CSE 341 -- Java Generics -- Mini-Exercises #2

1. Consider the Wild2.java example. There is a comment that we can't declare the method parameter to be a linked list of Object. (Well, at least with the current main method.) Why not?

2. Suppose that the compiler let this program through. What could go wrong then?

3. Suppose that we did change the method parameter to be a linked list of Object. How could you modify the main method so that this would compile?

For reference, here's the code again:

// CSE 341
// Example of using a wildcard.  Define a method that takes 
// a linked list of ? as a parameter.

import java.util.LinkedList;
import java.util.Iterator;
import java.awt.Point;

class Wild2 {

    // note that we couldn't declare the parameter as LinkedList<Object> !

    public static void printAll(LinkedList<?> s) {
	for (Object e : s) {
	    System.out.println(e);
	}
    }

    // another version using an iterator
    public static void printAll2(LinkedList<?> s) {
	Iterator<?> it = s.iterator();
	while (it.hasNext()) {
	    System.out.println(it.next());
	}
    }

    public static void main(String[] args) {
	LinkedList<Integer> ilist = new LinkedList<Integer>(); 
	ilist.add(Integer.valueOf(3)); 
	ilist.add(Integer.valueOf(5)); 
	ilist.add(null);
	printAll(ilist);
	printAll2(ilist);

	LinkedList<Point> plist = new LinkedList<Point>(); 
	plist.add(new Point(10,20)); 
	printAll(plist);
	printAll2(plist);
    }
}