Exception caused by list All elements are null-javaTutorial-php.cn

Exception caused by list All elements are null

巴扎黑

Release： 2017-06-26 10:22:04

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ArrayList allows adding null values, which can easily cause java.lang.NullPointerException exceptions when converting objects in the list.

Scenario:

Database select min(id) as id,min(name) as name from user where 1=2;

Query It's not that there is no record, but a record of null, null, which causes the User object to be null

List users = useDao.find(xxx); //size=1,All elements are null

At this time, an error will be reported when operating the user object!

Solution

Method 1: Remove null elements

Example:

 List<User> users = new ArrayList<User>();
        users.add(null);
        users.add(null);
        users.add(null);
        System.out.println("size:"+users.size()); //size:3for(User user:users){try {
                System.out.println("id:" + user.getId() + ",name:" + user.getName());
            }catch (Exception ex){
                System.out.println(ex); //java.lang.NullPointerException            }
        }users.remove(null); //移除第一个nullSystem.out.println("size:"+users.size()); //size:2    users.removeAll(Collections.singleton(null)); //移除所有的null元素System.out.println("size:"+users.size()); //size:0//不会进入循环for(User user:users){try {
                System.out.println("id:" + user.getId() + ",name:" + user.getName());
            }catch (Exception ex){
                System.out.println(ex);
            }
        }

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<span style="font-size: 14px">法二：保证数据库查询出来没有null值，即遇null值转换为默认值<br><br><br></span><span style='font-family: "Microsoft YaHei"'>mysql下使用ifnull/case when均可实现</span><br><span style='font-family: "Microsoft YaHei"'>select  ifnull(min(id),-1) as id,ifnull(min(name),'defaultName') as name from user where 1=2;</span>

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（有group by是分组统计，找不到记录就是没有记录）

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<span style="font-size: 14px"><em><br><br></em></span>

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