java实现两个list的left join操作

在日常开发中，会遇到将两个不同对象类型的list按照某个属性进行整合
例如 :
List<obj1> obj1List ，List<obj2> obj2List,需要按照obj1.id=obj2.id进行整合
select * from obj1List left join obj2List on obj1.id=obj2.id

• 第一步：将obj1List转换成map

  将list转成id为key，obj1为value的map对象

    public static <K, V> Map<K, V> listToMap(List<V> list, String keyMethodName, Class<V> c) {
        Map<K, V> map = new HashMap<K, V>();
        if (list != null) {
            try {
                Method methodGetKey = c.getMethod(keyMethodName);
                for (int i = 0; i < list.size(); i++) {
                    V value = list.get(i);
                    @SuppressWarnings("unchecked")
                    K key = (K) methodGetKey.invoke(list.get(i));
                    map.put(key, value);
                }
            } catch (Exception e) {
                throw new IllegalArgumentException("field can't match the key!");
            }
        }
  
        return map;
    }
  

• 第二步：根据key值将obj2List进行copy

  for (Obj2 obj: obj2List) {
            try {
                System.out.println(obj);
                BeanUtils.copyProperties(mapFromList1.get(obj.getId()), obj);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
  

整个过程时间复杂度为O(n) ☺