SOEN6441.genericsSOEN6441.genericsSOEN6441.generics

Concordia University Department of Computer Science and Software Engineering
ADVANCED PROGRAMING
PRACTICES
Java generics
Joey Paquet, 2006-2017
1
SOEN 6441 - Advanced Programming Practices

• Java Generics is a language feature that enables the definition of classes and
methods that are implemented independently of some type that they use as an
abstraction by accepting a type parameter.
• The goal is to define types and algorithms that apply in different contexts, but
where the interface and general functioning and implementation can be defined
such that it applies independently of the context of application.
• Generic types are instantiated to form parameterized types by providing actual
type arguments that replace the formal type parameters.
• For example, a class like LinkedList<T> is a generic type, that has a type
parameter T. Instantiations, such as LinkedList<String> or a
LinkedList<Integer>, are called parameterized types, and String and
Integer are the respective actual type arguments.
Genericprogramming:introduction
2

• M.D. McIlroy. Mass-Produced Software Components, Proceedings of the 1st
International Conference on Software Engineering, Garmisch Partenkirchen,
Germany, 1968.
• Barbara Liskov, Alan Snyder, Russell Atkinson, and Craig Schaffert. Abstraction
mechanisms in CLU. Commun. ACM 20, 8 (August 1977), 564-576.
doi=10.1145/359763.359789
• Joseph A. Goguen. Parameterized Programming. IEEE Trans. Software Eng. 10(5)
1984.
• David R. Musser, Alexander A. Stepanov. Generic Programming. In International
Symposium on Symbolic and Algebraic Computation (ISSAC 1988). Lecture Notes
in Computer Science 358, Springer-Verlag, 1989, pp 13-25.
• 1999: Sun Microsystems proposes to add generics to Java, based on GJ.
• 2001: Sun Microsystems releases a prototype including Java Generics.
• 2003: Java Generics included in Java 1.5.
Generics:history
3

• The Java compiler uses a technique called type erasure to translate
generic classes into executable code.
• Type erasure eliminates all generic type information at compile time.
• All the type information between angle brackets is thrown out so, for example,
a parameterized type like List<String> is converted into a List raw type.
• All remaining uses of type variables are replaced by the upper bound of the
type variable (or Object if there is no type bound).
• Whenever the resulting code isn’t type-correct, a cast to the appropriate type
is automatically inserted.
Javagenerics:implementation
4

• While Java generics syntactically look like C++ templates and are used to achieve
the same purpose, it is important to note that they are not implemented using
the same concepts, nor do they provide the same programming features.
• Java generics simply provide compile-time type safety and eliminate the need for
explicit casts when using type-abstract types and algorithms.
• Java generics use a technique known as type erasure, and the compiler keeps
track of the generic definitions internally, hence using the same class definition at
compile/run time.
• A C++ template on the other hand use template metaprogramming, by which
whenever a template is instantiated with a new type parameter, the entire code
for the template is generated adapted to the type parameter and then compiled,
hence having several definitions for each template instance at run time.
Javagenericsvs.C++templates
5

• A class or method that is defined
with a parameter for a type is
called a generic class/method or a
parameterized class/method
• For classes, the type parameter is
included in angular brackets after
the class name in the class
definition heading.
• For methods, the type parameter is
included before the method
definition.
• Methods can define/use additional
type parameters additional to their
class’ type parameters.
• The type parameters are to be used
like other types used in the
definition of a class/method.
• When a generic class is used, the
specific type to be plugged in is
provided in angular brackets.
• When a generic method is called,
its call’s parameter/return type are
plugged in.
Genericclasses/methods:definition
6
/**
* Generic class that defines a wrapper class around a single
* element of a generic type.
*/
public class Box<T extends Number> {
private T t;
public void set(T t) {
this.t = t;
}
public T get() {
return t;
}
/**
* Generic method that uses both the generic type of the class
* it belongs to, as well as an additional generic type that is
* bound to the Number type.
*/
public void inspect(){
System.out.println("T: " + t.getClass().getName());
}
public <U> void inspectWithAdditionalType(U u){
System.out.println("U: " + u.getClass().getName());
}
public static void main(String[] args) {
Box<Integer> integerBox = new Box<Integer>();
integerBox.set(new Integer(10));
integerBox.inspect();
integerBox.inspectWithAdditionalType("Hello world");
Integer i = integerBox.get();
}
}

