Generics and use with Vector

[Francesco Sisca]

i have a simple question...

this is a simple class implements a simple interface

public class PallinoNome implements PincoPallino {
	private String pallino;

	public PallinoNome() {
		this.setPallino("pallino");
	}

	@Override
	public String getPallino() {
		return this.pallino;
	}

	@Override
	public void setPallino(String pallino) {
		this.pallino = pallino;
	}

}

this is a simple interface

public interface PincoPallino {
	public String getPallino();

	public void setPallino(String pallino);
}

this is the main

import java.util.Vector;

public class StarterClass {

	public static void main(String[] args) {
		Vector<PallinoNome> pinchipallini = new Vector<PallinoNome>();
		aggiungiPallino(pinchipallini);
	}

	private static void aggiungiPallino(Vector<PincoPallino> pinchipallini) {
		// TODO Auto-generated method stub
	}
}

give me this error

The method aggiungiPallino(Vector<PincoPallino>) in the type StarterClass 
is not applicable for the arguments (Vector<PallinoNome>)

who tells me because i get this error and how i can solve?

[Matthew Brown]

Hi Francesco. Welcome to the Ranch!

You can't assign a Vector<PallinoNome> to a Vector<PincoPallino>. The problem is that if you did, you'd then be able to add instances of other classes that implement PincoPallino into a collection that was originally supposed to be a Vector<PallinoNome>. So you'd lose the type safety that generics are supposed to give you.

The general rule is, even if A is-a B, a GenericClass<A> is-not-a GenericClass<B>.

Your options to get round this are:

- Use either PincoPallino or PallinoNome as your generic type everywhere
- Make your method generic, and use a bounded type parameter
- Use a wildcard type parameter

Which is best depends on what you're trying to achieve, but the second and third options are more complicated. There's much more information about how they work in the Java tutorials at http://docs.oracle.com/javase/tutorial/java/generics/index.html

By the way, Vector is considered a legacy class nowadays. I'd strongly recommend using ArrayList instead.

[Francesco Sisca]

i use this solution.

import java.util.Vector;

public class StarterClass {

	public static void main(String[] args) {
		Vector<PallinoNome> pinchipallini = new Vector<PallinoNome>();
		aggiungiPallino(pinchipallini);
	}

	private static <E extends PincoPallino> void aggiungiPallino(
			Vector<E> pinchipallini) {

	}

}

thanks a lot for suggest