I am a junior, and I have basically finished learning java grammar, but I feel that I can't program and I don't have that kind of programming idea. What should I do?

Your problem is that you haven't practiced enough. No matter how good linguistics is, it is useless not to write code. If you think the project is difficult to do, I suggest you do more small topics similar to ACM to improve your algorithm ability. If you persist for a few months, you will find that your programming ability has been greatly improved. At that time, slowly start to do some small projects in object-oriented language, and you will find that those things are actually very easy.