Question

I have a static Hashmap, A. I am making a copy of it, B, for calculations. When I delete elements from B and print A, those elements are gone from A too. Is this expected?

    Static Map<String, ArrayList<String>> A;

    private void calculate(String id) {

    Map<String, ArrayList<String>> B= A;
    String maxKey = getMaxKey(B);

    int neighbour_matrix_length = B.size();

    for(int i=0 ; i< neighbour_matrix_length; i++){ 

    // remove all the visited nodes 
    ArrayList<String> visited = copy_neighbours_reachability_map.get(maxKey);

    for(int j = 0; j< visited.size() ; j++)
    {
    String temp = visited.get(i);
    Iterator<Entry<String, ArrayList<String>>> iter = B.entrySet().iterator();
    // iterate through the hashmap to remove elements
    while (iter.hasNext()) {
        Entry<String, ArrayList<String>> entry = iter.next();
        if(entry.getValue().contains(temp)){
        entry.getValue().remove(temp);
        }
    }
    }
    }
   // Elements from A are also getting deleted..
   System.out.println("neighbours_reachability_map is ");
   for (String key : A.keySet())
   {
    System.out.println( "Key : " + key + " - " + A.get(key).toString());
   }
   }

Am I missing something? I do not want elements from A to be deleted.

Answer 1

您正在做的只是共享对同一对象的引用，因此更改一个将改变另一个对象。这样做：

Map<String, ArrayList<String>> B = new HashMap<>(A);

将制作地图的副本，但是......地图中的值是列表，两个地图将保留对相同列表的引用（即使新旧地图是不同的对象）。所以你还需要复制列表：

Map<String, ArrayList<String>> B = new HashMap<>();

for (Map.Entry<String, ArrayList<String>> e : A.entrySet()) {
    B.put(e.getKey(), new ArrayList<String>(e.getValue()));
}

我们在这里做的是创建对象的深层复制，因此不会共享任何引用。

Answer 2

我正在制作它的副本B，用于计算。当我从B中删除元素并打印A时，这些元素也从A中删除。这是预期的吗？

是您预期的行为，因为当您说Map<String, ArrayList<String>> B= A;时，B会将相同 Map称为A（也就是说，B == A，这意味着它们共享相同的引用）。您可以使用类似

的内容将 A复制到B

Map<String, ArrayList<String>> B = new HashMap<>(A);

Java - Is a copy of a static variable also static?

2 个答案: