How to generify a Collection of Foo?

Question

Hi! I'm about to convert some legacy code to use generics. However, i'm puzzled by the following piece of code (out of my memory so please bear with me):
public SomeClass {
  public static void doSomething(Collection foos) {
    for (Iterator it = foos.iterator(); it.hasNext();) {
      Foo foo = (Foo) it.next();
      foo.doSomething();
}I have tried to change this to:
public SomeClass {
  public static void doSomething(Collection<Foo> foos) {
    for (Foo foo : foos) {
      foo.doSomething();
}Looks ok to me. However, i found the following uses of the method:
public class Bar extends Foo {
  public void doIt() {
    List bars = // create List of Bar...
    SomeClass.doSomething(bars);
}I was thinking of using Collection<? extends Foo> as parameter for SomeClass.doSomething, but will this work for Lists (or Sets)?
I'm thankful for any advice.
Markus

Answer 1

Howlingmad wrote:
public class Bar extends Foo {
public void doIt() {
List bars = // create List of Bar...
SomeClass.doSomething(bars);
Be sure to keep that List generic as well!
I was thinking of using Collection<? extends Foo> as parameter for SomeClass.doSomething, yes, you are on the right track. It should be like this:
class SomeClass {
    public static void doSomething(Collection<? extends Foo> foos) {
        for (Foo foo : foos) {
            foo.doSomething();
class Foo {
    void doSomething() {}
class Bar extends Foo {
    void doIt() {
        List<Bar> bars = new ArrayList<Bar>();
        SomeClass.doSomething(bars);
but will this work for Lists (or Sets)?Yes, it will work for any type of Collection of Foo's (a Bar is also a Foo, of course).
I'm thankful for any advice.
Markus