• When the class is compiled, type
erasure is applied on the type
parameter for each specific use of
the generic class or method:
• Every occurrence of the type
parameter is replaced with the
highest type applicable to the type
parameter.
• If a type bound was specified, this
type is applied. If no type bound
was specified, Object is used.
• If a value is extracted from a
generic class or returned from a
generic method that is of the type
parameter type, its type is
automatically casted to the type
used at instantiation.
Genericclasses/methods:typeerasure
7
/**
* Generic class that defines a wrapper class around a single
* element of a generic type.
*/
public class Box {
private Number t;
public void set(Number t) {
this.t = t;
}
public Number get() {
return t;
}
/**
* Generic method that uses both the generic type of the class
* it belongs to, as well as an additional generic type that is
* bound to the Number type.
*
*/
public void inspect(){
}
public void inspectWithAdditionalType(Object u){
System.out.println("U: " + u.getClass().getName());
}
Box integerBox = new Box();
integerBox.set(new Integer(10));
integerBox.inspect();
integerBox.inspectWithAdditionalType("Hello world");
Integer i = integerBox.get();
}
}

• So, what is the difference between a generic class and a class defined using
Object as the internal type? Consider a LinkedList class that can contain
elements of type Object:
• This seems interesting, until we get the elements from the list:
• As the elements are of type Object, we must explicitly cast them to use them as
objects of their own type after extraction.
• Do you see any problem with that?
Genericclasses:benefit
8
LinkedList list = new LinkedList();
list.add("abc"); // fine
list.add(new Date()); // fine as well
String s = (String)list.get(0); // cast required
Date d = (Date)list.get(1); // cast required

• The problem is that the compiler cannot check at compile time whether such
casts are valid or not. Upon execution, depending on what was actually stored as
the elements of the list, the runtime system might throw a
ClassCastException if the explicit casts are invalid.
• Using generic classes, we can define such a LinkedList and parameterize it for
every specific use and ensuring type safety for each different use of the generic
class:
• Thus, generic classes and the type erasure mechanism allow the programmer to:
• Define classes that are valid in different contexts of use.
• Ensure that they are used correctly in each specific context of use.
Genericclasses:benefit
9
LinkedList<String> stringList = new LinkedList<String>();
stringList.add("Hello"); // fine
// list.add(new Date(1,1)); // error
String s = stringList.get(0); // no cast needed
LinkedList<Integer> integerList = new LinkedList<Integer>();
integerList.add(new Integer(10)); // fine
// integerList.add("Hello"); // error
Integer i = integerList.get(0); // no cast needed

• When defining a generic class/method, a type bound can be stated
on any type parameter.
• A type bound can be any reference type, i.e. a class type that is
used to further describe a type parameter. It restricts the set of
types that can be used as type arguments and gives access to the
non-static methods of the type it mentions.
• A type parameter can be unbounded. In this case any reference type can be
used as type argument to replace the unbounded type parameter in an
instantiation of a generic type.
• Alternatively can have one or several bounds. In this case the type argument
that replaces the bounded type parameter in an instantiation of a generic type
must be a subtype of all bounds.
Generics:parametertypebounds
10

• The syntax for specification of type parameter bounds is:
i.e. a list of bounds consists of one class and/or several interfaces.
• The reason for imposing a type bound on a type parameter is that the code used
in the implementation code of the generic class is assuming that the type used is
of a certain type or any of its subtypes, or that it implements a certain interface.
• This can lead to rather complex class declarations such as:
• A pair class taking two parameters, where each parameter is of a certain type
that implements Comparable with itself and implements Cloneable.
• The class itself implements Comparable with itself and implements
Cloneable, i.e. instances of this class are Comparable with each other and are
Cloneable and the same is assumed of both parts of the Pair.
Generics:parametertypebounds
11
<TypeParameter extends AClass & AnInterface1 & ... & AnInterfaceN>
class Pair<A extends Comparable<A> & Cloneable ,
B extends Comparable<B> & Cloneable >
implements Comparable<Pair<A,B>>, Cloneable { ... }

• In the add() method:
• The method is generic over two type parameters A and B, which must be
subclasses of Number, which allows to use doubleValue().
• The argument to the method is a Pair; the type arguments to that Pair are
constrained by the type parameter bounds to add().
• In the swap() method:
• The type parameters are used to define the return type, as well as the
argument.
• Local variables in the method are declared in terms of the type parameters.
• The type parameters are used as type arguments in the constructor call.
Generics:parametertypes,interestingcases
12
class PairUtil {
public static <A extends Number, B extends Number> double add(Pair<A, B> p) {
return p.getFirst().doubleValue() + p.getSecond().doubleValue();
}
public static <A, B> Pair<B, A> swap(Pair<A, B> p) {
A first = p.getFirst();
B second = p.getSecond();
return new Pair<B, A>(second, first);
}
}
public class Pair <X, Y> {
private final X a;
private final Y b;
public Pair(X a, Y b) {
this.a = a;
this.b = b;
}
public X getFirst() {
return a;
}
public Y getSecond() {
return b;
}
}

