java.util.hashmap无法转换为java.util.list

Question

i'm trying to retrieve data from a hashmap with multiple values for 1 key and set it to a listview but im gettting the error java.util.hashmap cannot be cast to java.util.list. The code is as follows:

 ListView lv = (ListView)findViewById(R.id.list);
    //hashmap of type  `HashMap<String, List<String>>`
    HashMap<String, List<String>> hm = new HashMap<String, List<String>>();
    List<String> values = new ArrayList<String>();
    for (int i = 0; i < j; i++) {
        values.add(value1);
        values.add(value2);
        hm.put(key, values);
    }

and to retrieve the values and put in a listview

ListAdapter adapter = new SimpleAdapter(
                        MainActivitty.this, (List<? extends Map<String, ?>>) hm,
                        R.layout.list_item, new String[] { key,
                                value1,value2},
                        new int[] { R.id.id, R.id.value1,R.id.value2 });
                // updating listview
                lv.setAdapter(adapter);

how can i fix this?

Answer 1

Wrap the Map in a List to match the expected type List<? extends Map<String, ?> List<? extends Map<String, ?> of the constructor SimpleAdapter :

ListAdapter adapter = new SimpleAdapter(
                        MainActivitty.this, Arrays.asList(hm),
                        R.layout.list_item, new String[] { key,
                        value1,value2},
                        new int[] { R.id.id, R.id.value1,R.id.value2 });

Refer to this example

Answer 2

You must change the order how you add data. According to the documentation of SimpleAdapter , you must create a list of maps, where each entry of the list represents one row of data. The maps must contain the column names as key, and the column value as value.

So to create 3 rows with 3 columns you would do:

List<? extends Map<String, ?>> data = new ArrayList<? extends Map<String, ?>>();
for (int i=0; i < 3; i++) {
  Map<String, String> row = new HashMap<String, String>();
  row.put(key, "key in row " + i);
  row.put(value1, "value1 in row " + i);
  row.put(value2, "value2 in row " + i);
  data.add(row);
}

Then create your SimpleAdapter instance:

ListAdapter adapter = new SimpleAdapter(
                    MainActivitty.this, data,
                    R.layout.list_item, new String[] { key,
                            value1,value2},
                    new int[] { R.id.id, R.id.value1,R.id.value2 });