Welcome again
Have you read the
Java™ Tutorials about collections? Look at the sections about set interface, map interface, and list interface. Imagine you have a lot of telephone numbers. If you put them in a set, there will be one entry for each phone number and duplicates vanish and if you try to iterate the set they don't come out in any obvious order.
Enter 123 234 345 345 345 456
Contains 123 234 345 456
If you use a List and enter the phone numbers and iterate it, you get the first entered first and the last entered last, and if you enter the same number twice, the duplicates will appear.
Enter 123 234 345 234 345 456 567 234
Contains 123 234 345 234 345 456 567 234
If you use a Map, you have to enter two pieces of data, e.g. name and phone number. You would then find your map contains things like
Campbell↦8****2
Campbell's wife↦8****2
Campbell's daughter↦07***8
etc.
Does that help with the difference between the three? If you go through the tutorial link, you find there are special classes which allow you to get the contents of a set in a particular order, and all sorts of other things. If you use a Map or certain kinds of Set, you need to beware of putting mutable types in them; it is possible that they can change their state and vanish from sight. In the case of the Map, the name should be immutable. If the phone number object changes its state, that is all right.