CODEWITHSUNDEEP
javagenericstype-erasure
Lapo
Lapo

Reputation: 992

"both methods have same erasure" error using bounded type parameters

I'm using generics in Java for the first time, and I'm facing an issue I don't manage to overcome: why this compiles:

public interface Aa{}
public interface Bb{}
public interface Cc{}


public static <GenericAB extends Aa & Bb>
void method(GenericAB myABobject1, GenericAB myABobject2){}

public static <GenericAB extends Aa & Bb, GenericCA extends Cc & Aa>
void method(GenericAB myAbobject, GenericCA myCAobject){}

But this does not:

public interface Aa{}
public interface Bb{}
public interface Cc{}


public static <GenericAB extends Aa & Bb>
void method(GenericAB myABobject1, GenericAB myABobject2){}

public static <GenericAB extends Aa & Bb, GenericAC extends Aa & Cc>
void method(GenericAB myAbobject, GenericAC myACobject){}

And I get this error: both methods have same erasure.

I'm sorry if this is a stupid question, but I don't get why the order of interfaces in a bounded type parameter declaration seems to have importance. In reality I don't think it is the order which causes the error, but I don't get what does.

I'm reading this documentation by Oracle, it says which I must put the class as first parameter but Aa, Bb and Cc are all interfaces. Sorry for my english too.

Upvotes: 8

Views: 180

Answers (2)

Michal
Michal

Reputation: 1010

Because at runtime after type erasure both of the methods have the same method header

public static <GenericAB extends Aa & Bb> void method(GenericAB myABobject1, GenericAB myABobject2){}

becomes

public static void method(Aa myABobject1, Aa myABobject2){}

and 

public static <GenericAB extends Aa & Bb, GenericBC extends Aa & Cc>void method(GenericAB myAbobject, GenericBC myBCobject){}

becomes

public static void method(Aa myAbobject, Aa myBCobject){}

both resulting methods have the same signature which results in your error

EDIT after comments below parameters are fixed

Upvotes: 1

Radiodef
Radiodef

Reputation: 37845

It is the order that matters (§4.6):

The erasure of a type variable (§4.4) is the erasure of its leftmost bound.

GenericBC erases to either Aa or Cc, depending on which appears first (i.e. leftmost) in the bound.

Also see type erasure tutorial and type erasure, when and what happens Q&A for explanations of type erasure in general.

Upvotes: 4

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