一些實用的Java程式碼

往Map<Key,Collection>這種結構資料中插入值

    public static <K, V, C> void setMapOfCollectionValue(Map<K,C> map,K key, V value, Class collectonClass){
        if(map.containsKey(key)){
            ((Collection)map.get(key)).add(value);
        }else{
            try {
                Collection collection = (Collection)collectonClass.newInstance();
                collection.add(value);
                map.put(key, (C)collection);
            } catch (InstantiationException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        }
    }
 

物件克隆

1）在類中implements Cloneable實現clone方法 

2）先序列化物件再反序列化物件，implements Serializable, org.springframework.util.SerializationUtils，序列化工具kryo等

3）新建一個物件，然後將屬性拷貝到新物件上

org.springframework.cglib.beans.BeanCopier，org.springframework.beans.BeanUtils，org.apache.commons.beanutils.PropertyUtils

Bean複製的幾種框架效能比較

import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.BeanUtils;

@Slf4j
public class BeanUtil {
    public static <T> T clone(T source){
        T target = null;
        try {
            target = (T) source.getClass().newInstance();
        } catch (InstantiationException|IllegalAccessException e) {
            log.error("object clone error",e);
        }
        BeanUtils.copyProperties(source, target);
        return target;
    }
}
 

mongo查詢排序

mongo查詢出來的列表順序並不是按照in裡面的順序返回的

public static <T,R> List<T> sortEntityByIds(List<T> entities, List<R> ids, Function<T,R> idFunc){
        if(CollectionUtils.isEmpty(entities)){
            return entities;
        }
        Map<R,T> entitiesMap = new HashMap<>();
        for (T entity : entities) {
            entitiesMap.put(idFunc.apply(entity), entity);
        }
        List sortEntities = new ArrayList<>();
        for (R id : ids) {
            T entity = entitiesMap.get(id);
            if(null!=entity){
                sortEntities.add(entity);
            }
        }
        return sortEntities;
    }

List<User> users = userRepository.findByIds(userIds);

List<User> sortedUsers = sortEnityByIds(users, userIds, User::getId);