Java 8 generic type conversion fails where as it was passing with java 7

Question

I faced this issue and hence posting it as complete solution -

With Java 8, the below code will fail with Runtime exception. The problem is getInteger method is returning a generic Integer type and print method expects exact Object Type.

public static void main(String[] args) {
    print(getInteger());
}

private static <T> T getInteger() {
    return (T)new Integer(10);
}

private static void  print(Object...o1){
    for(Object o: o1){
       System.out.println(o);
    }
}

Answer 1

Your code mixes type inference with varargs.

In Java 7, it worked because there was no target type inference: the type of getInteger call was resolved as just Object, and then the object was boxed into an Object[] to fit the varargs call.

With Java 8, T is inferred from the target type as Object[]. Since you perform an unchecked cast in getInteger, this is completely valid, and mandated by the method signature resolution rules, which will consider varargs only if resolution failed without considering it. Here, that was not the case.

Lesson: by performing the unchecked type cast you have waived your right to expect type safety and correctness. You should be prepared to take care of it yourself.

Answer 2

As part of solution choose any of the following.

Break method call and argument pass in two calls –

Integer i = getInteger();

print(i);

Or

Call print method with Typed argument –

  print(Main.<Integer>getInteger());

I hope it will help.