How to Efficiently Convert List<SubClass> to List<BaseClass> in Java?

What is the most efficient way to convert a List<SubClass> to List<BaseClass> in Java without creating a new list?

List<BaseClass> convertedList = new ArrayList<BaseClass>(listOfSubClass);

In Java, casting a List of subclass instances to a List of base class references is not straightforward due to type safety. This article explains why and offers techniques to minimize overhead while maintaining references to original objects.

// Optimal method using bounded wildcard
public void processList(List<? extends BaseClass> list) {
    // Process the list, can reference as BaseClass
    for (BaseClass obj : list) {
        // Do something with obj
    }
}

Causes

• Java's type system prohibits direct casting of collections due to type safety.
• Generics in Java are invariant, meaning List<SubClass> cannot be treated as List<BaseClass> even though SubClass is a subclass of BaseClass.

Solutions

• Use a parameterized method that accepts List<SubClass> and processes it as List<BaseClass>.
• Leverage bounded wildcards such as List<? extends BaseClass> to refer to List<SubClass> as a list of BaseClass without needing to cast.

Mistake: Attempting to directly cast List<SubClass> to List<BaseClass> causes compile-time errors.

Solution: Replace direct casting with a wildcard approach, using List<? extends BaseClass>.

Mistake: Creating a new list and copying objects unnecessarily involves performance overhead.

Solution: Utilize bounded wildcards or generics in method parameters to operate on the original list.