Main method with generic parameter; why does it work?

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Category:Languages
public static <T extends String> void main(T[] args) {     System.out.println("Hello World!"); } 

I was curious to see if the above snippet of code would compile and run successfully, and it does! However, I also wondered what would happen if T extends String was replaced with T extends String & AutoClosable; String does not implement AutoClosable, so I didn't expect this to run successfully, but it still does!

public static <T extends String & AutoCloseable> void main(T[] args) {     System.out.println("This still works!"); } 

So my question is, why does this still run successfully?

Notes:

  • I'm testing this with Java 10.0.1
  • Intellij doesn't play nice with this method because it doesn't see it as an entry point to the program; I haven't tested it with other IDEs.
  • You're also able to pass arguments using the command-line just as you would with any other program.

 


It's because a generic parameter has a bound:

<T extends String>  <T extends String & AutoCloseable> 

And the bytecode after erasure is the same as for the regular main definition in both cases:

public static main([Ljava/lang/String;)V 

JLS §4.4. Type Variables:

The order of types in a bound is only significant in that the erasure of a type variable is determined by the first type in its bound, and that a class type or type variable may only appear in the first position.

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