GJ Specication

GJ Specication
Gilad Bracha, Sun Microsystems
Martin Odersky, University of South Australia
David Stoutamire, Sun Microsystems
Philip Wadler, Lucent Technologies
May 1998
1 Summary
We propose to add generic types and methods to the JavaTM programming language.
These are both explained and implemented by translation into the current language. The
translation closely mimics the way generics are emulated by programmers: In a nutshell,
it erases all type parameters, maps type variables to their bounds, and inserts casts as
needed. Some subtleties of the translation are caused by the handling of overriding.
The main benet of GJ over the current Java programming language lies in the added
expressiveness and safety that stems from making type parameters explicit and making
type casts implicit. This is crucial for using libraries such as collections in a
exible, yet
safe way.
GJ is designed to be fully backwards compatible with the current language, making the
transition from non-generic to generic programming very easy. In particular, one can
retrot existing library classes with generic interfaces without changing their code.
This paper gives a specication for GJ. It has a companion paper which gives an overview
and rationale of GJ's concepts [BOSW98]. The present paper is organized as follows.
Section 2 explains how parameterized types are declared and used. Section 3 explains
polymorphic methods. Section 4 explains how parameterized types integrate with exceptions.
Section 5 explains what changes in the Java programming language's expression
constructs. Section 6 explains how GJ is translated into the Java virtual machine (JVM).
Section 7 explains how generic type information is stored in classles. Where possible,
we follow the format and conventions the Java Language Specication (JLS) [GLS96].
2 Types
There are two new forms of types in GJ, parameterized types and type variables.
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2.1 Type Syntax
A parameterized type consists of a class or interface type C and a parameter section
hT1; : : : ; Tni. C must be the name of a parameterized class or interface, the types in the
parameter list hT1; : : : ; Tni must match the number of declared parameters of C, and
each actual parameter must be a subtype of the formal parameter's bound type.
In the following, whenever we speak of a class or interface type, we include the parameterized
version as well, unless explicitly excluded.
A type variable is an unqualied identier. Type variables are introduced by parameterized
class and interface declarations (Section 2.2) and by polymorphic method declarations
(Section 3.1).
Syntax (see JLS, Sec. 4)
ReferenceType ::= ClassOrInterfaceType
| ArrayType
| TypeVariable
TypeVariable ::= Identifier
ClassOrInterfaceType ::= ClassOrInterface TypeArgumentsOpt
ClassOrInterface ::= Identifier
| ClassOrInterfaceType . Identifier
TypeArguments ::= < ReferenceTypeList >
ReferenceTypeList ::= ReferenceType
| ReferenceTypeList , ReferenceType
Example 1 Parameterized types.
Vector<String>
Seq<Seq<A>>
Seq<String>.Zipper<Integer>
Collection<Integer>
Pair<String,String>
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// Vector<int> -- illegal, primitive types cannot be parameters
// Pair<String> -- illegal, not enough parameters
// Pair<String,String,String> -- illegal, too many parameters
2.2 Parameterized Type Declarations
A parameterized class or interface declaration denes a set of types, one for each possible
instantiation of the type parameter section. All parameterized types share the same class
or interface at runtime. For instance, the code
Vector<String> x = new Vector<String>();
Vector<Integer> y = new Vector<Integer>();
return x.getClass() == y.getClass();
will yield true.
Syntax (see JLS, Secs. 8.1, 9.1)
ClassDeclaration ::= ModifiersOpt class Identifier TypeParametersOpt
SuperOpt InterfacesOpt ClassBody
InterfaceDeclaration ::= ModifiersOpt interface Identifier
TypeParametersOpt ExtendsInterfacesOpt
InterfaceBody
TypeParameters ::= < TypeParameterList >
TypeParameterList ::= TypeParameterList , TypeParameter
| TypeParameter
TypeParameter ::= TypeVariable TypeBoundOpt
TypeBound ::= extends ClassType
| implements InterfaceType
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The type parameter section follows the class name and is delimited by < > brackets. It
denes one or more type variables that act as parameters. Type parameters have an
optional class or interface type as a bound; if the bound is missing, java.lang.Object
is assumed. The scope of a type parameter is all of the declared class, except any static
members or initializers, but including the type parameter section itself. Therefore,
type parameters can appear as parts of their own bounds, or as bounds of other type
parameters declared in the same section.
