autoboxing

Autoboxing automatically converts a primitive to the corresponding wrapper classes when needed if the generic type is specified in the declaration. 
```
import java.util.List;

public class Main {
    public static void main(String args[]) {
        List<Integer> heights = new ArrayList<>();
        heights.add(null);
        int h = heights.get(0);          // NullPointerException
    }
}
```