Iterating over a map entryset

Question

I need to iterate over the entry set of a map from which I do not know its parameterized types.

When iterating over such entryset, why this does not compile ?

public void myMethod(Map anyMap) {
    for(Entry entry : anyMap.entrySet()) {
        ...
    }
}

but this compile:

public void myMethod(Map anyMap) {
    Set<Entry> entries = anyMap.entrySet();
    for(Entry entry : entries) {
        ...
    }
}

and this also compiles (I cannot use this one since I do not know the types of the map):

public void myMethod(Map<String, String> stringMap) {
    for(Entry<String,String> entry : stringMap.entrySet()) {
        ...
    }
}

Answer 1

The error you get in your first one is:

Type mismatch: cannot convert from element type Object to Map.Entry

This is because the compiler converts your FOR-IN loop:

for (Entry entry : anyMap.entrySet()) {
}

To:

for (Iterator i = anyMap.entrySet().iterator(); i.hasNext();) {
    Entry e = i.next(); // not allowed
}

Your second example works, but only through cheating! You are doing an unchecked cast to get Set back into a Set<Entry>.

Set<Entry> entries = anyMap.entrySet(); // you get a compiler warning here
for (Entry entry : entries) {
}

Becomes:

Set<Entry> entries = anyMap.entrySet();
for (Iterator<Entry> i = entries.iterator(); i.hasNext(); ) {
    Entry e = (Entry) i.next(); // allowed
}

---

Update

As mentioned in comments, the type information is getting lost in both examples: because of the compiler's raw type erasure rules.

To provide backwards compatibility, ALL methods of raw type instances are replaced by their erased counterparts. So, because your Map is a raw type, it all gets erased. Including its Set<Map.Entry<K, V>> entrySet(); method: your raw type instance will be forced to use the erased version: Set entrySet().

Answer 2

It is because you use the raw type Map, therefore map.entrySet() gets you a non-parametrized Set which in return yields Object on iteration, not Entry.

A simple, but elegant solution is to use Map<?,?>, which will still allow you to pass ANY Map, but on the other hand forces map.entrySet() to have a return value of Set<Entry>:

public void test(Map<?,?> map) {        
    for(Entry e : map.entrySet()){
        Object key = e.getKey();
        Object value = e.getValue();
    }       
}