继承

function shipDensity(map){
monitorParent.call(this, map);

var that = this;
this._indexHistoryChart;

this.createStationMarker = function(){}

}

function monitorParent(map){
this._map = map;
this._markers = [];
this.isAlarmByStationID = function(alarmId){
return this._alarm&&!isEmpty(this._alarm)&&!isEmpty(this._alarm[alarmId]);
}
}