Most Efficient Way to Cast List<Subclass> to List<Baseclass>

Most efficient way to cast ListSubClass to ListBaseClass

The syntax for this sort of assignment uses a wildcard:

List<SubClass> subs = ...;
List<? extends BaseClass> bases = subs;

It's important to realize that a List<SubClass> is not interchangeable with a List<BaseClass>. Code that retains a reference to the List<SubClass> will expect every item in the list to be a SubClass. If another part of code referred to the list as a List<BaseClass>, the compiler will not complain when a BaseClass or AnotherSubClass is inserted. But this will cause a ClassCastException for the first piece of code, which assumes that everything in the list is a SubClass.

Generic collections do not behave the same as arrays in Java. Arrays are covariant; that is, it is allowed to do this:

SubClass[] subs = ...;
BaseClass[] bases = subs;

This is allowed, because the array "knows" the type of its elements. If someone attempts to store something that isn't an instance of SubClass in the array (via the bases reference), a runtime exception will be thrown.

Generic collections do not "know" their component type; this information is "erased" at compile time. Therefore, they can't raise a runtime exception when an invalid store occurs. Instead, a ClassCastException will be raised at some far distant, hard-to-associate point in code when a value is read from the collection. If you heed compiler warnings about type safety, you will avoid these type errors at runtime.

Cast the sub class List to base class List

No it will not work directly except you define your first list as:

List<? extends Animals> animal;

then you will be able to do:

List<Puppy> puppy = new ArrayList<Puppy>();
animal = puppy;

Convert ListDerivedClass to ListBaseClass

The way to make this work is to iterate over the list and cast the elements. This can be done using ConvertAll:

List<A> listOfA = new List<C>().ConvertAll(x => (A)x);

You could also use Linq:

List<A> listOfA = new List<C>().Cast<A>().ToList();

Convert List of Base Class to a List of Derived Class - possible?

Cast<T> method applies cast operation to all elements of the input sequence. It works only if you can do the following to each element of the sequence without causing an exception:

ParentClass p = ...
ChildClass c = (ChildClass)p;

This will not work unless p is assigned an instance of ChildClass or one of its subclasses. It appears that in your case the data returned from the server API contains objects of ParentClass or one of its subclasses other than ChildClass.

You can fix this problem by constructing ChildClass instances, assuming that you have enough information from the server:

List<ChildClass> childList = parentList
    .Select(parent => new ChildClass(parent.Name, ... /* the remaining fields */))
    .ToList();

Casting a list of an object to a list of super types

Java Generics are invariant. This means that, given two concrete classes, A and B, SomeClass<A> has no relationship to SomeClass<B>, even if B is a subtype of A.

So, List<RangeData> is not a supertype of List<IncomeRanges>. Instead, you should use an upper bounded wildcard, like this:

public RangeData findRangeData(List<? extends RangeData> list, int value) {
    ....code....
}

For more information, you can check the documentation on Generics, Inheritance, and Subtypes.

How do you cast a List of supertypes to a List of subtypes?

Simply casting to List<TestB> almost works; but it doesn't work because you can't cast a generic type of one parameter to another. However, you can cast through an intermediate wildcard type and it will be allowed (since you can cast to and from wildcard types, just with an unchecked warning):

List<TestB> variable = (List<TestB>)(List<?>) collectionOfListA;

Copying a ListBaseClass to ListDerivedClass

List<DerivedClass> result = 
    listBaseClass.ConvertAll(instance => (DerivedClass)instance);

Actually ConvertAll is good when you need to create new objects based on the original, when you just need to cast you can use the following

List<DerivedClass> result = 
    listBaseClass.Cast<DerivedClass>().ToList();

If not all of the items in your list can be cast to DerivedClass then use OfType instead

List<DerivedClass> result =
    listBaseClass.OfType<DerivedClass>().ToList();

Converting a collection of subtype to collection of supertype

Easiest way, assuming you don't need to modify the collection through as1:

Collection<A> as1 = Collections.unmodifiableCollection(bs);

If you do need to modify the collection, the only safe thing to do is to copy it:

Collection<A> as1 = new ArrayList<>(bs);

Insert a BaseClass instance into a List? extends BaseClass (Java)

Java keyword extends in generic type of reference is used to restrict actual generic type realization to be no less derived than the named type. Especially it can be a subclass. One should use it in case type is used to extract values and operate on them. You then use the interface of the base class not concerning the actual type (realization/implementation) of that class.

When you've got a specific class and need to pass (put) into collection, or other generic class object, the reference to point at that object should have super keyword, which restricts the actual object to be no more derived than the named type.

Code example below. Assume we have classes Base and Derived, where Derived extends Base, class Base have an interface method foo() and both have default constructor.

void service() {
    List<Base> theList = new ArrayList<Base>();

    produceObjects(theList);
    consumeObjects(theList);
}

void produceObjects(List<? super Base> consumerList) {
    consumerList.add(new Derived());
}

void consumeObjects(List<? extends Base> producerList) {
    for (Base base : producerList) {
        base.foo();
    }
}

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