why people use "Map m= new HashMap()" or "List l = new ArrayList()"?

Hi,

I saw in many places that people use:

Map m= new HashMap();

or

List l = new ArrayList();

I *rarely* see people do:

HashMap m= new HashMap();

or

ArrayList l = new ArrayList();

Is any specific reason for such a practice?

I noticed Map does not have clone(), but HashMap does.
.

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