List之根据某个字段在add的时候过滤掉重复的数据

下面是重写ArrayList，并保证ChooseCars里面alpha字段不重复的例子

public class DistinctList extends ArrayList<ChooseCars>{
 
    private static Map<String,Integer> distinct_map = new HashMap<String,Integer>();
    private Integer distinct_int_value = null;
 
    public void show(){
        for(String map_key:distinct_map.keySet()){
            System.out.println(map_key + "  " + distinct_map.get(map_key));
        }
    }
 
    public boolean add(ChooseCars distinct){
            if(distinct_map.containsKey(distinct.getAlpha())){
                Integer distinct_int_value = distinct_map.get(distinct.getAlpha());
                distinct_int_value = distinct_int_value+1;
                return true;
            }else{
                distinct_map.put(distinct.getAlpha(), 1);
                return super.add(distinct);
            }
    }
 
    public boolean normalAdd(ChooseCars distinct){
        return super.add(distinct);
    }
}