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477What Are Abstract Classes?
478The abstract Keyword for Classes
479Abstract Methods: Signature Without Body
480Abstract Method Syntax and Rules
481Concrete Methods in Abstract Classes
482Subclassing Abstract Classes
483Abstract Subclasses: Deferring Implementation
484Constructors in Abstract Classes
485Fields and Instance Variables in Abstract Classes
486When to Use Abstract Classes
487What Is an Interface? The Contract Metaphor
488Declaring an Interface: Syntax and Structure
489The implements Keyword: Class Adoption of Interfaces
490Implementing Interface Methods: Overriding Rules
491Interface Constants: public static final by Default
492Interfaces as Types: Polymorphic References
493Incomplete Implementation: Compiler Errors
494Empty Interfaces: Marker Interfaces
495Interfaces vs Abstract Classes: Initial Comparison
496Real-World Example: Implementing a Custom Interface
497Implementing Multiple Interfaces
498Interface Method Name Conflicts
499Conflicting Default Methods
500Using super to Call Specific Default Method
501Multiple Interfaces with Different Return Types
502Combining Abstract and Default Methods from Multiple Interfaces
503Static Methods Across Multiple Interfaces
504Practical Example: Multiple Capability Interfaces
505Best Practices for Designing Multiple Interfaces
506Why Default Methods Were Introduced
507Default Method Syntax
508Inheriting Default Methods
509Overriding Default Methods
510Multiple Inheritance of Default Methods
511The Diamond Problem and Resolution Rules
512Calling Superinterface Default Methods
513Default Methods and Abstract Classes
514Default Methods Cannot Access Instance State
515Real-World Examples: Collection Default Methods
516Static Methods in Interfaces: Syntax and Purpose
517Calling Static Interface Methods
518Static Methods vs Default Methods: Key Differences
519Use Cases for Static Interface Methods
520Private Methods in Interfaces: Java 9 Introduction
521Private Instance Methods in Interfaces
522Private Static Methods in Interfaces
523Visibility Rules: Private Methods Cannot Be Overridden
524Refactoring Default Methods with Private Helpers
525Abstract Classes: Is-A with Shared Implementation
526Interfaces: Contract-Based Design and Multiple Contracts
527State and Constructors: Abstract Classes Can Have Both
528Evolution and Backward Compatibility Concerns
529The Template Method Pattern with Abstract Classes
530Marker Interfaces vs Empty Abstract Classes
531Combining Both: Abstract Classes Implementing Interfaces
532Decision Criteria: A Practical Checklist

477What Are Abstract Classes?
478The abstract Keyword for Classes
479Abstract Methods: Signature Without Body
480Abstract Method Syntax and Rules
481Concrete Methods in Abstract Classes
482Subclassing Abstract Classes
483Abstract Subclasses: Deferring Implementation
484Constructors in Abstract Classes
485Fields and Instance Variables in Abstract Classes
486When to Use Abstract Classes
487What Is an Interface? The Contract Metaphor
488Declaring an Interface: Syntax and Structure
489The implements Keyword: Class Adoption of Interfaces
490Implementing Interface Methods: Overriding Rules
491Interface Constants: public static final by Default
492Interfaces as Types: Polymorphic References
493Incomplete Implementation: Compiler Errors
494Empty Interfaces: Marker Interfaces
495Interfaces vs Abstract Classes: Initial Comparison
496Real-World Example: Implementing a Custom Interface
497Implementing Multiple Interfaces
498Interface Method Name Conflicts
499Conflicting Default Methods
500Using super to Call Specific Default Method
501Multiple Interfaces with Different Return Types
502Combining Abstract and Default Methods from Multiple Interfaces
503Static Methods Across Multiple Interfaces
504Practical Example: Multiple Capability Interfaces
505Best Practices for Designing Multiple Interfaces
506Why Default Methods Were Introduced
507Default Method Syntax
508Inheriting Default Methods
509Overriding Default Methods
510Multiple Inheritance of Default Methods
511The Diamond Problem and Resolution Rules
512Calling Superinterface Default Methods
513Default Methods and Abstract Classes
514Default Methods Cannot Access Instance State
515Real-World Examples: Collection Default Methods
516Static Methods in Interfaces: Syntax and Purpose
517Calling Static Interface Methods
518Static Methods vs Default Methods: Key Differences
519Use Cases for Static Interface Methods
520Private Methods in Interfaces: Java 9 Introduction
521Private Instance Methods in Interfaces
522Private Static Methods in Interfaces
523Visibility Rules: Private Methods Cannot Be Overridden
524Refactoring Default Methods with Private Helpers
525Abstract Classes: Is-A with Shared Implementation
526Interfaces: Contract-Based Design and Multiple Contracts
527State and Constructors: Abstract Classes Can Have Both
528Evolution and Backward Compatibility Concerns
529The Template Method Pattern with Abstract Classes
530Marker Interfaces vs Empty Abstract Classes
531Combining Both: Abstract Classes Implementing Interfaces
532Decision Criteria: A Practical Checklist

Lesson 527 of 2,870·12. Abstract Classes and InterfacesPro lesson

State and Constructors: Abstract Classes Can Have Both

How abstract classes can maintain instance fields and constructors, unlike interfaces, for shared state across subclasses.

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