Practice Problems: Inheritance & Polymorphism

Transcription

1 Practice Problems: Inheritance & Polymorphism public class Foo public void method() System.out.println("foo "); public void method2() System.out.println("foo 2"); public String tostring() return "foo"; public class Bar extends Foo public void method2() System.out.println("bar 2"); public class Baz extends Foo public void method() System.out.println("baz "); public String tostring() return "baz"; public class Mumble extends Baz public void method2() System.out.println("mumble 2"); public class Polymorphism public static void main(string [] args) Foo[] pity = new Baz(), new Bar(), new Mumble(), new Foo() ; for (int i = 0; i < pity.length; i++) System.out.println(pity[i]); pity[i].method(); pity[i].method2(); System.out.println(); baz baz foo 2 foo foo bar 2 baz baz mumble 2 foo foo foo 2 *( ) method is inherited. Otherwise, method is overridden.. Tracing programs: The above is the program demonstrated in class. Now, what gets printed to the screen when we execute the following classes on the left?

2 public class A public int x = ; public void setx(int a) x=a; public class B extends A public int getb() setx(2); return x; public class C //trace program public static void main(string [] args) B b = new B(); System.out.println(a.x); System.out.println(b.getB()); public class A private int x = ; protected void setx(int a) x=a; protected int getx() return x; public class B extends A public int getb() setx(2); return getx(); public class C //trace program 2 public static void main(string [] args) B b = new B(); System.out.println(a.getX()); System.out.println(b.getB()); public class A protected int x = ; protected void setx(int a)x=a; protected int getx()return x; public class B extends A public int getb() setx(2); return x; public class C //trace program 3 public static void main(string [] args) B b = new B(); System.out.println(a.getX()); System.out.println(b.x); System.out.println(b.getB()); 2 direct access of attribute 2 use of method, instead of direct access of attribute, because the attribute is private! 2 protected

3 public class A protected int x = ; protected void setx(int a) x=a; protected int getx() return x; public class B extends A protected int x = 3; public int getx() return x; public int getb() return x; public class C //trace program 4 public static void main(string [] args) B b = new B(); System.out.println(a.getX()); System.out.println(b.getB()); System.out.println(b.getX()); System.out.println(a.x); System.out.println(b.x); public class A protected int x = ; protected void setx(int a) x=a; protected int getx() return x; public class B extends A protected int x = 3; public int getx() return x; public int getb() return x; public class C //trace program 5 public static void main(string [] args) A b = new B(); System.out.println(a.getX()); System.out.println(b.getX()); System.out.println(a.x); System.out.println(b.x); attribute shadowing, the above result is for comparison

4 public class A protected int x = ; protected void setx(int a) x=a; protected int getx() return x; public class B extends A protected int x = 3; public int getx() setx(2); return x; public int getb() return x; public class C //trace program 6 public static void main(string [] args) A b = new B(); System.out.println(a.getX()); System.out.println(b.getX()); System.out.println(a.x); System.out.println(b.x); public class Ham public void a() System.out.println("Ham a"); public void b() System.out.println("Ham b"); public String tostring() return "Ham"; public class Lamb extends Ham public void b() System.out.println("Lamb b"); public class Yam extends Lamb public void a() System.out.println("Yam a"); public String tostring() return "Yam"; 3 or in newer version of Java: 3 2 After setx(2), attribute of inherited part x = 2. Yam Spam a Lamb b Yam Yam a Lamb b Ham Ham a Ham b Ham Ham a Lamb b Check the order in the array declaration! public class Spam extends Yam public void a() System.out.println("Spam a"); public class Polymorphism2 //trace program 7

5 public static void main (String [] args) Ham[] food = new Spam(), new Yam(), new Ham(), new Lamb() ; for (int i = 0; i < food.length; i++) System.out.println(food[i]); food[i].a(); food[i].b(); System.out.println(); public class Ham int a = 0; int b = ; public void a() System.out.println("Ham + a); public void b() System.out.println("Ham + b); public String tostring() return "Ham + a + + b; public class Spam extends Ham int a = 2; public void a() System.out.println("Spam +a); public class Yam extends Spam int b = 3; public void a() System.out.println("Yam + a); public void b() System.out.println( Yam + b); public class Polymorphism3 //trace program 8 public static void main (String [] args) Ham[] food = new Spam(), new Yam(), new Ham(); for (int i = 0; i < food.length; i++) System.out.println(food[i]); food[i].a(); food[i].b(); Ham 0 Spam 2 Ham 0 Ham 0 Yam 2 Yam 3 0 Ham 0 Ham 0 Ham 0 Same tostring! Local method use local attribute. There is no polymorphism here. No type declaration twined. For instance, Spam 2. But for any attribute defined in multiple level inheritance, the closest one is inherited. For instance Yam 2. Attribute direct access via super class? Use the one in super class for instance, 0 and printed out at the end. System.out.println(food[i].a); System.out.println(food[i].b); System.out.println();

