plugin a 1v1 plugin and when the 1v1 starts, I want all players that are not in the 1v1, to not be able to see the players in the 1v1, and the players in the 1v1, can only see each others.
Here is my code

Code:java

for(Player online : Bukkit.getServer().getOnlinePlayers()) {
                    online.hidePlayer(plz);
                    online.hidePlayer(p);
                    if(!(battle.containsKey(online))) {
                        plz.hidePlayer(online);
                        p.hidePlayer(online);
                    }
                    if(!(battle.containsValue(online))) {
                        plz.hidePlayer(online);
                        p.hidePlayer(online);
                    }
                }

What can I do?

 

You can vanish everyone and then unvanish that 1 player?

 

DERP. Thank you!