Given this example from the generics tutorial.
List<String> list = new ArrayList<>();
list.add("A");
// The following statement should fail since addAll expects
// Collection<? extends String>
list.addAll(new ArrayList<>());
Why does the last line not compile, when it seems it should compile. The first line uses a very similar construct and compiles without a problem.
Please explain elaborately.
The explanation from the Type Inference documentation seems to answer this question directly ( unless I'm missing something else ).
Java SE 7 and later support limited type inference for generic instance creation; you can only use type inference if the parameterized type of the constructor is obvious from the context. For example, the following example does not compile:
List<String> list = new ArrayList<>();
list.add("A");
// The following statement should fail since addAll expects
// Collection<? extends String>
list.addAll(new ArrayList<>());
Note that the diamond often works in method calls; however, for greater clarity, it is suggested that you use the diamond primarily to initialize a variable where it is declared.
In comparison, the following example compiles:
// The following statements compile:
List<? extends String> list2 = new ArrayList<>();
list.addAll(list2);