[Feature] Add `toString()` to all java entity.

[kevinten10]

Add an overridden tosString() method for all java entity classes,
This will facilitate logging and debugging.

[seeflood]

Thanks for your suggestion!
I agree with u

[kevinten10]

Please assign to me.
By the way, I can read all the code.

[kevinten10]

I can also think about how to do #3

[seeflood]

@kevinten10 Thanks! It has been assigned to you