the puzzle of Collections.emptyList()

Question

int j = 0; 
List<Integer> i = j > 0 ? Collections.emptyList() : new ArrayList<Integer>(); // compiler error:cannot convert from List<capture#1-of ? extends Object> to List<Integer>

while,

List<Integer> li = Collections.emptyList(); // it works

Although i know the type erasure, i do not the reason of compiling failed!

Help, thx!

related question: http://stackoverflow.com/questions/306713/java-collections-emptylist-returns-a-listobject — Colin D, May 25 '12 at 12:38

score 7 · Accepted Answer · answered May 25 '12 at 12:35

7

Try this:

List<Integer> i = j > 0 ? Collections.<Integer>emptyList() : new ArrayList<Integer>();

answered May 25 '12 at 12:35

score 2 · Answer 2 · answered May 25 '12 at 12:41

2

In the first example you are not allowing Java to capture the <T> in public static <T> List<T> Collections.emptyList() since you are not assigning it directly to the var. Java's type inference is very weak and is not able to see through the conditional operator. In the second example you have a straightforward situation and T is successfully captured into Integer.

answered May 25 '12 at 12:41

Marko Topolnik

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i never though that the java's type inference is so weak! thx, too! – xiaok May 26 '12 at 05:28

score 0 · Answer 3 · answered May 25 '12 at 12:41

0

The reason the second works is that the compiler is able to figure out the exact type needed from the type of the variable where the result is stored. OTOH Java's type caclulation is not powerful enough to do the same in the first case

answered May 25 '12 at 12:41

Attila

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the puzzle of Collections.emptyList()

3 Answers3