Method of removing the same object element from the ArrayList container

I found the retainAll method on the Internet

Then add it to the code

Then I found that I can't. I checked a lot of things and found that the code is right. I just need to rewrite the equals method.

I don't know why, because the custom class book used in the small demo written earlier, as follows, can be used normally

Whatever, I'm going to find the source code of collection,

After overriding the equals method in employee.

As follows:

The above code can run normally.

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