Why are the types different in Java variable declarations? -

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• arraylist or list declaration in java 10 answers

what purpose of specifying different types on left , right side in java variable declarations?

for example, code standard:

list<string> l = new arraylist<>(); 

why don't:

arraylist<string> l = new arraylist<>(); 

or

object l = new arraylist<>(); // silly example shows 2 extremes 

• this list<string> l = new arraylist<>(); allows instantiate types of list, arraylist or linkedlistof string. can use methods of list
• arraylist<string> l = new arraylist<>(); can instantiate arraylist of string.
• object l = new arraylist<>(); cannot use list methods.