How to cast List<Object> to List

Answer 1

That's because although a Customer is an Object, a List of Customers is not a List of Objects. If it was, then you could put any object in a list of Customers.

Answer 2

Your best bet is to create a new List<Customer>, iterate through the List<Object>, add each item to the new list, and return that.

Answer 3

You can use a double cast.

return (List<Customer>) (List) getList();

Answer 4

You can do something like this

List<Customer> cusList = new ArrayList<Customer>();

for(Object o: list){        
    cusList.add((Customer)o);        
}

return cusList; 

Or the Java 8 way

list.stream().forEach(x->cusList.add((Customer)x))

return cuslist;

Answer 5

With Java 8 Streams:

Sometimes brute force casting is fine:

List<MyClass> mythings = (List<MyClass>) (Object) objects

But here's a more versatile solution:

List<Object> objects = Arrays.asList("String1", "String2");

List<String> strings = objects.stream()
                       .map(element->(String) element)
                       .collect(Collectors.toList());

There's a ton of benefits, but one is that you can cast your list more elegantly if you can't be sure what it contains:

objects.stream()
    .filter(element->element instanceof String)
    .map(element->(String)element)
    .collect(Collectors.toList());

Answer 6

Note that I am no java programmer, but in .NET and C#, this feature is called contravariance or covariance. I haven't delved into those things yet, since they are new in .NET 4.0, which I'm not using since it's only beta, so I don't know which of the two terms describe your problem, but let me describe the technical issue with this.

Let's assume you were allowed to cast. Note, I say cast, since that's what you said, but there are two operations that could be possible, casting and converting.

Converting would mean that you get a new list object, but you say casting, which means you want to temporarily treat one object as another type.

Here's the problem with that.

What would happen if the following was allowed (note, I'm assuming that before the cast, the list of objects actually only contain Customer objects, otherwise the cast wouldn't work even in this hypothetical version of java):

List<Object> list = getList();
List<Customer> customers = (List<Customer>)list;
list.Insert(0, new someOtherObjectNotACustomer());
Customer c = customers[0];

In this case, this would attempt to treat an object, that isn't a customer, as a customer, and you would get a runtime error at one point, either form inside the list, or from the assignment.

Generics, however, is supposed to give you type-safe data types, like collections, and since they like to throw the word 'guaranteed' around, this sort of cast, with the problems that follow, is not allowed.

In .NET 4.0 (I know, your question was about java), this will be allowed in some very specific cases, where the compiler can guarantee that the operations you do are safe, but in the general sense, this type of cast will not be allowed. The same holds for java, although I'm unsure about any plans to introduce co- and contravariance to the java language.

Hopefully, someone with better java knowledge than me can tell you the specifics for the java future or implementation.