Java 5 New Language Features

Transcription

1 Java 5 New Language Features Ing. Jeroen Boydens, BLIN prof dr. ir. Eric Steegmans Enterprise Programming Last session. Jeroen Boydens 1. JDK v1.4: assert 2. Generics 3. Autoboxing/unboxing 4. Covariant Overriding 5. Enhanced for loop 6. Typesafe enums 7. Varargs 8. import static APT: Annotation Processing Tool (in the small) 2 EP research 1

2 This Session. Eric Steegmans 1. Definition 2. Instantiation 3. Compilation 4. Raw Types 5. Polymorphism 6. Wildcards 7. Bounded Wildcards 8. Subclassing 9. Generic Methods Generics (in the large) 3 Generic Classes Generic classes or generic interfaces have one or more type parameters Formal type parameters can only be bound to reference types, not to primitive types Formal type parameters can be used (almost) everywhere a type can be used public class GenericClass<S,T> { public S somemethod(t p) { S localvar; 4 EP research 2

3 Definition Formal type parameters can not be used to create objects of formal parameter type create arrays of formal parameter type public class GenericClass<S,T> { public void somemethod() { // Illegal statements. new S( ); new S[10]; 5 Instantiation Supply actual reference types for formal arguments An actual argument may in turn be an instantiated generic class GenericClass<Integer,Person> myvariable = new GenericClass<Integer,Person>( ); ArrayList<Integer> mylist = new ArrayList<Integer>(); ArrayList<ArrayList<Integer>> mysuperlist = new ArrayList<ArrayList<Integer>>(); 6 EP research 3

4 Instantiation The Java compiler can now perform checks that had to be done at run-time otherwise Actual arguments must have the proper type Lots of type casts are avoided ArrayList<Integer> mylist = new ArrayList<Integer>(); //Arguments for method add when applied to mylist must have //static type Integer! mylist.add(27); //Type cast needed here in non-generic code! Integer myresult = mylist.get(2); 7 Compilation A generic class is compiled once and for all, resulting in a single class file All objects of instantiations of a generic class belong to the same class The class underlying all instantiations of a generic class is called a raw type For reasons of backward compatibility, raw types may still be used in programs Sun deprecates their usage in new code 8 EP research 4

5 Raw Types // The raw type for GenericClass public class GenericClass { public Object somemethod(object p) { Object localvar; // Declaration of a variable of raw type. GenericClass myrawobject = ; 9 Polymorphism Instantiations of a generic class never have a subtype/supertype relation This is true even if the one actual type is a subclass of the other actual type Integer is a subtype of Number; ArrayList<Integer> is not a subtype of ArrayList<Number> Strange enough, this rule does not apply to built-in in arrays Arrays of type Integer[] are thus assignable to variables of type Number[] 10 EP research 5

6 Wildcards Java offers wildcards to approach generic objects in a polymorphic way The most general wildcard stands for any type, and is denoted? ArrayList<?> is the supertype of all instantiations of the generic class ArrayList 11 Wildcards Methods with formal generic arguments against variables of wildcard-type Given ArrayList<?> mylist, mylist.add(new Integer()) is invalid Methods with formal generic return type against variables of wildcard-type Given ArrayList<?> mylist, Object o = mylist.get(0) is valid 12 EP research 6

7 Bounded Wildcards Bounded wildcards are wildcards with an upper bound or a lower bound A wildcard with upper bound T stands for type T or any sybtype of T A bounded wild card with upper bound T is denoted "? extends T " ArrayList<? extends Number> is the supertype of all instantiations of ArrayList involving Number or a subclass of Number 13 Bounded Wildcards A wildcard with lower bound lower bound T stands for type T or any supertype of T A bounded wild card with lower bound T is denoted "? super T " ArrayList<? super Number> is the supertype of all instantiations of ArrayList involving Number or a superclass of Number Restrictions that apply to unrestricted wildcards equally apply to bounded wildcards 14 EP research 7

8 Subclassing A generic type can be defined to inherit from another generic type The generic subclass actually inherits from an instantiation of its generic superclass A generic type can also inherit from an ordinary class or to implement an ordinary interface public class GenericSubClass<S,T> extends GenericSuperclass<S> implements GenericInterface<T>, Cloneable { { 15 Subclassing Formal type arguments may use upper bounded wildcards The raw type then includes type casts related to that upper bound public class GenericSubClass<S extends Number,T> extends GenericSuperclass<S> implements GenericInterface<T> { 16 EP research 8

9 Generic Methods A generic method is parameterized in one or more types Generic arguments express dependencies among types of formal arguments involved in a method Bounded wildcards in both directions are used for that purpose public <T> void genericmethod(collection<t> coll) { public static <T> void genericmethod (Collection<T> coll, Iterator<? super T>) { 17 Generic Methods The actual types of a generic method are derived from the actual arguments The compiler rejects invocations if no matching type is available 18 EP research 9