The Case for Runtime Types in Generic Java

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The Case for Run-time Types in Generic Java

• Eric E. Allen
• Robert Corky Cartwright
• Rice University

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Generic Types What are they and why bother?
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Generic Types What are they and why bother?

• Stack s new Stack()
• Integer i

s.pop()
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Solution 1 Insert Casts

• Stack s new Stack()
• Integer i

(Integer)s.pop()
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Solution 2 Duplicate Code
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Solution 3 Extend the Type System
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Generic Java

• public class VectorltT extends Objectgt
• public T elementAt(int i)
• public class StackltTgt extends VectorltTgt
• public T pop()

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Prior Attempts

• Hetereogeneous solution (copy code for each
  instantiation)
• Modify the JVM
• Type Erasure

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Type Erasure

• Generic types are used only for type checking
• After type checking, all generic types are erased
  to their non-parametric upper bounds

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Type Erasure

• TreeltTgt ? Tree
• TreeltIntegergt ? Tree
• T (in class Tree) ? Object

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Type Erasure Disadvantages

• No support for run-time type operations on
  generic types
• casting
• new operations
• instanceof operations

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NextGen

• Designed in tandem with GJ
• Extended from the GJ compiler
• No substantial overhead when compared to JSR-14
  (Allen, Cartwright, Stoler 2002)
• More expressiveness than GJ/JSR-14

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Type dependent operations are necessary for many
natural object-oriented designs in the context of
static type checking
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Examples

• The Singleton Design Pattern
• Interface Cloneable
• Functional mapping over arrays
• Debuggers and run-time development tools

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The Singleton Pattern
abstract class Tree class Leaf extends Tree
class Branch extends Tree Object value
Tree left Tree right
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The Singleton Pattern
class Leaf extends Tree public static final
Leaf ONLY new Leaf() private Leaf()
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The Singleton Pattern in NextGen
class LeafltTgt extends TreeltTgt public static
final LeafltTgt ONLY new LeafltTgt()
private Leaf()
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The Singleton Pattern in JSR-14
class LeafltTgt extends TreeltTgt public static
final LeafltIntegergt ONLY_INTEGER new
LeafltIntegergt() public static final
LeafltStringgt ONLY_STRING new
LeafltStringgt()
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Interface Cloneable

• Object clone()

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Interface Cloneable

• Vector v2 (Vector)v.clone()

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Interface Cloneable

• VectorltTgt v2 (VectorltTgt)v.clone()

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Functional Mapping over Arrays

• public interface MapperltA,Bgt
• public B map(A element)

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Functional Mapping over Arrays

• public class ArrayMapper
• ltA,Bgt public static B mapArrayltA,Bgt aMapper)
  
• B out new Bin.length
• for (int i 0 i lt in.length i)
• outi aMapper.map(ini)

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Limitations of Debugging with JSR-14

• All information about the instantiations of type
  variables is lost in JSR-14
• Debuggers cant report any information about the
  instantiations

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Limitations of other Run-time Development Tools

• Other testing harnesses (e.g., JUnit) cant test
  the values of these instantiations
• Read-eval-print-loops will display values as
  instances of erased type