Java generics: syntax clarification needed

Question

Suppose i have a

List<Integer> l = new ArrayList<Integer>();

and would like to have this list reversed with the help of

public static List<Object> reverseList(List<Object> o);

Thought process here is that one day i may be dealing with Integers and another with Doubles. Would be nice to have a generic method that is able to reverse my Lists

How should reverseList be declared for this to work? Please advise

NPE · Accepted Answer · 2011-07-11 16:08:13Z

3

One way to declare it is

public static <T> List<T> reverseList(List<T> o) {
  ...
}

You might also want to take a look at Collections.reverse.

edited Jul 11, 2011 at 16:08

answered Jul 11, 2011 at 15:58

NPE

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1 Comment

Matthew Gilliard Over a year ago

Your first example does not work - you will lose the type information. The second is correct.

Kal · Accepted Answer · 2011-07-11 16:00:09Z

2

You can do

public static <T extends Number> List<T> reverseList(List<T> o) {

answered Jul 11, 2011 at 16:00

Kal

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1 Comment

Andrzej Doyle Over a year ago

+1 for including a bound on the generic parameter - this seems like it might be relevant based on the question, and even if not it gives the syntax for later adaptation.

Matthew Gilliard · Accepted Answer · 2011-07-11 15:58:44Z

1

You can use this:

public static <T> List<T> reverse(List<T> in);

answered Jul 11, 2011 at 15:58

Matthew Gilliard

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