There are many ways to solve this problem. As far as I know, there are at least three ways, so I won't go into details here.
Adhering to the principle of being able to Ctrl+C/V and never typing code, here is the simplest solution.
Problem description: First, describe the problem. The picture below is a maven project made by my eclips. Obviously, there is a problem with the structure.
Reason: This situation is caused by the conflict between the default jdk version of the project and the jdk version of the development tool.
Solution: Since it is inconsistent, change it to be consistent.
Right-click the project, Build Path > Configure Buid Path > Libraries
Select the jdk version you want to use.
Finish > Apply > OK
After the modification, the project structure is normal, and the attached picture is as follows.