6 public class A private String x = "Ax"; protected String y = "Ay"; public String z = "Az"; public String tostring() return x + y + z; public static void main( String [] args) System.out.println(a); public class B extends A private String x = "Bx"; public String z = "Bz"; public String tostring() return x + y + z; public static void main( String [] args) B b = new B(); System.out.println(b); public class C extends A private String x = "Cx"; public static void main( String [] args) C c = new C(); System.out.println(c.x); System.out.println(c); public class D extends C //trace program 9 private String x = "Dx"; public String z = "Dz"; public static void main( String [] args) D d = new D(); System.out.println(d.x); System.out.println(d.y); System.out.println(d.z); System.out.println(d); C c = new D(); // Error: System.out.println(c.x); System.out.println(c.y); System.out.println(c.z); System.out.println(c); When A is executed, it displays: AxAyAz The println statement implicitly calls a.tostring(), which creates a string containing the concatenation of the variables x, y, and z. Once this concatenated String ("AxAyAz") is returned, it gets printed to the screen. When B is executed, it displays: BxAyBz The println statement implicitly calls b.tostring(), which refers to the overriding tostring() method defined in subclass B. This tostring method says to concatenate x+y+z (like the one defined in class A), but now it is referring to variables x, y, and z in the B class. Two of those variables are defined locally x and z. These variables hide (or "shadow") the x and z variables defined in class A. The third variable, y, is inherited from A. As a result, the concatenation in the tostring method produces a String that looks like "BxAyBz", which then gets printed.

7 When C is executed, it displays: Cx AxAyAz When the first println statement executes, it refers directly to c.x, which is defined locally. So that prints out "Cx". When the second println statement executes, it refers to C's inherited tostring method. The inherited tostring method is defined in class A, which returns the concatenation of x, y, and z from class A. So that returns "AxAyAz", which gets printed. When D is executed, it displays: Dx Ay Dz DxAyDz Ay Az DxAyDz The first 3 lines print the values of x, y, and z inside of d. Since x and z are defined inside of class D, those values get printed out. y is inherited from class A, so "Ay" gets printed out. The next line prints the return value of D's tostring method. The tostring method is defined locally to override the inherited one (unlike in the example for class C, where the tostring method is inherited instead of overridden). Because the tostring method is overridden, when it refers to x, y, and z, it refers to the variables inside of class D. So this returns "DxAyDz". Compare this to the inherited tostring method in class C, which returns "AxAyAz". The next two lines use a variable with static type C to refer to an object of dynamic type D. Notice that it is an error to try to print (or refer to) c.x. That's because 'x' is private in class C, and this code is written inside class D. Also notice that c.z is "Az", whereas d.z is "D.z". For fields, Java uses the value of the static type's field (in this case, the value of z from class C, which is inherited from class A and has value "Az"). The last line prints the value of c.tostring(). Java uses the value of a the static type's field, but the dynamic type's methods. Variable c has dynamic type D, because it refers to an object of type D. So Java uses the tostring method defined in class D, which returns the values of x, y, and z within class D (or "DxAyDz"). Notice the difference between how fields get handled, and how methods get handled. The field c.z refers to the field defined in class C (which is inherited from class A). The method c.tostring() refers to the method defined in class D, not class C. I have still not figured out any reason why Java does shadow things this way for fields. It's very confusing, and it can lead to very hard-to-fix bugs. In general, it is HIGHLY RECOMMENDED that you AVOID defining fields with the same name as a superclass's field. Sometimes though (like when you're extending a superclass from the Java API), you may not know what the superclass's fields are called, and in that case, you just have to guess.

8 2. Program Development a. Below is a partially filled-out Time class, which stores the hour and minute in private attributes. The constructor is also given. public class Time implements Comparable private int hour =0; private int minute =0; public Time (int h, int m) if (h<0 h>23) return; if(m<0 m>59) return; hour = h; minute = m; // additional (instance) methods will go here. Please complete the above Time class with methods compareto and tostring, in order to support the following application class and reach the desired result: import java.util.arrays; public class TimeCheck public static void main(string args[]) Time [] t = new Time(3, 5), new Time(6, 0), new Time(4, 29), new Time(, 59), new Time(23, ), new Time(2, 59), new Time (4, 9) ; Arrays.sort(t); for (Time f: t) System.out.println(f+",");

9 public class Time implements Comparable 2 private int hour =0; 3 private int minute =0; 4 public Time (int h, int m) 5 if (h<0 h>23) return; 6 if(m<0 m>59) return; 7 hour = h; 8 minute = m; // 0 pts 3 public int gethour() 4 return hour; 5 6 public int getminute() 7 return minute; // = =8 pts 2 public int compareto(object o) 22 if( o!=null && (o instanceof Time)) 23 Time r = (Time) o; 24 int t = hour * 60 + minute; 25 int t2 = r.hour * 60 + r.minute; 26 if (t<t2) return -; 27 else if(t>t2) return ; 28 else return 0; return -; 3 32 // 6 pts 33 public String tostring() 34 return "("+hour+", "+minute+")"; b. Given the super class Student (contained in the package that is available at Please complete the following class Records, in order to support the weighted score grading by the Test program and reach the desire result by using the input file trial. import javax.swing.joptionpane; import java.io.*; import java.util.*; public class Test public static void main(string [] args) Records [] t = new Records("CSC240", 2, 4, 3, 60, 0, 30), // 2 tests, 4 quizzes, and 3 assignments new Records("CSC32",, 3, 3, 25, 5, 60),

