Why developer use "Map m= new HashMap()" or "List l = new ArrayList()"?

[Rahul Sud]

Hi Techies,

Sorry if this question is repeated ...I have searched but havent got this one...

Why developer use "Map m= new HashMap()" or "List l = new ArrayList()" in place of HashMap hm= new HashMap()" or "ArrayList al = new ArrayList()"

According to me Map or List is interface and it is hidding the implementation from client.


Please give your inputs and share your knowledge for this question.


Thank You,
Rahul

[Jesper de Jong]

Have a look at this article: Program to an interface, not an implementation

The main reason to do this is to decouple your code from a specific implementation of the interface. By writing your code like this:

Map<String, Object> data = new HashMap<String, Object>();

then the rest of your code only knows that data is a Map<String, Object>, which is good, because it you can now easily change to a different implementation of interface Map if necessary. You'd only need to change that one line of code, for example to:

Map<String, Object> data = new TreeMap<String, Object>();

For the rest of your program, nothing changes - data is still a Map<String, Object>.

If you would have written:

HashMap<String, Object> data = new HashMap<String, Object>();

then changing it would have been much harder, because the rest of your code might have used methods that are specific to HashMap.

[Rahul Sud]

Thanks Jesper for good explanation