Generics in java just checking to make sure this is correct

Question

I'm struggling with these (similar) problems on the bounded quantification of generics in Java:

1. Write the header for a generic class named MyType. The class should have one type parameter. The type parameter’s upper bound should be the String class.

2. Write the header for a generic class named MyType. The class should have one type parameter. The type parameter’s lower bound should be the Integer class.

   public class myType<T extends String>{}
   

   and

   public class myType<T extends Integer>{}
   

is this correct im really having a hard time grasping the concept of generics in java

Answer 1

You don't use extends to define both the upper bound and the lower bound.

According to Wikipedia's article on Generics in Java...

To specify the upper bound of a type wildcard, the extends keyword is used...

So to define the upper bound, use extends:

public class myType<T extends String>{}

Again, according to Wikipedia's article on Generics in Java...

To specify the lower bounding class of a type wildcard, the super keyword is used.

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Addendum

I think whoever originally put this question together needs to sort a few things out.

• Java does not have header files. Look up Header Files on Wikipedia, and it immediately states that Java does not use them:

  Some newer languages (such as Java) dispense with header files...

• You cannot use a lower bound when defining a class - I believe it can only be done with a wildcard, which is not used when defining a class. The code public class MyClass<T super Integer> {} will give a syntax error: > expected

So all together, the question reeks of nonsense.