Java Wildcard and Generic Types

• A wildcard type is a type whose element type matches anything.

• A wildcard type is written Collection<?> (pronounced "collection of unknown"), and it is called a wildcard type for obvious reasons.

• A wildcard type is a subtype of the unknown type Shape.

• The type List<? extends Shape> is a bounded wildcard. This means that List<? extends Shape> is a subtype of both List<Shape> and Shape itself.

• The type List<Shape> is an unrestricted wildcard. This means that List<Shape> is a subtype of any type.

• The method drawAll() can only be called on lists of exactly Shape.

• It is no longer legal to write into shapes in the body of the method.

• For instance, this is not allowed:

You should be able to figure out why the code above is disallowed.

• The type of the second parameter to shapes.add() is ? extends Shape-- an unknown subtype of Shape.

• Since we don't know what type it is, we don't know if it is a supertype of Rectangle; it might or might not be such a supertype, so it isn't safe to pass a Rectangle there.

• Map<K,V> is an example of a generic type that takes two type arguments, representing the keys and values of the map.

• Again, note the naming convention for formal type parameters--K for keys and V for values.

• Summarize the key facts from the text above in 10 sentences:

• It is no longer legal to write into shapes in the body of the method.

• For instance, this is not allowed:

You should be able to figure out why the code above is disallowed.

• The type of the second parameter to shapes.add() is ? extends Shape-- an unknown subtype of Shape.

• Since we don't know what type it is, we don't know if it is a supertype of Rectangle; it might or might not be such a supertype, so it isn't safe to pass a Rectangle there.

• Map<K,V> is an example of a generic type that takes two type arguments, representing the keys and values of the map.

• Again, note the naming convention for formal type parameters--K for keys and V for values.

• Summarize the key facts from the text above in 10 sentences:

• It is no longer legal to write into shapes in the body of the method.

• For instance, this is not allowed:

You should be able to figure out why the code above is disallowed.

• The type of the second parameter to shapes.add() is ? extends Shape-- an unknown subtype of Shape.

• Since we don't know what type it is, we don't know if it is a supertype of Rectangle; it might or might not be such a supertype, so it isn't safe to pass a Rectangle there.

• Map<K,V> is an example of a generic type that takes two type arguments, representing the keys and values of the map.

• Again, note the naming convention for formal type parameters--K for keys and V for values.

• Summarize the key facts from the text above in 10 sentences:

• It is no longer legal to write into shapes in the body of the method.

• For instance