10 // test, 3 quizzes, and 3 assignments new Records("CSC5", 3, 0, 3, 60, 0, 40); // 3 tests, 0 quiz, and 3 assignments String filename = JOptionPane.showInputDialog("Enter the file name: "); File inputfile = new File (filename); try Scanner input = new Scanner(inputFile); String val; while(input.hasnext()) val = input.nextline(); String [] data = val.split(" "); int index = Integer.parseInt(data[0]); char opt = data[].tolowercase().charat(0); int [] s = new int[data.length-2]; for(int i = 0; i<s.length; i++) s[i] = Integer.parseInt(data[i+2]); switch(opt) case 't': t[index].set_test(s); break; case 'q': t[index].set_quiz(s); break; case 'a': t[index].set_assignment(s); break; input.close(); catch (FileNotFoundException e) System.out.println("file reading fails."); for(int i = 0; i<t.length; i++) Records tmp = t[i]; tmp.grading(); System.out.println(tmp);

11 In detail, the class Records should have the private attributes and the constructor as follows: import java.util.arrays; public class Records extends Student private int [] test = null; private int [] quiz = null; private int [] assignment = null; public Records (String name, int tn, int qn, int an, int tw, int qw, int aw) super(name, tw, qw, aw); test = new int [tn]; quiz = new int [qn]; assignment = new int [an]; This class should also have the following methods: ) set_test: that will accept a group of test scores in an integer array. The values will copy to the private attribute array test. 2) set_quiz: that will accept a group of quiz scores in an integer array. The values will copy to the private attribute array quiz. 3) set_assignment: that will accept a group of assignment scores in an integer array. The values will copy to the private attribute array assignment. 4) get_test: will calculate and return the average of test scores in attribute array test. 5) get_quiz: will calculate and return the average of quiz scores in attribute array quiz. 6) get_assignment: will calculate and return the average of assignment scores in attribute array assignment. 7) grading: will call the method letter_grading of the super class to set the GPA score (i.e., attribute of super class). 8) tostring: will return the information in a String, which carries both the course information and the scores of tests, quizzes, and assignments. GPA information is also needed.

12 import javax.swing.joptionpane; 2 import java.util.arrays; 3 import java.io.*; 4 import java.util.*; 5 6 public class Records extends Student 7 private int [] test = null; 8 private int [] quiz = null; 9 private int [] assignment = null; 0 public Records(String name, int tn, int qn, int an, int tw, int qw, int aw) 2 super(name, tw, qw, aw); 3 test = new int [tn]; 4 quiz = new int [qn]; 5 assignment = new int [an]; // = = 0 pts 9 public void set_test(int [] s) 20 if (s.length!=test.length) return; // the size is determined for(int i = 0; i<test.length; i++) 23 test[i] = s[i]; // = 0 pts; 28 public void set_quiz(int [] s) 29 if (s.length!=quiz.length) return; 30 3 for(int i = 0; i<quiz.length; i++) 32 quiz[i] = s[i]; // = 0 pts 37 public void set_assignment(int [] s) 38 if (s.length!=assignment.length) return; for(int i = 0; i<assignment.length; i++) 4 assignment[i] = s[i]; // 0 pts 46 public double get_test() 47 double total = 0.0; 48 if(test.length<=0) return 0; 49 for(int i = 0; i<test.length; i++) 50 total+= test[i]; 5 return total/test.length; // 0 pts 55 public double get_quiz() 56 double total = 0.0; 57 if(quiz.length<=0) return 0;

13 58 for(int i = 0; i<quiz.length; i++) 59 total+= quiz[i]; 60 return total/quiz.length; // 0 pts 64 public double get_assignment() 65 double total = 0.0; 66 if(assignment.length<=0) return 0; 67 for(int i = 0; i<assignment.length; i++) 68 total+= assignment[i]; 69 return total/assignment.length; // 0 pts 73 public void grading() 74 double [] tga = get_test(), get_quiz(), get_assignment(); 75 letter_grading(tga); // 5 pts 79 public String tostring() 80 String ret = ""; 8 if(test.length>0) 82 ret+="test score:"+arrays.tostring(test)+"\n"; 83 if(quiz.length>0) 84 ret+="quiz score:"+arrays.tostring(quiz)+"\n"; 85 if(assignment.length>0) 86 ret+="assignment score:"+arrays.tostring(assignment)+"\n"; 87 grading(); 88 ret+="gpa: "+get_gpa()+"\n"; 89 return ret;