Example 2 Mutually recursive type variable bounds.
interface ConvertibleTo<A> {
A convert();
}
class ReprChange<A implements ConvertibleTo<B>,
B implements ConvertibleTo<A>> {
A a;
void set(B x) { a = x.convert(); }
B get() { return a.convert(); }
}
Parameterized declarations can be nested inside other declarations.
Example 3 Nested parameterized class declarations.
class Seq<A> {
A head;
Seq<A> tail;
Seq() { this(null, null); }
boolean isEmpty() { return tail == null; }
Seq(A head, Seq<A> tail) { this.head = head; this.tail = tail; }
class Zipper<B> {
Seq<Pair<A,B>> zip(Seq<B> that) {
if (this.isEmpty() || that.isEmpty())
return new Seq<Pair<A,B>>();
else
return new Seq<Pair<A,B>>(
new Pair<A,B>(this.head, that.head),
this.tail.zip(that.tail));
}
}
}
class Client {{
Seq<String> strs =
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new Seq<String>("a", new Seq<String>("b", new Seq<String>()));
Seq<Number> nums =
new Seq<Number>(new Integer(1),
new Seq<Number>(new Double(1.5),
new Seq<Number>()));
Seq<String>.Zipper<Number> zipper = strs.new Zipper<Number>();
Seq<Pair<String,Number>> combined = zipper.zip(nums);
}}
2.3 Handling Consecutive Type Parameter Brackets
Consecutive type parameter brackets < and > do not need to be separated by whitespace.
This leads to a problem in that the lexical analyzer will map the two consecutive
closing angle brackets in a type such as Vector<Seq<String>> to the right-shift symbol
>>. Similarly, three consecutive closing angle brackets would be recognized as a unary
right-shift symbol >>>. To make up for this irregularity, we rene the grammar for types
and type parameters as follows.
Syntax (see JLS, Sec. 4)
ReferenceType ::= ClassOrInterfaceType
| ArrayType
| TypeVariable
ClassOrInterfaceType ::= Name
| Name < ReferenceTypeList1
ReferenceTypeList1 ::= ReferenceType1
| ReferenceTypeList , ReferenceType1
ReferenceType1 ::= ReferenceType >
| Name < ReferenceTypeList2
ReferenceTypeList2 ::= ReferenceType2
| ReferenceTypeList , ReferenceType2
ReferenceType2 ::= ReferenceType >>
| Name < ReferenceTypeList3
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ReferenceTypeList3 ::= ReferenceType3
| ReferenceTypeList , ReferenceType3
ReferenceType3 ::= ReferenceType >>>
TypeParameters ::= < TypeParameterList1
TypeParameterList1 ::= TypeParameter1
| TypeParameterList , TypeParameter1
TypeParameter1 ::= TypeParameter >
| TypeVariable extends ReferenceType2
| TypeVariable implements ReferenceType2
2.4 Subtypes, Supertypes, Member Types
In the following, assume a class declaration C with parameters A1; :::;An which have
bounds S1; :::; Sn. That class declaration denes a set of types ChT1; :::; Tni, where each
argument type Ti ranges over all subtypes of the corresponding bound type. That is,
each Ti is a subtype of
Si[A1 := T1; :::;An := Tn]
where [A := T] denotes substitution of the type variable A with the type T.
The denitions of subtype and supertype are generalized to parameterized types. Given
a class or interface declaration for ChA1; :::;Ani, the direct supertypes of the parameterized
type ChA1; :::;Ani are
 the type given in the extends clause of the class declaration if an extends clause
is present, or java.lang.Object otherwise, and
 the set of types given in the implements clause of the class declaration if an implements
clause is present.
The direct supertypes of the type ChT1; :::; Tni are DhU1; :::;Uki, where
 DhU1; :::;Uki is a direct supertype of ChA1; :::;Ani, and
  is the substitution [A1 := T1; :::;An := Tn].
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The supertypes of a type are obtained by transitive closure over the direct supertype
relation. The subtypes of a type T are all types U such that T is a supertype of U.
