PL Seminar Spring 2002

1
Making the future safe for the past (time
paradox)Adding Genericity to the Java
Programming Language
Gilad Bracha, Martin Odersky, David Stoutamire,
Philip Wadler
Appeared in OOPSLA 98
Presented by Anand Krishnan
Center for Distributed Object Computing Department
of Computer Science Washington University
February 13, 2002
2
Javas current support for genercity

• Generics by the idiom
• replace variable types by the top of type
  hieararchy
• Requires type casts
• Observer pattern introduced JDK 1.1
• Collection Classes introduced JDK 1.2
• JSR-000014 Adding Generics to the JavaTM
  Programming Language
• http//jcp.org/jsr/detail/14.jsp

3
Example uses of type parameterization

• Stack
• stacks of T objects
• Hashtable
• the T object must be hashable
• Priority Queue
• the T object must provide a consistent ordering
  relation
• Mix-in
• Deriving from a parameterized Base Class

4
Generic Legacy Problem

• Impact on legacy code due to addition of direct
  support for generics
• Pizza - backward compatible
• Pizza - Incompatibility between legacy type and
  parametric type
• rewrite legacy code or write adapter code
• problems include
• code bloat
• managing upgrade of dispersed legacy code
• legacy code available only as binaries

5
Styles of implementing Generics

• Syntactic instance - Stack Stack
• Semantic instance - data structure and compiled
  code associated with a syntactic instance

• Homogeneous
• the generic idiom - Instances of parameterized
  construct share same implementation
• possibilty of security breach
• Heterogeneous
• Data structure and compiled code associated with
  a syntactic instance
• different run time representations for methods
• support type parameterization in languages where
  all types cannot be referred to in a uniform way.
• Choice depends on language semantics
• eg. Parameterized class with a static variable

6
A GJ Example
interface ConvertibleTo A
convert()
interface ConvertibleTo B
convert()
class ReprChange,
B implements
ConvertibleTo
A a void set(B x) a x.convert() B
get() return a.convert()

• Bound
• Mutually Recursive

7
Translating GJ

• erasure
• parametric type delete the parameter
• non-parametric type - do nothing
• type parameter erase the bound
• erase all arg types and return types
• insert type casts where required
• insert bridge methods to ensure proper overriding
• bridging done so that bridge and original method
  share the same name

8
GJ - the bytecode meddler
class Interval implements Iterator
private int i, n public Interval (int l,
int u) i l n u public boolean
hasNext () return (i Integer next () return new Integer(i)
UponTranslation
interface Iterator public boolean hasNext
() public Object next ()
class Interval implements Iterator private
int i, n public Interval (int l, int u) i
l n u public boolean hasNext ()
return (i / () return new Integer(i) public
Object next / 2 / () return next / 1 /()

9
Covariant Overriding
class C implements Cloneable public C copy
() return (C)this.clone () class D extends
C implements Cloneable public D copy ()
return (D)this.clone ()
10
Invariant Subtyping
List ! Subtype( List )
String Subtype (Object )
11
Polymorphic Methods and Type Inference
public LinkedList singleton (A x)
public LinkedList doublet (A x, A y)

• Inference - choose the smallest type parameter
  that yields a valid call
• call to doublet with Integer and Float - A
  inferred as Number
• call to doublet with Integer and String -
  Inference Fails (common supertypes - Comparable
  and Serializable)
• Special Bottom Type - null type
• call to doublet with Integer and - A inferred
  as Integer

12
Security Implications
Problem
class SecureChannel extends Channel public
String read () public void add
(SecureChannel x) . class C
public LinkedList cs
...
13
Security Implications
Solution
Problem
class SecureChannelList extends
LinkedList
SecureChannelList () super()
public void add (SecureChannel x)
super.add(x) class C
public LinkedList cs
...
class SecureChannel extends Channel public
String read () public void add
(SecureChannel x) . class C
public LinkedList cs
...
14
Security Implications
Solution on Translation
Problem
class SecureChannelList extends LinkedList
SecureChannelList ()
super() public void add (Object
x) super.add((SecureChannel)x)
class C public
LinkedList cs ...
class SecureChannel extends Channel public
String read () public void add
(SecureChannel x) . class C
public LinkedList cs
...
15
Heterogeneous Translation -divorced from the JVM
security model

