This is one of the most commonly asked questions in Java Generics interviews. At first glance, both may look similar—but they behave very differently.
🔹 List<Object>
List<Object> means a list that can hold only Object type (and its subclasses explicitly added as Object).
✅ Example:
List<Object> list = new ArrayList<>();
list.add("Java");
list.add(10);
list.add(10.5);
✔ Key Characteristics:
- You can add any type of object
- You can read as Object
- It is a specific type, not flexible
🔹 List<?> (Wildcard)
List<?> means a list of unknown type.
✅ Example:
List<?> list = new ArrayList<String>();
✔ Key Characteristics:
- It can point to any type of list (
List<String>,List<Integer>, etc.) - You cannot add elements (except
null) - You can only read values as Object
🔹 Core Difference
| Feature | List<Object> |
List<?> |
|---|---|---|
| Type | Specific type | Unknown type (Wildcard) |
| Can hold | Only Object list | Any type of list |
| Add elements | ✅ Yes | ❌ No (except null) |
| Read elements | As Object | As Object |
| Flexibility | ❌ Less flexible | ✅ More flexible |
🔹 Important Concept
👉 List<Object> is not the parent of List<String>
List<Object> list = new ArrayList<String>(); // ❌ Compile-time error
👉 But wildcard works:
List<?> list = new ArrayList<String>(); // ✅ Valid
🔹 Real-Time Understanding
- Use
List<Object>when you want to store different types in the same list - Use
List<?>when you want to accept any type of list as input (read-only)
🔹 Interview Tip
👉 If the question is about flexibility → use ?
👉 If the question is about storing mixed data → use Object
🚀 Final Thoughts
Understanding the difference between List<Object> and List<?> helps you write more flexible and type-safe Java code, especially when working with APIs and collections.
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