Subtyping does not extend through parameterized types: T a subtype of U does not imply
that ChTi is a subtype of ChUi. To support translation by type erasure, we impose
the restriction that a class may not directly or indirectly implement an interface twice
such that the two implementations have dierent parameters. Hence, every superclass
and implemented interface of a parameterized type can be augmented by parameterization
to exactly one supertype. Here is an example of an illegal multiple inheritance of
an interface:
class B implements I<Integer>
class C extends B implements I<String>
A consequence of the parameterized types concept is that now the type of a class member
is no longer xed, but depends on the concrete arguments substituted for the class parameters.
Here are the relevant denitions. Assume again a class or interface declaration
of C with parameters A1; :::;An.
 Let M be a member declaration in C, whose type as declared is T. Then the type
of M in the type ChT1; :::; Tni is T[A1 := T1; :::;An := Tn].
 Let M be a member declaration in a supertype of ChT1; :::; Tni. Then the type of
M in ChT1; :::; Tni is the type of M in that supertype.
2.5 Raw Types
To facilitate interfacing with non-generic legacy code, it is also possible to use as a type
the erasure of a parameterized class without its parameters. Such a type is called a raw
type. Variables of a raw type can be assigned from values of any of the type's parametric
instances. For instance, it is possible to assign a Vector<String> to a Vector. The
reverse assignment from Vector to Vector<String> is unsafe from the standpoint of
the generic semantics (since the vector might have had a dierent element type), but is
still permitted in order to enable interfacing with legacy code. In this case, compilers
for GJ will issue a warning message that the assignment is deprecated.
The superclasses (respectively, interfaces) of a raw type are the raw versions of the
superclasses (interfaces) of any of its parameterized instances.
The type of a member declaration M in a raw type C is its erased type (see Section 6.1).
However, to make sure that potential violations of GJ's typing rules are always
agged,
some accesses to members of a raw type will result in \unchecked" warning messages.
The rules for generating unchecked warnings for raw types are as follows:
 A method call to a raw type generates an unchecked warning if the erasure changes
the argument types.
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 A eld assignment to a raw type generates an unchecked warning if erasure changes
the eld type.
No unchecked warning is required for a method call when only the result type changes,
for reading from a eld, or for a class instance creation of a raw type.
The supertype of a class may be a raw type. Member accesses for the class are treated
as normal, and member accesses for the supertype are treated as for raw types. In the
constructor of the class, calls to super are treated as method calls on a raw type.
Example 4 Raw types.
class Cell<A>
A value;
Cell (A v) { value=v; }
A get() { return value; }
void set(A v) { value=v; }
}
Cell x = new Cell<String>("abc");
x.value; // OK, has type Object
x.get(); // OK, has type Object
x.put("def"); // deprecated
3 Polymorphic Methods
3.1 Method Declarations
Syntax (See JLS 8.4)
MethodHeader :
ModifiersOpt TypeParametersOpt Type MethodDeclarator ThrowsOpt
| ModifiersOpt TypeParametersOpt VOID MethodDeclarator ThrowsOpt
Method declarations can have a type parameter section like classes have. The parameter
section precedes the result type of the method.
Example 5 Polymorphic methods.
static <Elem> void swap(Elem[] a, int i, int j) {
Elem temp = a[i]; a[i] = a[j]; a[j] = temp;
}
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void <Elem implements Comparable<Elem>> sort(Elem[] a) {
for (int i = 0; i < xs.length; i++)
for (int j = 0; j < i; j++)
if (a[j].compareTo(a[i]) < 0) <Elem>swap(a, i, j);
}
class Seq<A> {
<B> Seq<Pair<A,B>> zip(Seq<B> that) {
if (this.isEmpty() || that.isEmpty())
return new Seq<Pair<A,B>>();
else
return new Seq<Pair<A,B>>(
new Pair<A,B>(this.head, that.head),
this.tail.<B>zip(that.tail));
}
}
The denition of \having the same arguments" is extended to polymorphic methods as
follows:
Two method declarations M and N have the same arguments if either none of them
has type parameters and their argument types agree, or they have the same number of
type parameters, say hA1; :::;Ani for M and hB1; :::;Bni for N, and after renaming each
occurrence of a Bi in N0s type to Ai the bounds of corresponding type variables are the
same and the argument types of M and N agree.
It is illegal to declare two methods with the same name in a class if these methods have
the same argument types in some instantiation of the class.
