Method has the same erasure as another method in type

Question

Why is it not legal to have the following two methods in the same class   class Test     void add Set lt Integer gt  ii       void add Set lt String gt  ss        I get the compilation error      Method add Set  has the same erasure add Set  as another method in type Test    while I can work around it  I was wondering why javac doesn t like this   I can see that in many cases  the logic of those two methods would be very similar and could be replaced by a single   public void add Set lt   gt  set      method  but this is not always the case   This is extra annoying if you want to have two constructors that takes those arguments because then you can t just change the name of one of the constructors

User · Answer

Java generics uses type erasure  The bit in the angle brackets   lt Integer gt  and  lt String gt   gets removed  so you d end up with two methods that have an identical signature  the add Set  you see in the error   That s not allowed because the runtime wouldn t know which to use for each case   If Java ever gets reified generics  then you could do this  but that s probably unlikely now

User · Answer

The problem is that Set lt Integer gt  and Set lt String gt  are actually treated as a Set from the JVM  Selecting a type for the Set  String or Integer in your case  is only syntactic sugar used by the compiler  The JVM can t distinguish between Set lt String gt  and Set lt Integer gt

User · Answer

This is because Java Generics are implemented with Type Erasure   Your methods would be translated  at compile time  to something like   Method resolution occurs at compile time and doesn t consider type parameters   see erickson s answer   void add Set ii   void add Set ss     Both methods have the same signature without the type parameters  hence the error

User · Answer

Define a single Method without type like void add Set ii     You can mention the type while calling the method based on your choice  It will work for any type of set

User · Answer

It could be possible that the compiler translates Set Integer  to Set Object  in java byte code  If this is the case  Set Integer  would be used only at compile phase for syntax checking

User · Answer

I bumped into this when tried to write something like  Continuable lt T gt  callAsync Callable lt T gt  code         and Continuable lt Continuable lt T gt  gt  callAsync Callable lt Continuable lt T gt  gt  veryAsyncCode        They become for compiler the 2 definitions of Continuable lt  gt  callAsync Callable lt  gt  veryAsyncCode         The type erasure literally means erasing of type arguments information from generics  This is VERY annoying  but this is a limitation that will be with Java for while  For constructors case not much can be done  2 new subclasses specialized with different parameters in constructor for example  Or use initialization methods instead     virtual constructors   with different names     for similar operation methods renaming would help  like   class Test     void addIntegers Set lt Integer gt  ii       void addStrings Set lt String gt  ss        Or with some more descriptive names  self-documenting for oyu cases  like addNames and addIndexes or such

User · Answer

This rule is intended to avoid conflicts in legacy code that still uses raw types   Here s an illustration of why this was not allowed  drawn from the JLS  Suppose  before generics were introduced to Java  I wrote some code like this   class CollectionConverter     List toList Collection c            You extend my class  like this   class Overrider extends CollectionConverter    List toList Collection c            After the introduction of generics  I decided to update my library   class CollectionConverter      lt T gt  List lt T gt  toList Collection lt T gt  c            You aren t ready to make any updates  so you leave your Overrider class alone  In order to correctly override the toList   method  the language designers decided that a raw type was  override-equivalent  to any generified type  This means that although your method signature is no longer formally equal to my superclass  signature  your method still overrides   Now  time passes and you decide you are ready to update your class  But you screw up a little  and instead of editing the existing  raw toList   method  you add a new method like this   class Overrider extends CollectionConverter      Override   List toList Collection c           Override    lt T gt  List lt T gt  toList Collection lt T gt  c            Because of the override equivalence of raw types  both methods are in a valid form to override the toList Collection lt T gt   method  But of course  the compiler needs to resolve a single method  To eliminate this ambiguity  classes are not allowed to have multiple methods that are override-equivalent mdash that is  multiple methods with the same parameter types after erasure   The key is that this is a language rule designed to maintain compatibility with old code using raw types  It is not a limitation required by the erasure of type parameters  because method resolution occurs at compile-time  adding generic types to the method identifier would have been sufficient

[java] Method has the same erasure as another method in type

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