Your class diagram says almost nothing

transcript

You drew five boxes, every field inside, and lines between them. The interviewer asks one question. How many copies can a book have?

A box for every class. Every field inside. A plain line wherever two classes seem related. And no numbers anywhere.

Does the library create its members, or just know them? Silent. Who holds the reference, the loan or the copy? Silent. Nothing in it could be wrong. So nothing in it helps. And an interviewer may ask exactly those questions.

The fix is notation. Each line says what it means. A book creates and owns its copies. One book, one or more copies. An arrow from the loan to its copy shows who holds the reference. Now the diagram can be wrong, which means it is useful.

Choose each line by what the code does. Boxes are easy. The lines carry the design. Add the numbers, and it is complete.

Full breakdown in the lecture.