January 21, 2009 Generic Classes in Java
Department of Computer Science
Java Generics
E. Steegmans
K.U.Leuven
Overview
concepts
 Basic
erasure
 Type
Wild cards

Inheritance

Generic methods

Eric Steegmans - K.U.Leuven 1

January 21, 2009 Generic Classes in Java
Non-Generic Classes
No formal arguments involved in definitions of

classes and interfaces
Object is often used as the most general type for

variables, method arguments and return types
- Typical example: Container classes in Java API
Common practice before Java 1.5

Non-Generic Classes: Usage
List employees = new ArrayList();
Person albert = …;
Parrot filip = …;
employees.add(albert);
employees.add(filip);
// No compile-time checks nor runtime checks.
Person laurent = (Person) employees.get(1);
// Virtual machine throws ClassCastException.
Eric Steegmans - K.U.Leuven 2

January 21, 2009 Generic Classes in Java
Generics
Parameterized definitions of classes and

interfaces
Only reference types as formal arguments

- No other types such as integers, …
Convention: single letter arguments such as E

(element), T (type), K (key), …
Generic Classes
public class ArrayList
extends AbstractList
implements List, Cloneable, Serializable
{
public Arraylist();
public boolean add(Object elem);
public Object get(int index);
public boolean remove(Object o);
public boolean contains(Object o);
public Object[] toArray();
public Iterator iterator();
public List subList(int from, int to);
public boolean addAll(Collection c);
public boolean containsAll(Collection c);
public Class getClass();
public Object[] toArray(Object[] a);
…
}
Eric Steegmans - K.U.Leuven 3

January 21, 2009 Generic Classes in Java
Generic Classes
public class ArrayList<E>
{
public Arraylist();
public boolean add( E elem);
public E get(int index);
…
}
Generic Classes
public class ArrayList<E>
{
public Arraylist();
public boolean add(E elem);
public E get(int index);
public boolean remove(Object o);
public boolean contains(Object o);
public Object[] toArray();
…
}
Eric Steegmans - K.U.Leuven 4

January 21, 2009 Generic Classes in Java
Generic Classes
public class ArrayList<E>
extends AbstractList<E>
implements List<E>, Cloneable, Serializable
{
public Arraylist();
public boolean add(E elem);
public E get(int index);
public boolean remove(Object o);
public boolean contains(Object o);
public Object[] toArray();
public Iterator<E> iterator();
public List<E> subList(int from, int to);
…
}
Generic Classes
public class ArrayList<E>
extends AbstractList<E>
implements List<E>, Cloneable, Serializable
{
public Arraylist();
public boolean add(E elem);
public E get(int index);
public boolean remove(Object o);
public boolean contains(Object o);
public Object[] toArray();
public Iterator<E> iterator();
public List<E> subList(int from, int to);
public boolean addAll(Collection<? extends E> c);
public boolean containsAll(Collection<?> c);
public Class<? extends Object> getClass();
…
}
Eric Steegmans - K.U.Leuven 5

January 21, 2009 Generic Classes in Java
Generic Classes
public class ArrayList<E>
extends AbstractList<E>
implements List<E>, Cloneable, Serializable
{
public Arraylist();
public boolean add(E elem);
public E get(int index);
public boolean remove(Object o);
public boolean contains(Object o);
public Object[] toArray();
public Iterator<E> iterator();
public List<E> subList(int from, int to);
public boolean addAll(Collection<? extends E> c);
public boolean containsAll(Collection<?> c);
public Class<? extends Object> getClass();
public <T> T[] toArray(T[] a);
…
}
Generics: Usage
List<Person> employees =
new ArrayList<Person>();
Person albert = …;
Parrot filip = …;
employees.add(albert);
employees.add(filip);
// Compile-time checks.
Person laurent = employees.get(1);
// No type cast needed.
Eric Steegmans - K.U.Leuven 6

January 21, 2009 Generic Classes in Java
Generics: Advantages
Correctness 

Compile-time checks 

Complexity 

Type casts 

Readability 

Static information 

Overview
concepts
 Basic
erasure
 Type
cards
 Wild
Inheritance

Generic methods

Eric Steegmans - K.U.Leuven 7

January 21, 2009 Generic Classes in Java
Type Erasure
The Java compiler removes all information related

to generic types
The Java Virtual Machine only knows of “raw types”

- All instances of all instantiations of a generic class (interface)
belong to the same class (interface)
Advantage: binary compatibility

The introduction of generics does not require any

changes to the Java Virtual Machine
- All Java applications can still run on the same virtual machine
Type Erasure: Consequences
public class GenericClass<T> {
private T[] elems = new T[12];
// At the time an array is created, the
// type of its elements must be supplied.
}
Eric Steegmans - K.U.Leuven 8

January 21, 2009 Generic Classes in Java
Type Erasure: Consequences
public class GenericClass<T> {
private T[] elems = new T[12];
public void someMethod(Object elem) {
if (elem instanceof T);
// No runtime type information (RTTI)
// available concerning generic args.
}
}
Type Erasure: Consequences
public class GenericClass<T> {
private T[] elems = new T[12];
public void someMethod(Object elem) {
if (elem instanceof T);
new T();
// No information concerning constructor
// of generic argument.
}
}
Eric Steegmans - K.U.Leuven 9

January 21, 2009 Generic Classes in Java
Type Erasure: Consequences
public class GenericClass<T> {
private T[] elems = new T[12];
public void someMethod(Object elem) {
if (elem instanceof T);
new T();
T theElement = (T) elem;
// Unchecked cast from Object to T.
}
}
Type Erasure: Conclusion
Java 1.4: Run-time type safety