• JVM Security Model
• visibilty for top-level classes package wide
• public visibility

p.C
16
Heterogeneous Translation -divorced from the JVM
security model

• JVM Security Model
• visibilty for top-level classes package wide
• public visibility

p.C
- if D is public, place instantiation in p
17
Heterogeneous Translation -divorced from the JVM
security model

• JVM Security Model
• visibilty for top-level classes package wide
• public visibility

p.C
- if D is public, place instantiation in p
- else if body of C refers only to public
classes - place instantiation in q
- else cannot instantiate type
18
GJ Restrictions - Object and Array Creation

• For a type variable A
• new A () - illegal
• new An
• generates an unchecked warning
• indicates possible violation of GJs type
  soundness
• rather use Vector or ArrayList
• Use an array as a factory for arrays of similar
  type

19
GJ Restrictions - Object and Array Creation
class BadArray public static A
singleton (A x) return new A x
// unchecked warning public static
void main (String args) String a
singleton("zero")
20
GJ Restrictions - Casts Instance tests
class Convert public static
Collection up (LinkedList xs)
return (Collection)xs
Collection
LinkedList
public static LinkedList down
(Collection xs) if (xs instanceof
LinkedList) return
(LinkedList)xs else
throw new ConvertException()
21
GJ Restrictions - Casts Instance tests
class BadConvert public static Object
up (LinkedList xs) return
(Object)xs
Object
LinkedList
public static LinkedList down
(Object o) if (o instanceof
LinkedList) return
(LinkedList)o else
throw new ConvertException() // red
indicates compile time error
Solution - LinkedListString class extending
LinkedList - Wrapper class
LinkedListStringWrapper
22
Raw Types

• Parameterized Types stripped of its parameters
• Maintain consistency with legacy code
• Useful in cast and instance tests

Signature of member of object of Raw Type
Erasure of signature of member of object of
Parameterized Type

Value of parameterized Type is a Value of
Corrseponding RawType For eg Iterator
Param_Instance Iterator Raw_Instance
Param_Instance
Value of Raw Type is a Value of Corrseponding
Parameterized Type For eg Iterator
Raw_Instance Iterator Param_Instance
Raw_Instance //unchecked warning
23
Raw Types

• Method calls/Field assignments to Raw Types
  generate unchecked warning if erasure changes
  argument/field type

class Loophole public static String
loophole (Byte y) LinkedList xs
new LinkedList() LinkedList ys
xs ys.add(y) // unchecked warning
return xs.iterator().next()
24
Retrofitting Mode

• Retro-actively parameterize classes
• Attach generic types to prexisting class files,
  even if binaries
• Add signature attributes to Code

class LinkedList implements Collection
public LinkedList () public void add (A
elt) public Iterator iterator ()

• Rewriting of code can be done conveniently
• May involve recoding in certain cases
• Map method returning Key/Value has to be
  split

25
Other Flavors (thanks to Martin Linenweber)

• Poor mans genericity for java
• wrapper script for Java interpreter
• works during class loading
• MIT proposal
• homogeneous translation scheme
• change to JVM
• maintain runtime representation of parameteric
  types
• avoid casts
• backward compatible through raw types
• Stanford Proposal
• heterogeneous translation
• generic code compiled to special byte code format
• GJ expands on this with reflection

26
GJ Conclusions

• Easy to interface with legacy code
• backward compatible through raw types
• straightforward use of reflection on GJ programs
• forward compatible through retrofitting
• made forward compatible with NextGen
• maintains parameters at run time
• relaxes GJs restrictions

27
Questions ?
The Asimov Gang of Four
Making Java easier to type / 1 /and easier to
type / 2 /