Look the doc for the equals() method of ArrayList Returns true if and only if the specified object is also a list, both lists have the same size, and all corresponding pairs of elements in the two lists are equal. Since there are no elements, all the conditions satisfied and hence true. If you add elements to the both list (atleast … Read more
It’s because Java’s arrays (unlike generics) contain, at runtime, information about its component type. So you must know the component type when you create the array. Since you don’t know what T is at runtime, you can’t create the array.
You can’t create arrays with a generic component type. Create an array of an explicit type, like Object[], instead. You can then cast this to PCB[] if you want, but I don’t recommend it in most cases. If you want type safety, use a collection like java.util.List<PCB> instead of an array. By the way, if list is already a java.util.List, you should use … Read more
You could define callFriend this way: Then call it as such: This code has the benefit of not generating any compiler warnings. Of course this is really just an updated version of casting from the pre-generic days and doesn’t add any additional safety.
After type erasure, all that is known about T is that it is some subclass of Object. You need to specify some factory to create instances of T. One approach could use a Supplier<T>: Usage might look like this: Alternatively, you can provide a Class<T> object, and then use reflection.
I have to ask a question in return: is your GenSet “checked” or “unchecked”? What does that mean? Checked: strong typing. GenSet knows explicitly what type of objects it contains (i.e. its constructor was explicitly called with a Class<E> argument, and methods will throw an exception when they are passed arguments that are not of type E. See Collections.checkedCollection.-> in that case, you should … Read more
Yes, pretty much. List<T> is a generic class. It supports storing values of a specific type without casting to or from object (which would have incurred boxing/unboxing overhead when T is a value type in the ArrayList case). ArrayList simply stores object references. As a generic collection, List<T> implements the generic IEnumerable<T> interface and can be used easily in LINQ (without requiring any Cast or OfType call). ArrayList belongs to the days that C# didn’t have … Read more
You cannot overload operators in Java. The < operator only applies to primitive (or numeric) types, not reference types. Since T is a type variable that represents a reference type, you cannot use < on variables of type T. You have to use check the value returned and decide to do what you wish with … Read more
Using the generified version of class Class allows you, among other things, to write things like and then you can be sure that the Class object you receive extends Collection, and an instance of this class will be (at least) a Collection.
I’m new to Java and very confused. I have a large dataset of length 4 int[] and I want to count the number of times that each particular combination of 4 integers occurs. This is very similar to counting word frequencies in a document. I want to create a Map<int[], double> that maps each int[] to a running count … Read more