Generics

DCS – SWC 2 Generics In many situations, we want a certain functionality to work for a variety of types Typical example: we want to be able to store elements of a certain type in a container, say an ArrayList This ability does not pose any special requirements on the type itself

DCS – SWC 3 Generics public class ArrayList { public ArrayList() {…} public void add(Object e) {…} }

DCS – SWC 4 Generics Since all classes inherit from Object, this will work – in principle However, there is nothing to prevent us from storing elements of different types in the same ArrayList Will complicate processing of the elements Risk for errors at run-time

DCS – SWC 5 Generics A better (safer) solution is to use Generics In a generic class, one or more type parameters are included in the definition Example is the well- known ArrayList

DCS – SWC 6 Generics public class ArrayList { public ArrayList() {…} public void add(E e) {…} } E is the type parameter

DCS – SWC 7 Generics When we wish to use the class, we must instantiate it with a concrete type: –ArrayList We can use the ArrayList for all types, BUT we cannot put two objects of different types into the same ArrayList

DCS – SWC 8 Creating a generic class Creating a generic class is almost like creating an ordinary class However, one or more of the types are turned into type parameters Consider a class for storing a pair of objects, which may have different types Pair

DCS – SWC 9 Creating a generic class public class Pair { private T first; private S second; public Pair(T first, S second) { this.first = first; this.second = second; } public T getFirst() {return first;} public S getSecond() {return second;} }

DCS – SWC 10 Creating a generic method An ordinary method inside a class usually does not need a type parameter Type parameter for class is used However, this is not the case for static methods: public static void print(E[] a)

DCS – SWC 11 Creating a generic method When invoking a generic method, we do not need to specify a type parameter: Car[] carList; ArrayUtil.print(carList); The compiler can figure out what the concrete type of the parameter is

DCS – SWC 12 Type constraints Sometimes, we will only allow types that fulfill some properties Consider the min method: public static E min(E a, E b) { if (a < b) return a; else return b; }

DCS – SWC 13 Type constraints The previous code will only work for types that can be compared Type must implement the Comparable interface We must constrain the type parameter to fulfill this property

DCS – SWC 14 Type constraints public static E min(E a, E b) { if (a < b) return a; else return b; } See Advanced Topic 17.1 for more advanced issues on type constraints

DCS – SWC 15 Under the hood How does the Java Virtual Machine handle generics…? It substitutes the type variables with a concrete type, depending on the type constraints

DCS – SWC 16 Under the hood public class Pair { private T first; private S second; public Pair(T first, S second) {...} public T getFirst() {return first;} public S getSecond() {return second;} }

DCS – SWC 17 Under the hood public class Pair { private Object first; private Object second; public Pair(Object first, Object second) {...} public Object getFirst() {return first;} public Object getSecond() {return second;} }

DCS – SWC 18 Under the hood But will that always work…? Yes, because the compiler has checked that all generic types are used correctly It does however impose some limitations on the code in generic methods: E e = new E(); This is illegal…(why)?

DCS – SWC 19 Under the hood Why does it work like that? Enables pre-generics code to work together with code using generics Does it matter in practice? Not much, but it explains why some perhaps not-so-obvious limitations exist