Implement classes book and author according to the class diagrams below?

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Java

BlueJ Java question

Book:
fields:
-String bookName
-Author author
And
One constructor to initialize the above fields
One constructor to initialize bookName only

Author
fields
-String name
-String dob
And
One constructor to initialize the above fields

What I have tried:

I really don't know what to do :( How to use two classes

public class Book