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does not appear on the righthand side of an assignment, but is used as a argument to a method invocation. Why does it happen?

appear assignment invocation method righthand Used
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does not appear on the righthand side of an assignment, but is used as a argument to a method invocation. Why does it happen?

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For the type inference of new Callable <> () there is neither a constructor argument nor an assignment context. Hence the compiler infers that the missing type parameter of Callable is type Object . For the type inference of new FutureTask <> (new Callable <> ()) there is a constructor argument, namely Callable , which leads to the conclusion that the missing type parameter of FutureTask is type Object as well. This makes for a type mismatch when assignment compatibility is checked and an error message is emitted. The surprise perhaps is that the compiler does NOT consider the type information Future of the lefthand side expression of the assignment. It always takes a look at the constructor arguments first. The constructor of the FutureTask has an argument of type Callable and it permits a successful type inference the result of which is FutureTask ; the Future on the lefthand side is ignored for type inference because it is not needed for a success

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