无法将java.lang.Long强制转换为java.lang.Integer

Question

I want to read a map map from the Database (Firebase),and i compare it to another map motcle , if i find a duplicate key i increment the value of the key in map ,I am having a ClassCastException. the code:

public void addMotcle(final Map motcle, String Userid)
{
    mDatabase = FirebaseDatabase.getInstance().getReference("User");
    DatabaseReference user = mDatabase.child(Userid);
    final DatabaseReference keylist = user.child("motcle");
    keylist.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if(dataSnapshot.exists())
            {   
                //I tried to put the map in another map and change it but 
                //It is still the same error
                Map<String,Integer> map = (Map) dataSnapshot.getValue();
                Map<String,Integer>Hmap= new HashMap<>();
                Hmap.putAll(map);
                //if there is duplicate we incremente les mots clé
                for (Object key : motcle.keySet()) {
                    if (Hmap.containsKey(key)) {
                        Hmap.put((String)key,((Integer) Hmap.get(key))+1);
                    }
                    else
                    {
                        Hmap.put((String) key,1);
                    }
                }
               keylist.setValue(Hmap);

            }
            else
            {
             keylist.setValue(motcle);

            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });



}

Here is the Value that i want to write on in Firebase

the error is in the line map.put((String)key,map.get(key)+1);

why can't i write an Integer in the value if i declared map as Map<String,Integer> ?

Answer 1

When you do Map<String,Integer> map = (Map) dataSnapshot.getValue(); , you guarantee that map is a Map , but not that it's a Map<String, Integer> . Since generics are erased and not available at runtime, as long as dataSnapshot.getValue() is a Map of some kind, this cast will succeed.

Because you've declared Hmap as a Map<String,Integer> , when you get a value from the map, it will try to cast it to Integer , but there is nothing actually guaranteeing ahead-of-time that it will be an Integer . As a result, if the value is actually a Long , it will try to cast that Long to an Integer and fail (because Long does not extend Integer , and widening operations don't work on boxed types).

You need to find out what the actual types of the values of dataSnapshot.getValue() are, and cast it to the correct type of Map to avoid this error.

See here for more details: Weird problem about Java Generics operation

Answer 2

这是因为键列表是一个最终变量，您无法更改其值，因此可以像这样初始化它：-

DatabaseReference keylist = user.child("motcle");