Method overloading with generics

Question

Where I try to create two static overloading methods I got an compilation error. Can anyone explain this?

public class A {
 public static void a(Set<String> stringSet) {}
 public static void a(Set<Map<String,String>> mapSet) {}
}

Answer 1

The reason is type erasure. Generics are not stored in the classes, they are compile-time info only, so at runtime, the two methods are identical and hence there is a naming conflict.

Reference

• Generics section of the Sun Java Tutorial
• Java Generics and Collections (Book)

These three methods are actually identical (read: they produce identical bytecode):

public static void a(Set plainSet) {}
public static void a(Set<String> stringSet) {}
public static void a(Set<Map<String,String>> mapSet) {}

If you really want to have two separate methods, you must provide different method signatures (e.g. different method names, an additional parameter for one of the methods etc.)

Answer 2

From the point of view of the methods parameters Set<String> and Set<Map<String,String>> are the same, because all instances of a generic class have the same run-time class (Set in your case), regardless of their actual type parameters. Therefore you will get a erasure error. Also at runtime both will look like... public static void a(Set stringSet) {} AND public static void a(Set mapSet) {}

Answer 3

You got the compiler error because the methods are not overloaded properly. Both methods have a parameter of Set type that makes both the methods identical for he compiler.