Generic method return type problem

[Louie Cheung]

i wrote this code myself and i couldn't get it to compile with the <? super TestArrayList> method argument type, <? extends TestArrayList> doesn't work either is it because im adding to the list inside the method?, it seems that the problem is from the return type? please advice

import java.util.*;
 
public class TestArrayList {
  String a;
  public static void main(String[] args) {
    ArrayList<TestArrayList> output = new ArrayList<TestArrayList>();
    TestArrayList t1 = new TestArrayList("one");
    TestArrayList t2 = new TestArrayList("two");
    TestArrayList t3 = new TestArrayList("three");
    output.add(t1);
    output.add(t2);
    output.add(t3);
    System.out.println(output);
    output = processList(output);
    System.out.println(output);
  }
  public String toString(){
    return this.a;
  }
  TestArrayList(String s) {
    this.a = s;
  }
  TestArrayList() {}
  public static ArrayList<TestArrayList> processList(ArrayList<? super TestArrayList> input){
    TestArrayList t4 = new TestArrayList("four");
    input.add(t4);
    TestArrayList c1 = new child("Child one");
    input.add(c1);
    return input;
  }
}
 
class child extends TestArrayList {
  String a;
  child(String s) {
    super();
    this.a = s;    
  }
  public String toString(){
    return this.a;
  }
}

indenting repaired by Bu.
[ September 03, 2007: Message edited by: Burkhard Hassel ]

[Burkhard Hassel]

Howdy!

The problem lies here:

public static [B]ArrayList<TestArrayList>[/B] processList(
[B]ArrayList<? super TestArrayList> input[/B]) {
TestArrayList t4 = new TestArrayList("four");
input.add(t4);
TestArrayList c1 = new child("Child one");
input.add(c1);
return [B]input[/B];
}

processList's method declaration states that it will return an
ArrayList<TestArrayList>
but you tried to return an array list of unknown.
This is not possible.

Either change the return type to <? super TestArrayList> or the paramter type to
processList(ArrayList<TestArrayList> input) { ...



By the way, Louie,
I repaired the indenting of your original posting for the sake of readability.

Service of the house, you are a new rancher.

Welcome to the Ranch!






To indent your code properly by yourself:

At the posting page, mark the part of your posting that should be indented. Usually that will be your code (or some tabellaric output).

Hit the - Button below.
that's it!


Yours,
Bu.

[Louie Cheung]

thanks
i tried to change the return type to <? extends TestArrayList> but still i get compiler error like this:

java.util.ArrayList<capture of ? super TestArrayList>
output = processList(output);
^

why? i know changing the parameter to <TestArrayList> would work but i want to know how the wildcard works here, thanks a bunch

[Burkhard Hassel]

Louie Cheung tried

to change the return type to <? extends TestArrayList>

super...

Bu.

[Louie Cheung]

Originally posted by Burkhard Hassel:
Louie Cheung tried

super...

Bu.

yes i tried, and super too Burkhard Hassel

[Tony Smith]

Alright, I took a look and figurered out the problem:

You probably have this as the signature now:

public static ArrayList<? super TestArrayList> processList(ArrayList<? super TestArrayList> input)

but still the compiler complains about

output = processList(output);
^

Now go take a look at the output declaration:
ArrayList<TestArrayList> output

The reason that compiler complains is that ouput should be only refer to Type TestArrayList but the method return from processList is TestArrayList or its super types. Which creates conflict. One quick way to fix is also change the declaration of output to:
ArrayList<? super TestArrayList> output = new ArrayList<TestArrayList>();

or strip ? super from all of them

[Louie Cheung]

thanks burkhard/tony