What is a wildcard in Java Generics?

A wildcard is represented by the '?' symbol and is used to denote unknown types in generic code for added flexibility.

What is the PECS principle in generics?

PECS stands for Producer Extends, Consumer Super, guiding how to use wildcards when reading from or writing to collections.

Explain wildcard types in Java Generics | FullStackPrep.Dev

💡 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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Explain wildcard types in Java Generics