Returning a new instance of HashMap

Question

I have a HashMap that stores a custom Object and maps it to a certain ArrayList in a class. My class communicates with another class (think MVC style) and it passes a copy of that hashmap. So, in my "model", I would have:

public Map<AbstractArtistry, ArrayList<AbstractCommand>> getHashMap() { return new LinkedHashMap<AbstractArtistry, ArrayList<AbstractCommand>>(this.hashmap); }

However, my "controller", when it gets that, can still edit the AbstractArtistries inside of the model's this.hashmap. To avoid this, do I have to create a new instance of an Abstract Artistry over and over, or is there a cleaner way to do this? Meaning, would I have to loop over model.hashmap.keySet(), create a new instance of every artistry, insert it into a new hashmap (and do the same for all the values), and then return that new hashmap? Or is there a cleaner way?

Are you saying your map's key type is mutable? That's asking for trouble... — shmosel
– shmosel, Commented Apr 3, 2018 at 1:31
@shmosel Too late to change the design now, but just for educational purposes, why? — John Lexus
– John Lexus, Commented Apr 3, 2018 at 1:33
Wouldn't you also need to copy the lists? What about AbstractCommand? — shmosel
– shmosel, Commented Apr 3, 2018 at 1:39

shmosel · Accepted Answer · 2018-04-03 01:54:28Z

1

You can use streams to copy the map and replace the keys with defensive copies:

this.hashmap.entrySet() .stream() .collect(Collectors.toMap(e -> createCopy(e.getKey()), Map.Entry::getValue))

If you need to copy the values as well, you can run them through a similar function:

ArrayList<AbstractCommand> copyList(ArrayList<AbstractCommand> list) { return list.stream() .map(c -> copyCommand(c)) .collect(Collectors.toCollection(ArrayList::new)); }

edited Apr 3, 2018 at 1:54

answered Apr 3, 2018 at 1:43

shmosel

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10 Comments

John Lexus Over a year ago

How do they all fit together so I could return a new hashmap?

shmosel Over a year ago

@JohnLexus All of what? That snippet returns a map.

John Lexus Over a year ago

I am saying how do I combine the first and second snippets? The first snippet looks like it return a map with just keys and no values, while the second return a (map?) of the values, not with my AbstractArtistries as keys. Or am I reading this wrong (sorry new to Java)?

shmosel Over a year ago

I've updated the second snippet to return ArrayList. But ideally you should be sticking to the List interface.

shmosel Over a year ago

That's right. You would also have to create copyCommand(). Unless you don't need a deep copy, in which case you can just return new ArrayList<>(list).

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