Only trigger proximity detector when object enters the range, not when he moves within range

Question

I'm making a text-based radar for Minecraft. If a player comes within 20 blocks of you, it will say in chat. As of now, it spams the chat. How can I get it to only write to chat about that player ONCE? Even if you don't play the game, it should be easy to understand.

if (Camb.radar)
{
  for (Entity e: (List < Entity > ) mc.theWorld.loadedEntityList)
  {
    if (e instanceof EntityPlayer)
    {
      EntityPlayer player = (EntityPlayer) e;
      if (player == mc.thePlayer || mc.thePlayer.getDistanceToEntity(e) > 20.0)
        continue;
      mc.thePlayer.addChatMessage("\2479[CAMB] \247e" + player.getEntityName() + " has entered your 20 block radius!"); //Write to chat, only want this line done once for every player
    }
  }
}

Answer 1

You'll need to keep track of when the player leaves the range and set a flag, so you'll know when they're transitioning from "out of range" to "in range". Might also want to add a timer so that you can only alert once every N seconds.

Answer 2

You could try creating a List or Array of nearby players, and add them to that list when they are within 20 blocks. When you find an entity within range, check to see if its in your list. If not, notify and add it to the list, if so, game on :)

For removing items from your list, check the entities in your list and compare them to the players position. If they are out of range, remove them. This may need to happen in a separate loop.

Answer 3

If you make a class PlayerData, it can contain a hashmap of playernames mapped to booleans. You give each player a PlayerData object and then when somebody enters the radius of that player, you toggle his/her boolean.

public class PlayerData {
    public Player thePlayer;
    public HashMap<String,boolean> inRadius = new HashMap<String,boolean>();

    public PlayerData(Player thePlayer) {
       this.thePlayer = thePlayer;
    }

    public void checkRadius(P Entity player) {
    /**function that checks radius and if a new player is there, notify thePlayer*/
      if(inRadius.get(player.getEntityName()) == true || thePlayer == player || thePlayer.getDistanceToEntity(player) > 20.0) return;
      else {
        thePlayer.addChatMessage("whatever you want to say");
        inRadius.put(player.getEntityName(), true);
      }
      for(Iterator<String> key=inRadius.keySet().Iterator();key.hasNext()) {
        String name = key.next();
        /**Check to see if that player is still within 20 meters. If not, set value to false*/
        /** also be careful to check if player is online*/
      }
    }

}