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
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);
}
}
<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>
（有group by是分组统计，找不到记录就是没有记录）
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