Questions and Answers
Is it correct that the class ABC is declared as abstract without any abstract methods?
True
Why does the class AbstractClass show a compilation error?
Because abstract methods must not have a body.
Which class is instantiable?
What must class B implement to avoid compilation errors?
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What will be the value of the 'result' variable if the program runs correctly?
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Can we write explicit constructors in an abstract class?
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What is the error in the code where abstractMethod is private?
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Can we declare protected methods in an interface?
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What will be the output of the given abstract class program?
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What will be the output of the program using class X and its subclass Y?
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For every interface written in a Java file, a .class file will be generated after compilation.
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What is the error in the code declaring a private field in an interface?
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What will be the output of the code using interfaces and classes?
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Can a class implement more than one interface?
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What is causing the compile-time error in the implementation of interface X?
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Does the code compile successfully when trying to change the value of the interface field?
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Study Notes
Abstract Classes
- An abstract class can be declared without any abstract methods; it can still provide complete method implementations.
- A compilation error occurs if an abstract method contains a body; abstract methods must be declared without implementation.
Instantiation and Implementation
- Only non-abstract classes can be instantiated; thus, Class B (subclass of abstract Class A) is instantiable.
- When extending an abstract class, the subclass must implement all inherited abstract methods or also be declared abstract.
Between Abstract Classes and Interfaces
- Abstract classes can contain constructors; they are not limited in the number of constructors they can define.
- Abstract methods in an abstract class cannot be private; they must be accessible for implementation by subclasses.
Interfaces
- Protected methods cannot be declared in interfaces; only public methods are permitted.
- An interface generates a separate .class file after the compilation of each interface in Java.
Method Implementations
- Interface methods must always be declared public; reducing visibility during implementation results in a compilation error.
Field Modifiers
- Interface fields are inherently public, static, and final, meaning their values cannot be modified after initialization.
- Declaring a private field in an interface is prohibited due to legal modifier restrictions.
Example Code Behavior
- In the provided abstract class demonstration, calling methods results in the output: "FIRST", "THIRD", "SECOND" based on method chaining.
- In another example, constructing an instance of Class Y from abstract Class X outputs "ONE", "TWO", and executing the abstract method outputs "THREE".
General Knowledge
- A class can implement multiple interfaces, promoting polymorphism and code reusability.
- Ensuring method signatures maintain public visibility when implementing interface methods is crucial to avoid compilation errors.
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Test your knowledge about abstract classes and interfaces with these flashcards. Explore concepts like abstract methods, class declarations, and their implications in programming. Perfect for students learning Java or object-oriented programming.
