Abstract Classes and Interfaces Flashcards
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Questions and Answers

Which of the following class definitions defines a legal abstract class?

  • public class A { void method(); }
  • abstract class A { abstract void unfinished(); } (correct)
  • class A { abstract void unfinished(); }
  • public abstract class A { void method(); }

    • Which of the following declares an abstract method in an abstract Java class?

  • void method();
  • public void method();
  • public abstract void method(); (correct)
  • abstract void method();

    • Which of the following statements regarding abstract methods are true?

  • A subclass of a non-abstract superclass can be abstract. (correct)
  • An abstract class can be used as a data type. (correct)
  • An abstract class can be extended. (correct)
  • A subclass can override a concrete method in a superclass to declare it abstract. (correct)

    • Which of the following statements regarding abstract methods are true?

    <p>It is possible to declare an abstract class that contains no abstract methods.</p> Signup and view all the answers

    Suppose A is an abstract class, B is a concrete subclass of A, and both A and B have a no-arg constructor. Which of the following is correct?

    <p>B b = new B();</p> Signup and view all the answers

    What is the output of running the class Test?

    <p>ABCDE</p> Signup and view all the answers

    What occurs during the casting of numberRef to Double?

    <p>A runtime class casting exception occurs.</p> Signup and view all the answers

    What happens during this assignment: Number[] numberArray = new Integer; numberArray = new Double(1.5);?

    <p>Compile error.</p> Signup and view all the answers

    What is the reason for the compile error in the code provided?

    <p>x does not have the compareTo method.</p> Signup and view all the answers

    Why does the program have a compile error related to member access and casting?

    <p>Member access operator is executed before the casting operator.</p> Signup and view all the answers

    Which of the following statements are correct?

    <p>Double i = 4.5;</p> Signup and view all the answers

    What is the compile error related to java.util.Calendar?

    <p>The error occurs because java.util.Calendar is an abstract class.</p> Signup and view all the answers

    Assume Calendar calendar = new GregorianCalendar().__________ returns the month of the year.

    <p>calendar.get(Calendar.MONTH)</p> Signup and view all the answers

    Assume Calendar calendar = new GregorianCalendar().__________ returns the week of the year.

    <p>calendar.get(Calendar.WEEK_OF_YEAR)</p> Signup and view all the answers

    Assume Calendar calendar = new GregorianCalendar().__________ returns the number of days in a month.

    <p>calendar.getActualMaximum(Calendar.DAY_OF_MONTH)</p> Signup and view all the answers

    Which of the following is a correct interface?

    <p>interface A { void print();}</p> Signup and view all the answers

    Which of the following is incorrect?

    <p>Abstract classes can be instantiated.</p> Signup and view all the answers

    Study Notes

    Abstract Classes

    • An abstract class can define abstract methods that must be implemented by subclasses.
    • An abstract class may contain no abstract methods at all.
    • Constructors are allowed in abstract classes, and they can have private access.
    • Abstract classes can be extended by concrete classes or other abstract classes.
    • A subclass of a non-abstract superclass can itself be abstract.

    Abstract Methods

    • An abstract method cannot exist in a non-abstract class.
    • A subclass can override a concrete method from a superclass and declare it as abstract, requiring overriding in further subclasses.
    • Abstract methods must be declared with the keyword "abstract."

    Concrete Classes

    • A concrete subclass of an abstract class must implement all abstract methods in its superclass to be instantiated.
    • If a class has a valid no-argument constructor, it can be instantiated even if it inherits from an abstract superclass.

    Java Interfaces

    • Interfaces can contain method declarations but cannot have constructors.
    • A correct interface definition must declare one or more methods with no implementations.

    Java Exceptions

    • A runtime class casting exception occurs if an object is cast to an incompatible type, such as trying to cast from Number to Double.
    • Compile-time errors can arise when using methods that do not exist for the object type, as seen with compareTo for Number.

    Java Collections and Array Behavior

    • An array of a specific type (e.g., Integer[]) cannot accept an object of a different type (e.g., Double).
    • Attempting to assign an array of one type to another incompatible type will produce a runtime error.

    Calendar Class in Java

    • The Calendar class in Java is abstract, and cannot be instantiated directly; it requires a subclass like GregorianCalendar.
    • Methods in the Calendar class can retrieve specific date information, such as the month, week of the year, or maximum days in a month.

    Common Statements about Java Types

    • Type declarations can vary; for example, a statement like Double i = 4.5; is valid, indicating polymorphism within Java.
    • Incorrect type usages, such as declaring a variable of an abstract class type without initialization, can lead to compile-time errors.

    Compile Errors

    • Certain operations result in compile errors due to misunderstandings of method availability and the order of operations regarding casting and member access.

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    Description

    Test your knowledge of abstract classes and interfaces in Java with these informative flashcards. Each card presents a scenario or definition to help reinforce your understanding of these fundamental concepts in object-oriented programming.

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