How to compile this without warnings?

Question

Hello,
I am learning generics, and I grasped concepts but still get stuck with this:
Properties props = System.getProperties();
List propsList = new ArrayList(props.keySet());
Collections.sort(propsList);
for (Object key : propsList) {
     out.println(key + "  =  " + props.getProperty((String)key));
}I get 3 warnings from the compiler and was not able to get rid of them. Thing is that I don't understand what to do with Set<K> which is returned from props.keySet(). I tried to make List<String>, List<? extends Object>, List<? super String>, it was only guessing, so I would appreciate quick explanation how to make this compile without warnings.
And warnings I get are:
warning: [unchecked] unchecked call to ArrayList(java.util.Collection<? extends E>) as a member of the raw type java.util.ArrayList
List propsList = new ArrayList(props.keySet());
^
warning: [unchecked] unchecked conversion
found : java.util.List
required: java.util.List<T>
Collections.sort(propsList);
^
warning: [unchecked] unchecked method invocation: <T>sort(java.util.List<T>) in java.util.Collections is applied to (java.util.List)
Collections.sort(propsList);
Thank you!

Answer 1

ndario wrote:
OK, List<String> is what I want, but I can't get it as
List<String> propsList = new ArrayList<String or whatever>(props.keySet());
The constructor you use here does not allow for what you intent to do, right. But all the constructor does is to create a new list and add all elements from the set. This can also be done by creating the list first and adding the set's element in a loop, ensuring to convert each element to the desired type.
Thing is that I am not looking for a solution to this problem (there are many ways to sort keys of Properties) but trying to learn something about generics. I was interested what needs to be done to compile the code snippet from the first post without warnings. Is it possible at all?Sure, I already told you what you need to do:
1. Define the list as I did in my first posting
2. Define your own Comparator<Object> to be passed as second parameter to sort().