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<E>

DCS – SWC 6

Generics

public class ArrayList<E>

{

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<BankAccount>– ArrayList<Car>

• 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<String, Car>

DCS – SWC 9

Creating a generic classpublic class Pair<T,S>

{

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 <E> 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> 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 extends Comparable> 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<T, S>

{

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