Collections copy error

[John Jai]

	public static void main(String...args){
		List<String> testList = new ArrayList<String>();
		testList.add("John");
		testList.add("Paul");
		testList.add("Jai");		
		List<Object> copyList;		
		Collections.copy(testList,copyList);	
	}

I get a compilation error -> The method copy(List<? super T>, List<? extends T>) in the type Collections is not applicable for the arguments (List<String>, List<Object>)

what does List<? super T> & List<? extends T> signifies... i am simply copying from List<String> to List<Object> which is in my terms a more distinct hierarchy to the most generic Object list which Java should allow?

[John Jai]

My mistake.. it should have been Collections.copy(copyList,testList);

Sorry!

[John Jai]

I modified the code but get IndexOutOfBoundsException: Source does not fit in dest

List<Object> copyList = new ArrayList<Object>();		
		Collections.copy(copyList,testList);

[Matthew Brown]

Have you read the Javadocs for that method? They explain why it won't work.

However, you should be able to get the effect you want like this:

List<Object> copyList = new ArrayList<Object>(testList);

[John Jai]

Thanks it worked. can you tell me why second one is throwing indexoutofbounds exception even if i declare list of big size?

List<Object> copyList2 = new ArrayList<Object>(testList);
		System.out.println(copyList2);
		List<Object> copyList = new ArrayList<Object>(100);		
		Collections.copy(copyList,testList);

[Rob Spoor]

You declared it with a big capacity (the number of elements it can hold before its internal structure needs to change). The size (the actual number of elements) is still 0. You'll first need to add several objects before the size increases.

[Matthew Brown]

That's capacity, not size. The target list actually has to contain that many elements.

I'm not entirely sure why the method is written like that - it seems a bit pointless - but it's how it's defined.

[John Jai]

Mathew & Rob - thanks!

it worked after doing below

copyList.add("");copyList.add("");copyList.add("");
Collections.copy(copyList,testList);