A Type-Passing Approach for the Implementation of Parametric Methods in Java

class List { Object head; List tail; List(Object head, List tail){ this.head=head; this.tail=tail; } void setHead(Object head){this.head=head;} void setTail(List tail){this.tail=tail;} Object getHead(){ return head;} List getTail(){ return tail;} void setIfPossible(Object o){ if ($snip1(o)) head=$snip2(o); } abstract boolean $snip1(Object o); abstract Object $snip2(Object o);boolean $snip1(Object o); abstract Object $snip2(Object o); } interface $List$_String_${} class $$List$_String_$ extends List implements $List$_String_${ $$List$_String_$(Object head, List tail){ super(head,tail); } boolean $snip1(Object o){ return o instanceof String; } Object $snip2(Object o){ return (String)o;} } // Object o=new List("1",null); Object o=new $$List$_String_$("1",null); // List l=(List)o; List l=(List)(($List$_String_$)o); FIGURE 3. NextGen translation of class List. by the GJ compiler. However, from the draft specification on generics released [26], it is clear that Sun is open to having future versions of the compiler extending the current implementation of GJ so as to tackle the issue of exact types. In fact, some solutions have been proposed to avoid the problem. Basically, they extend the behaviour of GJ so that the necessary information on the instantiation of the type parameters is somehow accessible at run-time. In this way, these proposals are able to apply type-casts and instance tests to parametric types and type variables as well, supporting the true integration of parametric polymorphism in Java.