如何在Java中以相反的顺序迭代hashmap

Question

我试了几个小时,但没有找到任何最好的方法来以相反的顺序实现hashmap的迭代,这是我的hashmap.

      Map<Integer, List<String>> map = new HashMap<Integer, List<String>>();

             for(Integer key : map.keySet()) {
                List<String> value = map.get(key);
                List<Map<String,?>> security = new LinkedList<Map<String,?>>();  
                for(int ixy = 0; ixy < value.size()-1; ixy++){
                    security.add(createItem(value.get(ixy), value.get(ixy+1))); 
                }
                adapter.addSection(Integer.toString(key), new SimpleAdapter(getApplicationContext(), security, R.layout.list_complex, new String[] { ITEM_TITLE, ITEM_CAPTION }, new int[] { R.id.list_complex_title, R.id.list_complex_caption }));  
            }

我也见过TreeMap的例子,

             Map<Integer, List<String>> sortedMap = new TreeMap<Integer, List<String>>(map);

但是树形图也按升序给出,我想要的是降序.

Answer 1

以相反顺序实现hashmap迭代迭代的最佳方法

HashMap没有定义其元素的任何特定顺序.因此,也没有定义"反向"顺序.

对于a TreeMap,你可以使用descendingMap().

Answer 2

Hashmap没有特定的顺序.但是你可以使用TreeMap.

也许这个简单的例子可以帮助你:

Map<Integer, String> map = new TreeMap<Integer, String>();
        map.put(1, "abc1");
        map.put(2, "abc2");
        map.put(3, "abc3");

        ArrayList<Integer> keys = new ArrayList<Integer>(map.keySet());
        for(int i=keys.size()-1; i>=0;i--){
            System.out.println(map.get(keys.get(i)));
        }

Answer 3

HashMap不会在键之间维护eny顺序.

TreeMap按其自然顺序或按构造地图时传递的比较器强加的顺序对其键进行排序.因此,如果您想以相反的顺序排序Integer键,请以这种方式构造TreeMap:

Map<Integer, List<String>> sortedMap = 
    new TreeMap<Integer, List<String>>(Collections.reverseOrder());

Answer 4

Map<Integer, List<String>> sortedMap = new TreeMap<Integer, List<String>>(Collections.reverseOrder());

Collections.reverseOrder() keeps the map sorted in descending order.

Answer 5

你可以使用TreeMap#descendingKeySet方法.

Map<Integer, List<String>> map = new TreeMap<Integer, List<String>>();

for(Integer key : map.descendingKeySet()) {
    List<String> value = map.get(key);
    List<Map<String,?>> security = new LinkedList<Map<String,?>>();  
    for(int ixy = 0; ixy < value.size()-1; ixy++){
        security.add(createItem(value.get(ixy), value.get(ixy+1))); 
    }
    adapter.addSection(Integer.toString(key), new SimpleAdapter(getApplicationContext(), security, R.layout.list_complex, new String[] { ITEM_TITLE, ITEM_CAPTION }, new int[] { R.id.list_complex_title, R.id.list_complex_caption }));
} 

参考:

https://docs.oracle.com/javase/8/docs/api/java/util/TreeMap.html#descendingKeySet--