3.2 Overriding
The denition of overriding is adapted straightforwardly to parameterized types:
A class or interface ChA1; :::;Ani may contain a declaration for a method with the same
name and the same argument types as a method declaration in one of the supertypes
of ChA1; :::;Ani. In this case, the declaration in C is said to (directly) override the
declaration in the supertype.
GJ requires that the result type of a method is a subtype of the result types of all
methods it overrides. This is more general than the Java programming language, which
requires the result types to be identical. See Section 6.2 for an implementation scheme
to support this generalization.
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Example 6 The following declarations are legal in GJ.
class C implements Cloneable {
C copy() { return (C)clone(); }
...
}
class D extends C implements Cloneable {
D copy() { return (D)clone(); }
...
}
4 Exceptions
To enable a direct mapping into the JVM, type parameters are not allowed in catch
clauses or throws lists. It is still possible to have parameterized extensions of Throwable
but one uses the raw class name of such an extension in a catch clause or throws list.
The name of a parameterized class in a throws list indicates that any of its instances
might be thrown. Analogously, the name of a parameterized class in a catch clause will
cover any parameterized instance of the class.
Example 7 Parameterized exceptions.
class ExcParam<A> extends Exception {
A value;
ExcParam(A value) { this.value = value; }
}
class ExcInteger extends ExcParam<Integer> {
ExcInteger (int i) { super(new Integer(i)); }
int intvalue () { return value.intvalue(); }
}
class Test {
void throwExc (int i) throws ExcParam, ExcInteger {
throw i==0 ? new ExcParam<String>("zero") : new ExcInteger(i);
}
int tryExc (int i) {
try {
throwExc(i)
} catch (ExcInteger ex1) {
return ex1.intvalue();
} catch (ExcParam ex2) {
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return 0;
}
}
}
5 Expressions
5.1 Class Instance Creation Expressions
A class instance creation expression for a parameterized class consists of the fully parameterized
type of the instance to be created and arguments to a constructor for that
type.
Syntax (see JLS, Sec. 15.8)
ClassInstanceCreationExpression ::= new Name TypeArgumentsOpt ( ArgumentListOpt
)
Example 8 Class instance creation expressions.
// with explicit type parameters
new Vector<String>();
new Pair<Seq<Integer>,Seq<String>>(
new Seq<Integer>(new Integer(0), new Seq<Integer>()),
new Seq<String>("abc", new Seq<String>()));
5.2 Array Creation Expressions
The element type in an array creation expression is a fully parameterized type. It is
deprecated to create an array whose element type is a type variable.
Syntax (see JLS, Sec. 15.9)
ArrayCreationExpression ::= new PrimitiveType DimExprs DimsOpt
| new ClassOrInterfaceType DimExprs DimsOpt
| new PrimitiveType Dims ArrayInitializer
| new ClassOrInterfaceType Dims ArrayInitializer
Example 9 Array creation expressions.
new Vector<String>[n]
new Seq<Character>[10][20][]
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5.3 Cast Expressions
The target type for a cast can be a parameterized type.
Syntax (see JLS, Sec. 15.15)
CastExpression ::= ( PrimitiveType DimsOpt ) UnaryExpression
| ( ReferenceType ) UnaryExpressionNotPlusMinus
The usual rules for casting conversions (Spec, Sec. 5.5) apply. Since type parameters are
not maintained at run-time, we have to require that the correctness of type parameters
given in the target type of a cast can be ascertained statically. This is enforced by
rening the casting conversion rules as follows:
A value of type S can be cast to a parameterized type T if one of the following two
conditions holds:
 T is a subtype of S, and there are no other subtypes of S with the same erasure
(see Section 6.1) as T.
 T is a supertype of S.
Note that even when parameterized subtypes of a given type are not unique, it will
always be possible to cast to the raw type given by their commmon erasure.
Example 10 Assume the declarations
class Dictionary<A,B> extends Object { ... }
class Hashtable<A,B> extends Dictionary<A, B> { ... }
Dictionary<String,Integer> d;
Object o;
Then the following are legal:
(Hashtable<String,Integer>)d // legal, has type: Hashtable<String,Integer>
(Hashtable)o // legal, has type: Hashtable
But the following are not:
(Hashtable<Float,Double>)d // illegal, not a subtype
(Hashtable<String,Integer>)o // illegal, not unique subtype
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5.4 Type Comparison Operator
Type comparison can involve parameterized types. The rules of casting conversions, as
dened in Section 5.3, apply.
