Generic types are not covariant : Generics allow you to use only one type

All the Object-based collection types (known as raw types) in classic Java are now retrofitted as generic types and accept type parameters. Generic types allow you to express your intent as being restricted to a particular type when creating and using it...

TreeSet<String> names = new TreeSet<String>();
names.add("Fred");
names.add("Wilma");
names.add("Pebbles");

SortedSet<String> is called the parameterized type and String is the actual type argument. Type-safety checking will be performed at compile time...

names.add(new Integer(100)); //compilation error

The compiler will also reject the following...

TreeSet<Object> names = new TreeSet<String>();
TreeSet<Object> alias = (TreeSet<Object>)names;

This is because generic types are not covariant – a TreeSet of String references is not a TreeSet of Object references.
With generic types, retrieved elements do not require explicit conversion...