将两个通配符哈希图合并为一个通配符哈希图

Question

I want to combine two hash maps with wild cards into a single hash map which still contains dynamic values.

I have SharedPreferences in two different places in my Android application - I need to combine the entire set into a single hash map to pass to a function.

I have tried putAll() and merge functions, but the wild cards keep causing issues.

Currently I have two functions which return the values, but I really need to combine them into a single function.

private HashMap<String, ?> getAppPrefs() {
   SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
   return (HashMap<String, ?>) prefs.getAll();
}

private HashMap<String, ?> getScorePrefs() {
   SharedPreferences scoreprefs= this.getSharedPreferences("scoreprefs", 0);
   return (HashMap<String, ?>) scoreprefs.getAll();
}

My expected result is something like:

private HashMap<String, ?> getAllPrefs() {
   SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
   HashMap<String, ?> map1 = (HashMap<String, ?>) prefs.getAll();   
   SharedPreferences scoreprefs= this.getSharedPreferences("scoreprefs", 0);
   HashMap<String, ?> map2 = (HashMap<String, ?>) prefs.getAll();
   HashMap<String, ?> map3 = map1.merge(map2);  // <-- Or something else
   return map3;
}

If I use merge, I get:

error: no suitable method found for merge(HashMap<String,CAP#1>)
  HashMap<String, ?> map3 = map1.merge(map2);
                                ^
method Map.merge(String,CAP#2,BiFunction<? super CAP#2,? super CAP#2,? extends CAP#2>) is not applicable
  (actual and formal argument lists differ in length)
method HashMap.merge(String,CAP#2,BiFunction<? super CAP#2,? super CAP#2,? extends CAP#2>) is not applicable
  (actual and formal argument lists differ in length)
  where CAP#1,CAP#2 are fresh type-variables:
CAP#1 extends Object from capture of ?
CAP#2 extends Object from capture of ?

Answer 1

With the unknown wildcard ( <?> ), the only assumption that can safely be made is that instances are a subtype of Object.

We cannot take objects of an unknown type and put them in another collection of another unknown type because the two types might be incompatible. However, we can put them in a collection of <Object> as this is always safe, and then declare that this collection is of a third unknown type.

For instance, with the type of your example:

public static HashMap<String, ?> merge(HashMap<String ,?> map1, HashMap<String, ?> map2) {
    HashMap<String, Object> merged = new HashMap<>();
    merged.putAll(map1);
    merged.putAll(map2);
    return merged;
}