JAVA练习题含答案-answer to practice 7.doc

Chapter 7Inheritance13Chapter 7InheritancenMultiple Choice1) Inheritance is the process by which a new class known as a _ - is created from another class, called the _.(a) base class, derived class(b) derived class, base class(c) inherited class, base class(d) base class, inherited classAnswer: B 2) Inheritance promotes code _.(a) reinvention(b) reuse(c) repeats(d) all of the aboveAnswer: B3) The keyword extends indicates:(a) encapsulation(b) polymorphism(c) inheritance(d) none of the aboveAnswer: C 4) A derived class is also called a(a) sub class(b) super class(c) base class(d) all of the aboveAnswer: A5) A super class is also called a(a) derived class(b) dominant class(c) sub class(d) base classAnswer: D6) What does a derived class automatically inherit from the base class?(a) instance variables(b) static variables(c) public methods(d) all of the aboveAnswer: D7) If the final modifier is added to the definition of a method, this means:(a) The method may be redefined in the derived class.(b) The method may be redefined in the sub class.(c) The method may not be redefined in the derived class.(d) None of the above.Answer: C 8) A base class is synonymous with a:(a) Child class(b) Parent class(c) Derived class(d) Sub classAnswer: B 9) The special syntax for invoking a constructor of the base class is:(a) super()(b) base()(c) parent()(d) child()Answer: A 10) An object of a derived class has the type of the derived class, and it also has the type of the base class, and more generally, has the type of every one of its _ classes.(a) descendant(b) child(c) ancestor(d) subAnswer: C 11) In using the keyword this in place of super(), the invocation of this must be the _ action taken by the constructor.(a) first(b) last(c) it does not matter(d) none of the aboveAnswer: A 12) A method or instance variable modified by protected:(a) can not be accessed by name inside its own class definitions.(b) can not be accessed by name inside any class derived from it.(c) can not be accessed by name in the definition of any class in the same package.(d) can not be accessed by name in any other class (that is, other than classes named in a-c.).Answer: D 13) If an instance variable is not modified by public, protected or private then it is said to have:(a) Package access(b) Default access(c) Friendly access(d) All of the aboveAnswer: D 14) The class _ is an ancestor class of all Java classes.(a) String(b) Object(c) Math(d) JFrameAnswer: B 15) The Object class contains the method:(a) getClass()(b) toString()(c) equals()(d) all of the aboveAnswer: D 16) The equals method for a class should have _ as the type of its one parameter.(a) String(b) Object(c) Integer(d) DoubleAnswer: B nTrue/False1) A derived class contains only public instance variables and public methods from the base class.Answer: False 2) Inheritance refers to a very specialized form of a class.Answer: False 3) A derived class is a class defined by adding instance variables and methods to an existing class.Answer: True 4) When you define a derived class, you give only the added instance variables and the added methods as well as all the methods from the base class.Answer: False 5) The keyword extends indicates polymorphism.Answer: False 6) Overriding is when a derived class redefines a method from the base class.Answer: True 7) A constructor for a derived class begins with an invocation of a constructor for the base class.Answer: True8) The call to the base class constructor (super) must always be the last action taken in a constructor definition.Answer: False 9) You may substitute the keyword this for super() to call a constructor of the derived class.Answer: True 10) An instance variable (or method) that is private in a base class is accessible by name in the definition of a method in any other class.Answer: False 11) Private methods of the base class are not available for use by derived classes.Answer: True nShort Answer/Essay1) Define a base class to represent a Clock. Your class should have instance variables for hours, minutes and seconds.Answer:public class Clockprivate int hour;private int minute;private int second;public Clock()/initialize to default valueshour = 0;minute = 0;second = 0;public Clock(int h, int m, int s)setHour(h);setMinute(m);setSecond(s);/*Valid integers for hour is in the range of 0 - 24*/public void setHour(int h)if(h = 0) & (h = 0) & (m = 0) & (s = 0)length = l;elseSystem.out.println(Fatal error: Length may not be negative!);public void setWidth(double w)if(w = 0)width = w;elseSystem.out.println(Fatal error: Width may not be negative!);public double getLength()return length;public double getWidth()return width;public double getArea()return length * width;public double getPerimeter()return 2 * (length + width);public String toString()return Length = + length + nWidth = + width;public boolean equals(Object o)if(o = null)return false;else if(getClass() != o.getClass()return false;elseRectangle otherRectangle = (Rectangle) o;return(length = otherRectangle.length) & (width = otherRectangle.width);6) Create a test driver to test the functionality of your Rectangle class created in number 14 above.Answer:public class RectangleTestpublic static void main(String args)Rectangle r1 = new Rectangle();Rectangle r2 = new Rectangle(5,6);Rectangle r3 = r2;System.out.println(r1.toString() + n + r2.toString() +n + r3.toString();r1.setLength(8);r1.setWidth(10);System.out.println(The area of r1 = + r1.getArea();System.out.println(The perimeter of r1 = + r1.getPer