Explain wildcard types in Java Generics
💡 Concept: Wildcard Types in Generics
Wildcards in Java Generics use the ? symbol to represent unknown types. They make generic code more flexible by allowing you to work with related but not identical types.
📘 Quick Intro
There are three main wildcard forms: ? (unbounded), ? extends T (upper-bounded), and ? super T (lower-bounded). They’re used to relax method parameter types while maintaining type safety.
🧠 Analogy
Think of ? extends T like accepting any fruit that’s a kind of Apple. ? super T is like accepting any container that can hold Apples. And ? means you’ll accept any random item regardless of type.
🔧 Technical Explanation
List<?>: accepts any type of list (read-only).List<? extends Number>: can read from a list of Number or its subclasses, but cannot write.List<? super Integer>: can write Integer or its subclasses, but cannot read specific types.- Wildcard capture and PECS (Producer Extends, Consumer Super) principle guide their use.
- Wildcards improve API design flexibility while preserving type safety.
🎯 Use Cases
- ✅ API methods that operate on generic types without knowing exact type
- ✅ Reading from generic producers (
? extends) - ✅ Writing to generic consumers (
? super) - ✅ Supporting multiple subclasses in method signatures
💻 Example: Wildcard Types
import java.util.List;
public class WildcardExample {
// Upper bounded wildcard (Producer)
public static void printNumbers(List<? extends Number> list) {
for (Number num : list) {
System.out.println(num);
}
}
// Lower bounded wildcard (Consumer)
public static void addIntegers(List<? super Integer> list) {
list.add(10);
list.add(20);
}
}
❓ Interview Q&A
Q1: What does ? represent in generics?
A: An unknown type.
Q2: When should you use ? extends T?
A: When you only need to read from the collection.
Q3: When should you use ? super T?
A: When you need to write to the collection.
Q4: What is PECS in Java generics?
A: Producer Extends, Consumer Super.
Q5: Can you add elements to List<? extends T>?
A: No.
Q6: Can you add to List<? super T>?
A: Yes.
Q7: Can List<?> accept any type?
A: Yes, but you can’t add to it.
Q8: Are wildcards optional in generics?
A: Yes, but they add flexibility.
Q9: Does List<Object> accept List<String>?
A: No, generics are not covariant.
Q10: Can wildcards be used in method return types?
A: Yes, but cautiously.
📝 MCQs
Q1. What does '?' mean in generics?
- Any class
- Generic list
- Unknown type
- Null
Q2. What does '? extends T' allow?
- Write to list
- Only null
- Read from list
- Nothing
Q3. What does '? super T' allow?
- Only read
- Write to list
- Only sort
- Type erasure
Q4. Can you add to List<?>?
- Yes
- No
- Only once
- Only null
Q5. Which principle explains wildcard usage?
- SOLID
- PECS
- POJO
- MVC
Q6. What is '?' used for?
- Interface
- Default type
- Wildcard type
- Class type
Q7. Can List<? extends Number> accept List<Integer>?
- No
- Yes
- Only List<Number>
- Only List<Object>
Q8. Is List<String> a subtype of List<Object>?
- Yes
- No
- Sometimes
- Always
Q9. Is List<? super Integer> a consumer?
- No
- Yes
- Never
- Only static
Q10. Can you use wildcard in method return?
- No
- Yes
- Only public
- Only void
💡 Bonus Insight
Use “extends” when you're only reading from a structure and “super” when you're only writing. Think “Producer Extends, Consumer Super” (PECS) to decide which wildcard to use.
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