How to Cast a List of Supertypes to a List of 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;

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.

Most efficient way to cast List SubClass to List BaseClass

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.

Java-8 stream - cast list of sub types to list of super types

You can collect the items into lists of the more general type by specifying the type parameter to use for toList:

Collectors.<ScheduleContainer>toList()

Adding list of subtypes to list of supertypes in Kotlin

The problem is that imageList may only contain instances of type Image. However, attachableList contains instances of type Attachable. Since other classes than Image may implement Attachable, you cannot safely add all elements from attachableList to imageList.

If you're sure that attachableList only contains instances of type Image, using it as Image is fine (though there's also .filterIsInstance<Image>(). I don't see a way around this using the variance features.

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);

Dart - Casting List SuperType to List SubType using generics

In Dart (and indeed in many languages) generics screws with the concept of inheritance. You would think that if Bar inherits from Foo, that List<Bar> would also be castable to List<Foo>.

This is not actually going to be the case because of how generics work. When you have a generic class, every time you use that class with a different type, that type is treated as a completely separate class. This is because when the compiler compiles those types, class MyGenericType<Foo> extends BaseClass and class MyGenericType<Bar> extends BaseClass are basically converted to something like class MyGenericType_Foo extends BaseClass and class MyGenericType_Bar extends BaseClass.

Do you see the problem? MyGenericType_Foo and MyGenericType_Bar are not descendants of one another. They are siblings of each other, both extending from BaseClass. This is why when you try to convert a List<Entity> to List<Vehicle>, the cast doesn't work because they are sibling types, not a supertype and subtype.

With all this being said, while you cannot directly cast one generic type to another based on the relationship of the generic type parameter, in the case of List there is a way to convert one List type to another: the cast method.

List<Entity> entityList = <Entity>[...];
List<Vehicle> vehicleList = entityList.cast<Vehicle>(); // This cast will work

One thing to note though, if you are casting from a supertype generic to a sub-type generic and not all the elements of the list are that new type, this cast will throw an error.

Convert List Object to List CustomClass

If you know in advance that all of your objects are actually CustomClass objects, you can perform an unsafe cast:

@SuppressWarnings("unchecked")
List<CustomClass> list = (List<CustomClass>)(List<?>)getObjects();

This is the fastest solution; practically in runtime nothing is performed except the variable assignment. However if you're wrong and your list actually contains other objects, you may have an unexpected ClassCastException later.

Java 8 stream - cast list items to type of subclass

It's possible, but you should first consider if you need casting at all or just the function should operate on subclass type from the very beginning.

Downcasting requires special care and you should first check if given object can be casted down by:

object instanceof ScheduleIntervalContainer

Then you can cast it nicely by:

use functional method to cast like
ScheduleIntervalContainer.class::cast

So, the whole flow should look like:

collection.stream()
    .filter(obj -> obj instanceof ScheduleIntervalContainer)
    .map(ScheduleIntervalContainer.class::cast)
    // other operations

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