Syntax (see JLS, Sec. 15.19)
RelationalExpression
::= ...
| RelationalExpression instanceof Type
Example 11 Type comparisons.
class Seq<A> implements List<A> {
static boolean isSeq(List<A> x) {
return x instanceof Seq<A>
}
static boolean isSeq(Object x) {
return x instanceof Seq
}
static boolean isSeqArray(Object x) {
return x instanceof Seq[]
}
}
Example 12 Type comparisons and type casts with type constructors.
class Pair<A, B> {
A fst; B snd;
public boolean equals(Object other) {
return
other instanceof Pair &&
equals(fst, ((Pair)other).fst) &&
equals(snd, ((Pair)other).snd);
}
private boolean equals(Object x, Object y) {
return x == null && y == null || x != null && x.equals(y);
}
}
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5.5 Polymorphic Method Invocation
Polymorphic method invocations do not have special syntax. Type parameters of polymorphic
methods are elided; they are inferred from value parameters according to the
rules given in Section 5.6.
Syntax (See JLS 15.11)
MethodInvocation ::= MethodExpr ( ArgumentListOpt )
MethodExpr ::= MethodName
| Primary . Identifier
| super . Identifier
Example 13 Polymorphic method calls with implicit type parameters (see Example 5)
swap(ints, 1, 3) <Integer>swap(ints, 1, 3)
sort(strings) <String>sort(strings)
strings.zip(ints) strings.<Integer>zip(ints)
Optional Extension. If desired, one can also admit explicit parameterization for
methods. In that case, the parameter section of a polymorphic method invocation
would immediately precede the method name.
Syntax (See JLS 15.11)
MethodInvocation ::= MethodExpr ( ArgumentListOpt )
MethodExpr ::= MethodId
| Primary . MethodId
| super . MethodId
MethodId ::= TypeArgumentsOpt MethodName
Example 14 Polymorphic method calls with explicit type parameters (see Example 5)
<Integer>swap(ints, 1, 3)
<String>sort(strings)
strings.<Integer>zip(ints)
The convention of passing parameters before the method name is made necessary by
parsing constraints: with the more conventional \type parameters after method name"
convention the expression f (a<b,c>(d)) would have two possible parses. The alternative
of using [ ] brackets for types poses other problems.
5.6 Type Parameter Inference
Type parameters of a call to a polymorphic method are inferred from the call's value
parameters. To make inference work, we make use of a bottom type Null, which is
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a subtype of every reference type. This type already exists in the Java programming
language as the type of null. We postulate that the subtyping relationship between
Null and reference types is promoted through constructors: If T and U are identical
types except that where T has occurrences of Null U has occurrences of other reference
types, then T is a subtype of U.
Type parameter inference inserts the most specic parameter types such that
1. each actual argument type is a subtype of the corresponding formal argument
type,
2. no type variable that occurs more than once in the method's result type is instantiated
to a type containing Null.
It is an error if most specic parameter types do not exist or are not unique.
Example 15 Type parameter inference.
Assume the polymorphic method declarations:
static <A> Seq<A> nil() { return new Seq<A>(); }
static <A> Seq<A> cons(A x, Seq<A> xs) { return new Seq<A>(x, xs); }
Then the following are legal expressions:
cons("abc", nil()) // of type: Seq<String>
cons(new IOException(), cons(new Error(), nil()))
// of type: Seq<Throwable>
nil(); // of type: Seq<Null>
cons(null, nil()); // of type: Seq<Null>
The second restriction above needs some explanation. Note that a covariance rule applies
to types containing Null. For any type context TC, type U, TC[Null] is a subtype of
TC[U]. General covariance leads to unsound type systems, so we have to argue carefully
that our type system with its restricted form of covariance is still sound. The soundness
argument goes as follows: since one cannot declare variables of type TChNulli, all one
can do with a value of that type is assign or pass it once to a variable or parameter of
some other type. There are now three possibilities, depending on the variable's type:
 The variable's type is an unparameterized supertype of the raw type TC. In this
case the assignment is clearly legal.