• A wildcard is a syntactic construct that is used to denote a family of
types in a generic class/method instantiation. There are three
different kinds of wildcards:
• " ? " : the unbounded wildcard. It stands for the family of all types.
• " ? extends SuperType " : a wildcard with an upper bound. It stands for
the family of all types that are SuperType itself or subtypes of SuperType.
• " ? super SubType " - a wildcard with a lower bound. It stands for the
family of all types that are SubType itself or supertypes of SubType.
JavaGenerics:wildcards
13
B
C D
A
E
F G
? extends B
? super G

• The method printCollection() receives a parameter that is a Collection
of elements of any type. It can do so because it does not apply any type-specific
operation on the Collection it receives as a parameter.
• The method copy() receives as parameters two lists, where the type of
elements in the source List must be a subtype of type of elements in the
destination List. Failure to impose such a restriction may allow to attempt to
copy the elements of a list into a list of elements of an unrelated type, leading to
an illegal type cast. It must do so because it uses get() and set(), which are
bound to the type of values stored in the List.
JavaGenerics:wildcards
14
public void printCollection( Collection<?> c ){
for (Object o : c){
System.out.println(o);
}
}
public static <T> void copy( List<? super T> dest, List<? extends T> src) {
for (int i=0; i<src.size(); i++)
dest.set(i,src.get(i));
}
}

• If a generic class is referred to without using any type parameter, it refers to a
raw type, i.e. a class that has been subject to type erasure, taking the generic
type’s uppermost allowable type as type argument. In the example above, a raw
Basket is a Basket of Object.
• The class hierarchies of the parameter types are not transposed onto the generic
classes that use them as parameters.
• Wildcards must be used in a context where they can be verified to instantiate to
a specific type at runtime, e.g. new Basket<?>() is invalid as it attempts to
create a Basket containing a value of unknown type.
Genericclasses:instantiationvs.typeparameter
15
Basket b = new Basket(); // OK but using raw type!
Basket b1 = new Basket<Fruit>(); // OK but using raw type!
Basket<Fruit> b2 = new Basket<Fruit>(); // OK !
// Type mismatch: cannot convert from Basket<Fruit> to Basket<Apple>
Basket<Apple> b3 = new Basket<Fruit>(); // WRONG !!!
// Type mismatch: cannot convert from Basket<Apple> to Basket<Fruit>
Basket<Fruit> b4 = new Basket<Apple>(); // WRONG !!!
Basket<?> b5 = new Basket<Apple>(); // OK!
// 1. Cannot instantiate the type Basket<?>
// 2. Type mismatch: cannot convert from Basket<?> to Basket<Apple>
Basket<Apple> b6 = new Basket<?>(); // WRONG !!!
class Basket<E> {...}
class Fruit {...}
class Apple extends Fruit {...}
class Orange extends Fruit {...}

Javagenericclassexample:genericPairclass
16
/**
* Immutable generic pair class
*/
public class Pair<TypeOfFirst, TypeOfSecond>{
private final TypeOfFirst first;
private final TypeOfSecond second;
public Pair(){}
public Pair(TypeOfFirst first, TypeOfSecond second){
this.first = first;
this.second = second;
}
public Pair(Pair<TypeOfFirst, TypeOfSecond> newPair){
this.first = newPair.getFirst();
this.second = newPair.getSecond();
}
public TypeOfFirst getFirst() {
return this.first;
}
public TypeOfSecond getSecond() {
return this.second;
}
public String toString(){
return first.getClass().getName() + ":" + first.toString() + " , "
+ second.getClass().getName() + ":" + second.toString();
}
}

Javagenericclassexample:genericPairclass
17
public class PairDriver {
Pair<String, Integer> p1 = new Pair<String, Integer>("Hello", 1);
System.out.println(p1);
ArrayList<Integer> v1 = new ArrayList<Integer>();
for (int x = 1; x <= 3; x++)
v1.add(new Integer(x));
ArrayList<String> v2 = new ArrayList<String>();
v2.add(new String("un"));
v2.add(new String("deux"));
v2.add(new String("trois"));
ArrayList<Pair<Integer, String>> v3 = new ArrayList<Pair<Integer, String>>();
for (int x = 0; x <= 2; x++)
v3.add(new Pair<Integer, String>(v1.get(x), v2.get(x)));
for (Pair<Integer, String> p : v3)
System.out.println(p);
}
} java.lang.String:Hello , java.lang.Integer:1
java.lang.Integer:1 , java.lang.String:un
java.lang.Integer:2 , java.lang.String:deux
java.lang.Integer:3 , java.lang.String:trois

