Next question: what do these errors tell you? The first errors you will encounter are:
Line 8: Look at line 8 in your example. These compilation errors are thrown because line 8 isn't valid Java code. Try to find out which statements to use, and how to order/use them correctly.
Line 9: After that line 9 will become a problem, you are returning an object type instead of an actual object.
Try to fix these two lines first. That should fix the compilation errors, and you should be able to continu the exercise.
If you still encounter any problems, we're here to help you out.
Let's look at the code you wrote carefully:
You have the method signature correct for an accessor in line 6. In line 7 you create a local variable transactions that hides the member variable on line 4. This is probably not what you want to do. Line 8 uses a type ArrayList_new. There is no such type in the standard library. Did you create it? Then in line 9 you return the type, not the value. This is a syntax error.
Most accessors are three lines long, so you are complicating things. I don't know what a "deep copy" has to do with this code.
All things are lawful, but not all things are profitable.
You might find the List#addAll() method to be useful. You can use it to make a new list but the contents of the new list will refer to the same objects of type "Long". Having the same reference to a Long is not a problem because a Long is immutable and cannot be modified.