 The variable's type is TC0hTi for some type T and supertype TC0 of TC. Now,
the only value of Null is null, which is also a value of every reference type T.
Hence, any value of type TChNulli will also be a value of type TC0hTi, so the
assignment is sound.
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 The variable is a parameter p whose type is a type variable, A. Then code that
accesses p in the method body is insensitive to the type of the actual parameter, so
the method body itself cannot give rise to type errors. Furthermore, by restriction
(2.) above, the method's formal result type will contain at most one occurrence of
A, so the actual type of the method application is again of the form TC0hNulli,
where TC0 is a type context.
Without restriction (2.) type soundness would be compromised, as is shown by the
following example.
Example 16 An unsafe situation for type parameter inference.
Pair<A,A> duplicate(A x) { return new Pair<A,A>(x, x); }
void crackIt(Pair<Seq<String>,Seq<Integer>> p) {
p.fst.head := "hello";
Integer i = p.snd.head;
}
crackIt(duplicate(cons(null, nil()))); // illegal!
This will eectively assign a String to an Integer. The problem is that the duplicate
method returns the same value under two type parameters which then get matched
against dierent types. I.e.
duplicate(cons(null, nil()))
has type
Pair<Seq<Null>,Seq<Null>>
but the two Seq<Null> parameters really stand for the same object, hence it is unsound
to widen these types to dierent Seq types. The GJ compiler will report an error for
the call
crackIt(duplicate(cons(null, nil())));
The error message will state that an uninstantiated type parameter appears several
times in the result type of a method.
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6 Translation
In the following we explain how GJ programs are translated to JVM bytecodes. In a
nutshell, the translation proceeds by erasing all type parameters, mapping type variables
to their bounds, and inserting casts as needed. Some subtleties of the translation are
caused by the handling of overriding.
6.1 Translation of Types
As part of its translation process, a GJ Compiler will map every parameterized type to
its type erasure. Type erasure is a mapping from GJ types to conventional (unparameterized)
types. We write jTj for the erasure of type T. The erasure mapping is dened
as follows.
 The erasure of a parameterized type ChT1; :::; Tni is C.
 The erasure of a nested type T:C is jTj:C.
 The erasure of an array type T[] is jTj[].
 The erasure of a type variable is the erasure of the type variable's bound.
 The erasure of every other type is the type itself.
6.2 Translation of Methods
Each method T m(T1; :::; Tn) is translated to a method with the same name whose return
type and argument types are the erasures of the corresponding types in the original
method. In addition, if a method declaration T m(T1; :::; Tn) is inherited (and possibly
overridden) in an extension where a type parameter of the class or interface is instantiated,
such that the method now has a dierent type erasure, a bridge method will be
generated. The type of the bridge method is the type erasure of the method in the base
class or interface. In the bridge method's body all arguments to the method will be cast
to their type erasures in the extending class, after which the call will be forwarded to
the original method or an overriding instance.
Example 17 Bridge methods.
class C<A> { abstract A id(A x); }
class D extends C { String id(String x) { return x; } }
This will be translated to:
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class C { abstract Object id(Object x); }
class D extends C {
String id(String x) { return x; }
Object id(Object x) { return id((String)x);
}
Note that the translation scheme can produce methods with identical names and argument
types, yet with dierent result types, all declared in the same class. Here's an
example:
Example 18 Bridge methods with the same parameters as normal methods.
class C<A> { abstract A next(); }
class D extends C<String> { String next() { return ""; } }
This will be translated to:
class C { abstract Object next(); }
class D extends C<String> {
String next/*1*/() { return ""; }
Object next/*2*/() { return next/*1*/(); }
}
A Java compiler would reject that program because of the double declaration of next.
But the bytecode representation of the program is legal, since the bytecode always
refers to a method via its the full signature and therefore can distinguish between the
two occurrences of next. Since we cannot make the same distinction in the Java source,
we resorted to indices in /* ... */ comments to make clear which method a name refers
to.
The same technique is used to implement method overriding with covariant return
types1.
Example 19 Overriding with covariant return types.
class C { C dup(){...} }
class D extends C { D dup(){...} }
This translates to:
class C { C dup(); }
class D {
D dup/*1*/(){...}
C dup/*2*/(){ return dup/*1*/(); }
}
1covariant return types were at some time before version 1.0 part of the Java programming language
but got removed later
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Since our translation of methods erases types, it is possible that dierent methods with
identical names but dierent types are mapped to methods with the same type erasure.
