New to Generics

[S Chan]

Hi all I am new to Generics but I want to try it out building a class that uses generics. I am stuck at something so I hope someone would give me a hand. Thank you.

Basically, I want the supported type to be a solid class of interface java.lang.Comparable, so I thought may be this would work:

public class MyList<E extends Comparable<?>> implements List<E> {
private E item;
...
}

Then later on, in one of my defined methods, I want the argument to run a method which is defined in the Comparable:

	@Override
	public boolean add(E item) {
		...
		int c = this.item.compareTo(item);
		...
	}

Eclipse complains about the type is not applicable for the arguments (E)

I wonder what is wrong here.

[Paul Clapham]

You know perfectly well what type is going to be passed to the Comparable. So don't put ? there. This is what you want:

MyList<E extends Comparable<E>> implements List<E>

[S Chan]

I see. That solves the problem. Thank you for your help!

Actually before I had ? , I was trying with ? extends E and ? super E .

I had read the documents but didn't quite understand what they are and when to use them.

Can someone be kind and explains them to me in plain English?

[S Chan]

I find it very confusing because I don't know when to use ? and when to use E.

Let's take an example, I have the following:

	@Override
	public boolean containsAll(Collection<?> c) {
		List<?> clones = new ArrayList<?>();
		for (Object i:c) {
			clones.add(i);
		}
		Collections.sort(clones);
		...
	}

I know I shouldn't use new ArrayList<?>() here but how can I make sure clones is a list of ? type (defined by the argument)?

Also, even thought I have

public class SortedList<E extends Comparable<E>> implements List<E>

for the class declaration, why can't I have

 Collections.sort(clones); 

in the containsAll method?

[S Chan]

Would this be a better way of re-writing containsAll?

	@SuppressWarnings("unchecked")
	@Override
	public boolean containsAll(Collection<?> c) {
		List<E> clones = new ArrayList<E>();
		for (Object i:c) {
			if (i instanceof Comparable<?>) {
				clones.add((E)i);
			} else {
				return false;
			}
		}
		Collections.sort(clones);
	...
	}

Need some expert advice here. I am so bad at this... sigh...

[Campbell Ritchie]

Since the E class might not implement Comparable, but might extend a class which implements Comparable, the actual type parameter should be
<E extends Comparable<? super E>>

[Rob Spoor]

Campbell is right. Without the ? super part you wouldn't be able to use java.sql.Timestamp for instance, as it extends java.util.Date and therefore implements Comparable<java.util.Date> - not Comparable<java.sql.Timestamp>.

[S Chan]

Ah! That rings the bell!

Thank you for the explanations and advices!

[Campbell Ritchie]

Well done ... and you're welcome.