Instantiate an Interface [duplicate]

Answer 1

If you write:

SimpleDoc mydoc = new SimpleDoc();

all the further code may depend on details exposed by the implementing class SimpleDoc. But if you write:

Doc mydoc = new SimpleDoc();

the further code my only depend on aspects exposed by Doc, which make the code even work if you decide in the future to write:

Doc mydoc = new ComplexDoc();

A good example for the differences is List, which has at least two implementations:

ArrayList
LinkedList

If you write:

List list = new ArrayList();

you are free to replace it later with:

List list = new LinkedList();

without breaking the code relying on the variable list (assuming you did not used casts or reflection to access implementation specific features of list).