Quick Answer
Abstract class: can have state (instance fields), constructors, concrete methods. Used for "is-a" relationships with shared implementation. A class can extend only one. Interface: no state (only constants), no constructors, all methods abstract by default (Java 8+ adds default/static). A class can implement many interfaces. Use abstract class for template base, interface for multiple capabilities/contracts.
Answer
Abstract classes can have fields, constructors, and concrete methods. Interfaces declare abstract methods (plus default/static methods). Java allows single inheritance for classes but multiple inheritance through interfaces.
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Verified Answer
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