Difference between and in Java

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<? extends Fruit> 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<? extends Fruit>

Now lets see

List<? super Melon> 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.