Such a case, if it arises, is considered to be an error in the original GJ program. There
are three rules to prevent signature clashes caused by the translation. Say a method
declaration M indirectly overrides a method declaration M0 if there is a (possibly empty)
path of method declarations
M = M0;M1; :::;Mn = M0
such that Mi overrides Mi+1. Then one must have:
Rule 1 Methods declared in the same class with the same name must have dierent
erasures.
Rule 2 If a method declaration M in class C has the same name and type erasure as
a method declaration M0 in a superclass D of C then M in C must indirectly override
M0 in D.
Rule 3 If a method declaration M in a superclass D of C has the same name and type
erasure as a method declaration M0 in an interface I implemented by C then there must
be a method declaration M00 in C or one of its superclasses that indirectly overrides M
in D and implements M0 in I.
Example 20 Name clash excluded by Rule 2.
class C<A> { A id (A x) {...} }
class D extends C<String> {
Object id(Object x) {...}
}
This is illegal since C.id and D.id have the same type erasure, yet D.id does not override
C.id. Hence, Rule 2 is violated.
Example 21 Name clash excluded by Rule 3.
class C<A> { A id (A x) {...} }
interface I<A> { A id(A x); }
class D extends C<String> implements I<Integer> {
String id(String x) {...}
Integer id(Integer x) {...}
}
This is also illegal, since C.id and I.id have the same type erasure yet there is no
single method in D that indirectly overrides C.id and implements I.id. Hence, Rule 3
is violated.
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6.3 Translation of Expressions
Expressions are translated unchanged except that casts are inserted where necessary.
There are two situations where a cast needs to be inserted.
1. Field access where the eld's type is a type parameter. Example:
class Cell<A> { A value; A getValue(); }
...
String f(Cell<String> cell) {
return cell.value;
}
Since the type erasure of cell.value is java.lang.Object, yet f returns a String, the
return statement needs to be translated to
return (String)cell.value;
2. Method invocation, where the method's return type is a type parameter. For instance,
in the context of the above example the statement
String x = cell.getValue();
needs to be translated to:
String x = (String)cell.getValue();
7 The Signature Classle Attribute
Classles need to carry GJ's additional type information in a backwards compatible way.
This is accomplished by introducing a new \Signature" attribute for classes, methods
and elds. The structure of this attribute is as follows:
"Signature" (u4 attr-length, u2 signature-index)
When used as an attribute of a method or eld, a signature gives the full GJ type of
that method or eld. When used as a class attribute, a signature indicates the type
parameters of the class, followed by its supertype, followed by all its interfaces.
The type syntax in signatures is extended to parameterized types and type variables.
There is also a new signature syntax for formal type parameters. The syntax extensions
for signature strings are as follows:
Syntax
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MethodOrFieldSignature ::= TypeSignature
ClassSignature ::= ParameterPartOpt
super_TypeSignature interface_TypeSignatures
TypeSignatures ::= TypeSignatures TypeSignature
|
TypeSignature ::= ...
| ClassTypeSignature
| MethodTypeSignature
| TypeVariableSignature
ClassTypeSignature ::= 'L' Ident TypeArgumentsOpt ';'
| ClassTypeSignature '.' Ident ';' TypeArgumentsOpt
MethodTypeSignature ::= TypeArgumentsOpt '(' TypeSignatures ')' TypeSignature
TypeVariableSignature ::= 'T' Ident ';'
TypeArguments ::= '<' TypeSignature TypeSignatures '>'
ParameterPart ::= '<' ParameterSignature ParameterSignatures '>'
ParameterSignatures ::= ParameterSignatures ParameterSignature
|
ParameterSignature ::= Ident ':' bound_TypeSignature
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References
[BOSW98] Gilad Bracha, Martin Odersky, David Stoutamire, and Philip Wadler. Making
the future safe for the past: Adding Genericity to the Java Programming
Language. Submiited to OOPSLA98, 1998.
[GLS96] James Gosling, Bill Joy, and Guy Steele. The Java language specication. Java
Series, Sun Microsystems, ISBN 0-201-63451-1, 1996.
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