stuff

AcceptCommand.java

public class AcceptCommand implements CommandExecutor
{
    private ChallengeMain plugin;

    public AcceptCommand(ChallengeMain plugin)
    {
        this.plugin = plugin;
    }

    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args)
    {
        if ( !(sender instanceof Player ) ) return true;
        Player playerSender = (Player) sender;
        if (args.length != 1)
        {
            sender.sendMessage( "Usage: /accept <player>" );
            return true;
        }
        Player player = Bukkit.getPlayer( args[ 0 ] );
        if ( player.isOnline() )
        {
            if ( plugin.getChallenges().containsKey( playerSender.getName() ) )
            {
                Map<String, Long> innerMap = plugin.getChallenges().get( playerSender.getName() );
                if ( innerMap.containsKey( player.getName() ) )
                {
                    long time = innerMap.get( player.getName() ) +
                            plugin.getExpireTime()  - ( System.currentTimeMillis() / 1000L );
                    if ( time > 0 )
                    {
                        // teleport, etc
                    } else
                    {
                        innerMap.remove( player.getName() );
                    }
                }
            }
        } else
        {
            // handle offline
        }
        return true;
    }
}

ChallengeCommand.java

public class ChallengeCommand implements CommandExecutor
{
    private ChallengeMain plugin;

    public ChallengeCommand(ChallengeMain plugin)
    {
        this.plugin = plugin;
    }

    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args)
    {
        if ( !(sender instanceof Player) ) return true;
        Player playerSender = ( Player ) sender;
        if ( args.length != 1 )
        {
            sender.sendMessage( "Usage: /challenge <player>" );
            return true;
        }
        Player player = Bukkit.getPlayer( args[ 0 ] );
        if ( player.isOnline() )
        {
            long time = handleAdd( playerSender, player );
            if ( time > 0L )
            {
                playerSender.sendMessage( "you have to wait to " + time + " challege again!" );
            } else if ( time == -1 ) {
                playerSender.sendMessage( "you challenged " + player.getName() );
            }
        } else
        {
            // TODO: handle offline player
        }
        return true;
    }

    private long handleAdd(Player challenger, Player challenged)
    {
        Map<String, Long> innerMap;
        if ( plugin.getChallenges().containsKey( challenged.getName() ) )
        {
            innerMap = plugin.getChallenges().get( challenged.getName() );

            if ( innerMap.containsKey( challenger.getName() ) )
            {
                long time = (innerMap.get( challenger.getName() ) +
                        plugin.getExpireTime() ) - ( System.currentTimeMillis() / 1000L );
                if ( time < 0 )
                {
                    time = -1L;
                    innerMap.put( challenger.getName(), System.currentTimeMillis() / 1000L );
                }
                return time;
            }
        } else
        {
            innerMap = new HashMap<>();
            plugin.getChallenges().put( challenged.getName(), innerMap );
        }
        innerMap.put( challenger.getName(), System.currentTimeMillis() / 1000L );
        return -1L;
    }
}

ChallengeMain.java

public class ChallengeMain extends JavaPlugin
{
    private Map<String, Map<String, Long>> challenges = new HashMap<>();
    private long expireTime = 15; // seconds

    @Override
    public void onEnable()
    {
        getCommand( "challenge" ).setExecutor( new ChallengeCommand( this ) );
        getCommand( "accept" ).setExecutor( new AcceptCommand( this ) );
    }

    public Map<String, Map<String, Long>> getChallenges()
    {
        return challenges;
    }

    public long getExpireTime()
    {
        return expireTime;
    }
}