Two problems with generics in Java

Since generics were out and I started using them, there were always a few cases in which I couldn't make them do what I wanted. I always thought it was my problem, and that I didn't understand what was going on... Turns out: it's not. There are at least two things that are implemented in a way that break what I thought were very safe expectations. Inference Before Generics, you would write something like: List list = Collections.EMPTY_LIST; Adding generics you get a warning: List list1 = Collections.EMPTY_LIST; GenericsGotchas.java:[19,40] [unchecked] unchecked conversion found : java.util.List required: java.util.List The problem is that we have a raw type on the right, and a generic type on the other. We could suppress the warning every single time, but there is a better option: using a generic method and let the compiler infer and match the types List list2 = Collections.emptyList(); This compiles without a problem. But this does not: List list3 = new ArrayList(Collections.emptyList()); GenericsGotchas.java:[21,29] cannot find symbol symbol : constructor ArrayList(java.util.List) location: class java.util.ArrayList anotherExpression(expression); is always valid. Unfortunately, it's not true in this corner case. So, when I am refactoring my code, or decorating an expression, I have now one more thing to remember and to pay attention to. Casting generics type Castin...

Date: April, 09 2010
Url: http://www.java.net/blog/carcassi/archive/2010/04/09/two-problems-generics-java-0

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