Java 中 HashMap 的深拷贝

Question

I am having difficulty making a deep copy of a HashMap. I tried the code below from How to copy HashMap (not shallow copy) in Java , but List at List.copyOf is giving me an error (Cannot resolve symbol 'List') and I'm not sure what to replace it with. If there is any other way to make a deep copy of HashMap, I would love to hear.

private HashMap<Character, ArrayList<int[]>> board;

public Vertex(HashMap<Character, ArrayList<int[]>> inputboard) {
        board = new HashMap<>();
        this.board = copy(inputboard);
}

public HashMap<Character, ArrayList<int[]>> copy (HashMap<Character, ArrayList<int[]>> original) {  
        HashMap<Character, ArrayList<int[]>> copy = original.entrySet().stream()
                .collect(Collectors.toMap(e -> e.getKey(), e -> List.copyOf(e.getValue())));
        return copy;
}

Before I used the above code, I also tried this from How to copy HashMap (not shallow copy) in Java but it did not make a deep copy

private HashMap<Character, ArrayList<int[]>> board;

public Vertex(HashMap<Character, ArrayList<int[]>> inputboard) {
        board = new HashMap<>();
        this.board = copy(inputboard);
}

public HashMap<Character, ArrayList<int[]>> copy (HashMap<Character, ArrayList<int[]>> original) {
       HashMap<Character, ArrayList<int[]>> copy = new HashMap<>();
       for (Map.Entry<Character, ArrayList<int[]>> entry : original.entrySet()) {
            copy.put(entry.getKey(), new ArrayList<>(entry.getValue()));
       }
       return copy;
}

Answer 1

Both ways you showed doesn't copy the int[] , ie not copying as deep as you want.

The first way actually returns a Map<Character, List<int[]>> rather than the HashMap<Character, ArrayList<int[]>> you want. I would recommend that you program to interfaces and change your method to return a Map<Character, List<int[]>> instead, then you can use streams like this:

public Map<Character, List<int[]>> copy (HashMap<Character, ArrayList<int[]>> original) {
    Map<Character, List<int[]>> copy = original.entrySet().stream()
        .collect(Collectors.toMap(
            Entry::getKey,
            e -> e.getValue().stream().map(x -> Arrays.copyOf(x, x.length)).collect(Collectors.toList())
        ));
    return copy;
}

If you really don't want to change the return type, you'd have to pass more arguments to specify the concrete types you want:

public HashMap<Character, ArrayList<int[]>> copy (HashMap<Character, ArrayList<int[]>> original) {
    HashMap<Character, ArrayList<int[]>> copy = original.entrySet().stream()
        .collect(Collectors.toMap(
            Entry::getKey,
            e -> e.getValue().stream().map(x -> Arrays.copyOf(x, x.length)).collect(Collectors.toCollection(ArrayList::new)),
            (x, y) -> x,
            HashMap::new
        ));
    return copy;
}

Otherwise, you can use the traditional for loops:

public HashMap<Character, ArrayList<int[]>> copy (HashMap<Character, ArrayList<int[]>> original) {
    HashMap<Character, ArrayList<int[]>> copy = new HashMap<>();
    for (Map.Entry<Character, ArrayList<int[]>> entry : original.entrySet()) {
        ArrayList<int[]> listCopy = new ArrayList<>();
        for (int[] array: entry.getValue()) {
            listCopy.add(Arrays.copyOf(array, array.length));
        }
        copy.put(entry.getKey(), listCopy);
    }
    return copy;
}

Notice that in all three code snippets, I have used Arrays.copyOf to copy the int[] .