11review(inheritance andpolymorphism)
- 1. Chapter 11 Inheritance and Polymorphism 1. (a) The printout is A’s no-arg constructor is invoked (b) The default constructor of B attempts to invoke the default of constructor of A, but class A's default constructor is not defined. 2. All false. (1) A subclass is an extension of a superclass and normally contains more details information than its superclass. (2) If a subclass’s constructor explicitly invoke a superclass’s constructor, the superclass’s no-arg constructor is not invoked. (3) You can only override accessible instance methods. (4) You can only override accessible instance methods. 3. The following lines are erroneous: { radius = radius; // Must use this.radius = radius } class B extends Circle (missing extends) { Circle(radius); // Must use super(radius) length = length; // Must use this.length = length } public double getArea() { return getArea()*length; // super.getArea() } 4. Use super() or super(args). This statement must be the first in the constructor in the subclass. 5. Use super.method() or super.method(args). 6. Method overloading defines methods of the same name in a class. Method overriding modifies the methods that are defined in the superclasses. 7. It is method overridden. 8. It will be a syntax error.
- 2. 9. It is method oveloading. 10. Yes, because these two methods are defined in the Object class; therefore, they are available to all Java classes. The subclasses usually override these methods to provide specific information for these methods. The toString() method returns a string representation of the object; the equals() method compares the contents of two objects to determine whether they are the same. 11. B’s constructor is invoked A’s constructor is invoked The default constructor of Object is invoked, when new A(3) is invoked. The Object’s constructor is invoked before any statements in B’s constructor are executed. 12. (a) (circle instanceof GeometricObject1) => true (object1 instanceof GeometricObject1) => true (circle instanceof Circle1) => true (object1 instanceof Circle1) => false (b) Yes, because you can always cast from subclass to superclass. (c) Causing a runtime exception (ClassCastExcpetion) 13. Is fruit instanceof Fruit true? true Is fruit instanceof Orange true? false Is fruit instanceof Apple true? true Is fruit instanceof GoldDelicious true? true Is fruit instanceof Macintosh true? false Is orange instanceof Orange true? true Is orange instanceof Fruit true? true Is orange instanceof Apple true? false Suppose the method makeApple is defined in the Apple class. Can fruit invoke this method? Yes
- 3. Can orange invoke this method? No Suppose the method makeOrangeJuice is defined in the Orange class. Can orange invoke this method? Yes. Can fruit invoke this method? No. Is the statement Orange p = new Apple() legal? No Is the statement Macintosh p = new Apple() legal? No Is the statement Apple p = new Macintosh() legal? Yes 14. Object apple = (Apple)fruit; Causes a runtime ClassCastingException. 15. The output is false if the Circle class in (A) is used. The Circle class has two overloaded methods: equals(Circle circle) defined in the Circle class and equals(Object circle) defined in the Object class, inherited by the Circle class. At compilation time, circle1.equals(circle2) is matched to equals(Circle circle), because equals(Circle circle) is more specific than equals(Object circle). The output is true if the Circle class in (B) is used. The Circle class overrides the equals(Object circle) method defined in the Object class. At compilation time, circle1.equals(circle2) is matched to equals(Circle circle) and at runtime, the equals(Object circle) method implemented in the Circle class is invoked. 16. How do you create an ArrayList? Use ArrayList list = new ArrayList(); How do you append an object to a list? list.add(object); How do you insert an object at the beginning of a list? list.add(0, object); How do you find out the number of objects in a list? list.size();
- 4. How do you remove a given object from a list? list.remove(object); How do you remove the last object from the list? list.remove(list.size() - 1); How do you check whether a given object is in a list? list.contains(object); How do you retrieve an object at a specified index from a list? list.get(index); 17. Error 1: String city = list.get(0); Should be written as String city = (String)list.get(0); Error 2: list.set(3, "Dallas"); is wrong because there is no element at index 3 in the list. Error 3: list.get(3) is wrong because there is no element at index 3 in the list. 18. default visibility modifier. 19. protected. 20. If the question marks are replaced by blanks, can class B be compiled? Yes. If the question marks are replaced by private, can class B be compiled? No. If the question marks are replaced by protected, can class B be compiled? Yes. 21. If the question marks are replaced by blanks, can class B be compiled? No. If the question marks are replaced by private, can class B be compiled? No.
- 5. If the question marks are replaced by protected, can class B be compiled? Yes. 22. Use the final keyword. 23. Find these terms in this chapter. 24. Indicate true or false for the following statements: 1. True. 2. False. (But yes in a subclass that extends the class where the protected datum is defined.) 3. True. 4. A final class can have instances. Answer: True 5. A final class can be extended. Answer: False 6. A final method can be overridden. Answer: False 7. You can always successfully cast a subclass to a superclass. Answer: True 8. You can always successfully cast a superclass to a subclass. Answer: False 25. Matching a method signature and binding a method implementation are two separate issues. The declared type of the reference variable decides which method to match at compile time. The compiler finds a matching method according to parameter type, number of parameters, and order of the parameters at compile time. A method may be implemented in several subclasses. The JVM dynamically binds the implementation of the method at runtime, decided by the actual class of the object referenced by the variable. 26. See the text.
- 6. If the question marks are replaced by protected, can class B be compiled? Yes. 22. Use the final keyword. 23. Find these terms in this chapter. 24. Indicate true or false for the following statements: 1. True. 2. False. (But yes in a subclass that extends the class where the protected datum is defined.) 3. True. 4. A final class can have instances. Answer: True 5. A final class can be extended. Answer: False 6. A final method can be overridden. Answer: False 7. You can always successfully cast a subclass to a superclass. Answer: True 8. You can always successfully cast a superclass to a subclass. Answer: False 25. Matching a method signature and binding a method implementation are two separate issues. The declared type of the reference variable decides which method to match at compile time. The compiler finds a matching method according to parameter type, number of parameters, and order of the parameters at compile time. A method may be implemented in several subclasses. The JVM dynamically binds the implementation of the method at runtime, decided by the actual class of the object referenced by the variable. 26. See the text.