Based on Bert F's answer I would like to explain my understanding.
Lets say we have 3 classes as
public class Fruit{}
public class Melon extends Fruit{}
public class WaterMelon extends Melon{}
Here We have
List fruitExtendedList = …
//Says that I can be a list of any object as long as this object extends Fruit.
Ok now lets try to get some value from fruitExtendedList
Fruit fruit = fruitExtendedList.get(position)
//This is valid as it can only return Fruit or its subclass.
Again lets try
Melon melon = fruitExtendedList.get(position)
//This is not valid because fruitExtendedList can be a list of Fruit only, it may not be
//list of Melon or WaterMelon and in java we cannot assign sub class object to
//super class object reference without explicitly casting it.
Same is the case for
WaterMelon waterMelon = fruitExtendedList.get(position)
Now lets try to set some object in fruitExtendedList
Adding fruit object
fruitExtendedList.add(new Fruit())
//This in not valid because as we know fruitExtendedList can be a list of any
//object as long as this object extends Fruit. So what if it was the list of
//WaterMelon or Melon you cannot add Fruit to the list of WaterMelon or Melon.
Adding Melon object
fruitExtendedList.add(new Melon())
//This would be valid if fruitExtendedList was the list of Fruit but it may
//not be, as it can also be the list of WaterMelon object. So, we see an invalid
//condition already.
Finally let try to add WaterMelon object
fruitExtendedList.add(new WaterMelon())
//Ok, we got it now we can finally write to fruitExtendedList as WaterMelon
//can be added to the list of Fruit or Melon as any superclass reference can point
//to its subclass object.
But wait what if someone decides to make a new type of Lemon lets say for arguments sake SaltyLemon as
public class SaltyLemon extends Lemon{}
Now fruitExtendedList can be list of Fruit, Melon, WaterMelon or SaltyLemon.
So, our statement
fruitExtendedList.add(new WaterMelon())
is not valid either.
Basically we can say that we cannot write anything to a fruitExtendedList.
This sums up List
Now lets see
List melonSuperList= …
//Says that I can be a list of anything as long as its object has super class of Melon.
Now lets try to get some value from melonSuperList
Fruit fruit = melonSuperList.get(position)
//This is not valid as melonSuperList can be a list of Object as in java all
//the object extends from Object class. So, Object can be super class of Melon and
//melonSuperList can be a list of Object type
Similarly Melon, WaterMelon or any other object cannot be read.
But note that we can read Object type instances
Object myObject = melonSuperList.get(position)
//This is valid because Object cannot have any super class and above statement
//can return only Fruit, Melon, WaterMelon or Object they all can be referenced by
//Object type reference.
Now, lets try to set some value from melonSuperList.
Adding Object type object
melonSuperList.add(new Object())
//This is not valid as melonSuperList can be a list of Fruit or Melon.
//Note that Melon itself can be considered as super class of Melon.
Adding Fruit type object
melonSuperList.add(new Fruit())
//This is also not valid as melonSuperList can be list of Melon
Adding Melon type object
melonSuperList.add(new Melon())
//This is valid because melonSuperList can be list of Object, Fruit or Melon and in
//this entire list we can add Melon type object.
Adding WaterMelon type object
melonSuperList.add(new WaterMelon())
//This is also valid because of same reason as adding Melon
To sum it up we can add Melon or its subclass in melonSuperList and read only Object type object.
