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Java

910What Are Bounded Type Parameters?
911The extends Keyword in Generic Type Parameters
912Bounded Type Parameters with Classes
913Bounded Type Parameters with Interfaces
914Accessing Members Through Bounded Type Parameters
915Bounded Type Parameters in Generic Methods
916Why extends Works for Both Classes and Interfaces
917Recursive Type Bounds: The Enum Pattern
918Type Erasure and Bounded Type Parameters
919Common Use Cases for Bounded Type Parameters
920What Are Multiple Bounds in Generics?
921Syntax: Combining extends with &
922Class Bound Must Come First
923Multiple Interface Bounds Without a Class
924Accessing Members from All Bounds
925Use Case: Combining Comparable and Serializable
926Intersection Types in Method Signatures
927Common Pitfalls: Overly Restrictive Bounds
928The Unbounded Wildcard: ? Basics
929Wildcard Capture and Reading from <?> Collections
930Upper Bounded Wildcards: <? extends T> Syntax
931Reading from <? extends T> Collections
932Why You Cannot Write to <? extends T> Collections
933Lower Bounded Wildcards: <? super T> Syntax
934Writing to <? super T> Collections
935Reading from <? super T> Collections Returns Object
936Comparing extends vs super Wildcards
937Wildcards in Method Parameters and Return Types
938Common Wildcard Mistakes and Compiler Errors
939The PECS Principle: Producer Extends, Consumer Super
940Producers: When to Use '? extends T'
941Consumers: When to Use '? super T'
942Covariance with '? extends T': Read-Only Semantics
943Contravariance with '? super T': Write-Only Semantics
944Real-World Example: Collections.copy and PECS
945PECS in Method Signatures: Designing Flexible APIs
946Common PECS Mistake: Using extends for Consumers
947PECS and Return Types: When Not to Use Wildcards
948Practice: Refactoring Code to Apply PECS
949Unbounded Wildcards: The ? Type
950When to Use Unbounded Wildcards
951Reading from Unbounded Wildcard Collections
952Writing Restrictions with Unbounded Wildcards
953Unbounded Wildcards vs Raw Types
954Introduction to Capture Conversion
955Capture Conversion in Action
956Capture Helper Methods Pattern
957Common Capture Conversion Pitfalls
958Cannot Instantiate Generic Types with Primitives
959Cannot Create Instances of Type Parameters
960Cannot Create Arrays of Parameterized Types
961Cannot Use Type Parameters in Static Context
962Cannot Use instanceof with Parameterized Types
963Cannot Catch or Throw Generic Exception Types
964Cannot Overload Methods Differing Only by Type Parameters
965Cannot Create Generic Arrays: T[] vs List<T>
966Reified Generics in Other Languages: What Java Lacks

Java

910What Are Bounded Type Parameters?
911The extends Keyword in Generic Type Parameters
912Bounded Type Parameters with Classes
913Bounded Type Parameters with Interfaces
914Accessing Members Through Bounded Type Parameters
915Bounded Type Parameters in Generic Methods
916Why extends Works for Both Classes and Interfaces
917Recursive Type Bounds: The Enum Pattern
918Type Erasure and Bounded Type Parameters
919Common Use Cases for Bounded Type Parameters
920What Are Multiple Bounds in Generics?
921Syntax: Combining extends with &
922Class Bound Must Come First
923Multiple Interface Bounds Without a Class
924Accessing Members from All Bounds
925Use Case: Combining Comparable and Serializable
926Intersection Types in Method Signatures
927Common Pitfalls: Overly Restrictive Bounds
928The Unbounded Wildcard: ? Basics
929Wildcard Capture and Reading from <?> Collections
930Upper Bounded Wildcards: <? extends T> Syntax
931Reading from <? extends T> Collections
932Why You Cannot Write to <? extends T> Collections
933Lower Bounded Wildcards: <? super T> Syntax
934Writing to <? super T> Collections
935Reading from <? super T> Collections Returns Object
936Comparing extends vs super Wildcards
937Wildcards in Method Parameters and Return Types
938Common Wildcard Mistakes and Compiler Errors
939The PECS Principle: Producer Extends, Consumer Super
940Producers: When to Use '? extends T'
941Consumers: When to Use '? super T'
942Covariance with '? extends T': Read-Only Semantics
943Contravariance with '? super T': Write-Only Semantics
944Real-World Example: Collections.copy and PECS
945PECS in Method Signatures: Designing Flexible APIs
946Common PECS Mistake: Using extends for Consumers
947PECS and Return Types: When Not to Use Wildcards
948Practice: Refactoring Code to Apply PECS
949Unbounded Wildcards: The ? Type
950When to Use Unbounded Wildcards
951Reading from Unbounded Wildcard Collections
952Writing Restrictions with Unbounded Wildcards
953Unbounded Wildcards vs Raw Types
954Introduction to Capture Conversion
955Capture Conversion in Action
956Capture Helper Methods Pattern
957Common Capture Conversion Pitfalls
958Cannot Instantiate Generic Types with Primitives
959Cannot Create Instances of Type Parameters
960Cannot Create Arrays of Parameterized Types
961Cannot Use Type Parameters in Static Context
962Cannot Use instanceof with Parameterized Types
963Cannot Catch or Throw Generic Exception Types
964Cannot Overload Methods Differing Only by Type Parameters
965Cannot Create Generic Arrays: T[] vs List<T>
966Reified Generics in Other Languages: What Java Lacks

← Java

Lesson 928 of 2,870·22. Generics: Bounds and WildcardsPro lesson

The Unbounded Wildcard: ? Basics

Understand what <?> means, when to use unbounded wildcards, and why they differ from raw types and Object.

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