How to Handle Casting with Java Generics Interfaces

What are the best practices for casting between Java Generics interfaces?

List<SomeType> myList = new ArrayList<SomeType>();
SomeType item = myList.get(0); // No explicit cast needed.

In Java, generics enable types (classes and interfaces) to be parameterized, meaning you can use them with different types while ensuring type safety at compile time. However, when dealing with casting and generics, it’s crucial to understand how type erasure works and how it can affect your code.

// Example of bounded wildcards
public static <T extends Number> void printList(List<T> list) {
    for (T number : list) {
        System.out.println(number);
    }
}

Causes

• Casting issues arise from type erasure, which removes generic type information during runtime.
• Improperly using raw types can lead to ClassCastException.
• Assuming that generic types retain type information at runtime may cause unexpected behavior.

Solutions

• Always use parameterized types instead of raw types to maintain type safety.
• Utilize bounded wildcards when you need flexibility with types, e.g., < ? extends SomeType > for covariance or < ? super SomeType > for contravariance.
• When casting, ensure that the object is of the expected type before casting by using the instanceof operator.

Mistake: Using raw types instead of generics.

Solution: Always define your collections with type parameters, e.g., List<String> not List.

Mistake: Casting without checking types, which leads to runtime exceptions.

Solution: Use instanceof before casting to ensure compatibility.