Javagenericclassexample:genericcomparablePairclass
18
public class ComparablePair<TypeOfFirst extends Comparable<TypeOfFirst>, TypeOfSecond extends Comparable<TypeOfSecond>>
implements Comparable<ComparablePair<TypeOfFirst, TypeOfSecond>> {
private TypeOfFirst first;
private TypeOfSecond second;
public ComparablePair() {}
public ComparablePair(TypeOfFirst first, TypeOfSecond second) {
this.first = first;
this.second = second;
}
public ComparablePair(ComparablePair<TypeOfFirst, TypeOfSecond> newComparablePair) {
this.first = newComparablePair.getFirst();
this.second = newComparablePair.getSecond();
}
public String toString() {
return first.getClass().getName() + ":" + first.toString() + " , "
+ second.getClass().getName() + ":" + second.toString();
}
public TypeOfFirst getFirst() { return this.first; }
public TypeOfSecond getSecond() { return this.second; }
public int compareTo(ComparablePair<TypeOfFirst, TypeOfSecond> otherComparablePair) {
int compareFirst = first.compareTo(otherComparablePair.getFirst());
int compareSecond = second.compareTo(otherComparablePair.getSecond());
if (compareFirst != 0) {
return compareFirst;
} else {
return compareSecond;
}
}
}

Javagenericclassexample:genericcomparablePairclass
19
public class ComparablePairDriver {
ArrayList<ComparablePair<Integer, String>> alcp = new ArrayList<ComparablePair<Integer, String>>();
alcp.add(new ComparablePair<Integer, String>(3,"trois"));
alcp.add(new ComparablePair<Integer, String>(4,"quatre"));
alcp.add(new ComparablePair<Integer, String>(1,"un"));
alcp.add(new ComparablePair<Integer, String>(1,"one"));
alcp.add(new ComparablePair<Integer, String>(1,"one"));
ComparablePair<Integer, String> previousalcp = null;
for (ComparablePair<Integer, String> p : alcp) {
System.out.println(p);
if (previousalcp != null) System.out.println(p.compareTo(previousalcp));
previousalcp = new ComparablePair<Integer, String>(p);
}
}
}
java.lang.Integer:3 , java.lang.String:trois
java.lang.Integer:4 , java.lang.String:quatre
1
java.lang.Integer:1 , java.lang.String:un
-1
java.lang.Integer:1 , java.lang.String:one
-6
java.lang.Integer:1 , java.lang.String:one
0

• Angelika Langer. Java Generics FAQ
• Gilad Bracha. Generics in the Java Programming Language.
• Paul Gibson. Generics (in Java)
• David R. Musser, Alexander A. Stepanov. Generic Programming. In International
Symposium on Symbolic and Algebraic Computation (ISSAC 1988). Lecture Notes
in Computer Science 358, Springer-Verlag, 1989, pp 13-25.
• Ronald Garcia, Jaakko Jarvi, Andrew Lumsdaine, Jeremy G. Siek, and Jeremiah
Willcock. A Comparative Study of Language Support for Generic Programming.
SIGPLAN Not. 38, 11 (October 2003), 115-134. doi=10.1145/949343.949317
• Charles W. Krueger. Software reuse. ACM Comput. Surv. 24, 2 (June 1992), 131-
183. doi=10.1145/130844.130856
• Ross Tate, Alan Leung, Sorin Lerner. Taming Wildcards in Java’s Type System.
Technical Report. Cornell University.
• Joseph A. Goguen. Parameterized Programming. IEEE Trans. Softw. Eng. 10, 5
(September 1984), 528-543. doi=10.1109/TSE.1984.5010277
References
20

• Mads Torgersen, Christian Plesner Hansen, Erik Ernst, Peter von der Ahé, Gilad
Bracha, and Neal Gafter. Adding wildcards to the Java programming language. In
Proceedings of the 2004 ACM symposium on Applied computing (SAC '04). ACM,
New York, NY, USA, 1289-1296. doi=10.1145/967900.968162
• java.boot.by. SCJP Tiger Study Guide. Collections/Generics.
• java2novice.com. Java Generics Sample Code.
References
21

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