Class List cannot be converted to Collection

Question

It may be a really stupid question , I have a java code which is as follows :

public Collection<Object> getCollection(){
     List<Map<String,Object> listMap = getListMap();
     return listMap;
}

I am getting an exception saying

cannot assign a value of Java.util.list<java.util.map<java.lang.String,java.lang.Object>>  
to java.util.Collection<java.lang.Object>

Of course the error must be valid , But why is that ? And how can i overcome this ?

have a look at [this](http://stackoverflow.com/q/2745265/2764279) question. — earthmover, Feb 07 '17 at 10:21
This error seems not matching with the exposed code. I would expect to "cannot convert..." but not "cannot assign" — davidxxx, Feb 07 '17 at 10:23
Okay , I am very weak in generics ideally from my understanding this would have worked. Anyways , i will cast it for now. Thank you — Bibin Mathew, Feb 07 '17 at 10:23
Possible duplicate of [Is List a subclass of List? Why aren't Java's generics implicitly polymorphic?](http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicitly-p) — Tom, Feb 07 '17 at 10:29

score 1 · Answer 1 · answered Feb 07 '17 at 10:29

When using generics, avoid using references to Object. Instead use the wild card ? For example, in your case, change the Collection<Object> to Collection<?>.

So your method becomes,

public Collection<?> getCollection(){
     List<Map<String,Object>> listMap = getListMap();
     return listMap;
}

Hope this helps!

score 0 · Answer 2 · answered Feb 07 '17 at 10:24

0

change your method signature to Collection<Map<String,Object>>

 public Collection<Map<String,Object>> getCollection(){
     List<Map<String,Object>> listMap = getListMap();
     return listMap;
}

answered Feb 07 '17 at 10:24

Sasikumar Murugesan

4,412
10
51
74

score 0 · Answer 3 · answered Feb 07 '17 at 11:10

0

This Material will be useful for understanding generics Inheritance: Java Documentation . Try this (it should work fine):

public Collection<Object> getCollection() {
    return new ArrayList<>(getListMap());
}

answered Feb 07 '17 at 11:10

Petya

312
2
6

Class List cannot be converted to Collection

3 Answers3