Java programs may include attempts to assign values of

improper type
- Most attempts are already caught at compile-time
During the execution of a program, a variable can never

store a value of the wrong type
- The virtual machine checks the correctness of type casts
Eric Steegmans - K.U.Leuven 10

January 21, 2009 Generic Classes in Java
Type Erasure: Conclusion
Java 1.5: No runtime type safety for generic types

Without proper care

- Instantiations of generic classes may store elements of the
wrong type
- It is not impossible for a list of persons to store other objects such
as bank accounts, books and trees
- Instantiations of generic classes may return objects of a type
that differs from the return type
With proper care

- A generic class can make it impossible for clients to store
elements of the wrong type
- Methods of a generic class always return objects of the type
they promise
Overview
Basic concepts

erasure
 Type
cards
 Wild
 Inheritance
Generic methods

Eric Steegmans - K.U.Leuven 11

January 21, 2009 Generic Classes in Java
Polymorphism in Java 1.4
If S is a subtype of T, an object of type S can be

assigned to a variable of type T
Objects of type Woman and objects of type Man can be

assigned to variables of type Person
Can we assign an array of type S[] to a variable

of type T[]
Polymorphism in Java 1.5
Can we assign an object of type G<S> to a

variable of type G<T>?
Assuming G is a generic class or interface, and S is a

true subtype of T
- Can we assign an object of type List<Woman> to a variable of
type List<Person>?
Eric Steegmans - K.U.Leuven 12

January 21, 2009 Generic Classes in Java
Unbounded Wild Cards
A variable of type G<?> is a supertype for any

instantiation of the generic class or interface G
A variable of type List<?> can be assigned objects of

type List<Person>, List<Integer>, ...
The object referenced by a variable of type G<?>

is more or less read-only
Methods of G<E> with arguments of type E cannot be

invoked against variables of type G<?>
- Invocations of methods without arguments of type E may still
cause state changes
Variables of type G<?> act as if the class or interface is

instantiated with Object
- Invocations of methods declared to return an object of type E,
return an object of type Object
Bounded Wild Cards
A variable of type G<? extends X> is a super-

type for instantiations with type X or subtype of X
A variable of type List<? extends Person> can be

assigned objects of type List<Man>, List<Woman>, ..
This type of wild card specifies an upper bound for

actual types at the time of instantiation
The object referenced by a variable of type

G<? extends X> is to some extent read-only
Methods of G<E> with arguments of type E cannot be

invoked against variables of type G<? extends X>
Variables of type G<? extends X> act as if the class

or interface is instantiated with X
Eric Steegmans - K.U.Leuven 13

January 21, 2009 Generic Classes in Java
Bounded Wild Cards
A variable of type G<? super X> is a supertype

for any instantiation with type X or supertype of X
A variable of type List<? super Man> can be

assigned objects of type List<Man>, List<Person>
This type of wild card specifies a lower bound for actual

types at the time of instantiation
The object referenced by a variable of type

G<? super X> is to some extent write-only
Methods of G<E> with actual argument of type E can

only be invoked with actual arguments of type X
Invocations of methods declared to return an object of

type E, return an object of type Object
Raw Types
A variable of type G is a supertype for any

instantiation of the generic class or interface G
A variable of type List can be assigned objects of type

List<Account>, List<Person>
Be careful in approaching generic instantiations by

means of raw types!
Methods of G<E> with actual argument of type E can be

invoked against variables of type G
- The Java compiler only issues warnings for possible type
mismatches
Invocations of methods declared to return an object of

type E, return an object of type Object
Eric Steegmans - K.U.Leuven 14

January 21, 2009 Generic Classes in Java
Generic Arguments: Bounds
A generic class or interface G<T extends C>

can only be instantiated with C or a subtype of C
All methods of class or interface C can be invoked

against variables of type T
Multiple bounds on generic arguments are

possible as in G<T extends X & Y>
Wild Cards: Conclusion
Wild cards are needed in polymorphic

assignments only because of type erasure
Other languages such as C++, C# and Eiffel do not

need wild cards in variable declarations
Approaching generic instantiations with raw types is

asking for (big) problems
Wild cards are useful in specifying restrictions on

generic arguments
Eric Steegmans - K.U.Leuven 15

January 21, 2009 Generic Classes in Java
Overview
Basic concepts

Type erasure

cards
 Wild
 Inheritance
methods
 Generic
Generic Subclasses
A generic class may extend a single class and

may implement several interfaces
Superclasses and superinterfaces may be instantiations

of generic classes, resp. generic interfaces
- Typically, formal arguments of the generic subclass are used to
instantiate generic superclasses and superinterfaces
A generic interface may extend several interfaces

Eric Steegmans - K.U.Leuven 16

January 21, 2009 Generic Classes in Java
Polymorphism
If S<E> inherits from T<E>, can we assign an

object of type S<C> to a variable of type T<C>?
Can an object of type SortedSet<Integer> be

assigned to a variable of type Set<Integer>?
Frameworks
Generics (can) considerably improve the quality of

frameworks
Unfortunately they may also considerably increase the

complexity of frameworks
Wild cards are not powerful enough to express all

restrictions
Eric Steegmans - K.U.Leuven 17

January 21, 2009 Generic Classes in Java
The Composite Pattern
Overview
Basic concepts

Type erasure

Wild cards

 Inheritance
methods
 Generic
Eric Steegmans - K.U.Leuven 18

January 21, 2009 Generic Classes in Java
Generic Methods: Definition
A generic method is parameterized in one or more

types
As for generic classes, generic methods can only be

parameterized in reference types
- The generic parameters precede the return type of the method
Generic arguments can express dependencies

among types of ordinary formal arguments
Bounded wildcards in both directions are used for that

purpose
The actual types of generic methods are derived

from their actual arguments
The compiler rejects invocations if no matching type is

available
Eric Steegmans - K.U.Leuven 19