Chapter 10. Further Abstraction Techniques

Transcription

1 Chapter 10 Further Abstraction Techniques In the previous chapter, we saw how Java checks the usage of methods. We also saw that if the method is not defined in the superclass, the compiler will not work. There is a way to define the method in the superclass without really defining it. We can define an abstract method in the superclass. An abstract method does not have a body, and leave the implementation to the subclasses. If you think about the displaystudent() method, that we defined in the Student superclass: public void displaystudent() System.out.println("The name of the student is: " + name); if (studenttype.equals("gr")) System.out.println("The student is a Graduate student"); System.out.println("The student is an Undergraduate student"); // End of Display Student And the displaystudent( ) methods that we defined in the Graduate and Undergraduate subclasses: public void displaystudent() super.displaystudent(); boolean fin = true; for( int i=0; i < NUM_OF_TESTS; i++) int score = gettestscore(i); if (score == -1) fin = false; System.out.println("The score for test " + (i+1) + " is " + score); if (fin) computecoursegrade(); System.out.println("The course grade is " + getcoursegrade()); // End of displaystudent And public void displaystudent() super.displaystudent();

2 boolean fin = true; for( int i=0; i < NUM_OF_TESTS; i++) int score = gettestscore(i); if (score == -1) fin = false; System.out.println("The score for test " + (i+1) + " is " + score); if (fin) computecoursegrade(); System.out.println("The course grade is " + getcoursegrade()); // End of displaystudent With the new idea of abstract methods, they could be simplified. We define two abstract methods in the Student superclass: abstract public void displaystudent(); abstract public int getnumberoftests(); Then the two subclasses implement these two methods in the following way: In the Graduate subclass we will have: public void displaystudent() System.out.println("The name of the student is: " + getname()); System.out.println("The student is a Graduate student"); boolean fin = true; for( int i=0; i < NUM_OF_TESTS; i++) int score = gettestscore(i); if (score == -1) fin = false; System.out.println("The score for test " + (i+1) + " is " + score); if (fin) computecoursegrade(); System.out.println("The course grade is " + getcoursegrade()); // End of displaystudent And

3 public int getnumberoftests() return NUM_OF_TESTS; In the Undergraduate class, the implementation will be: public void displaystudent() System.out.println("The name of the student is: " + getname()); System.out.println("The student is an Undergraduate student"); boolean fin = true; for( int i=0; i < NUM_OF_TESTS; i++) int score = gettestscore(i); if (score == -1) fin = false; System.out.println("The score for test " + (i+1) + " is " + score); if (fin) computecoursegrade(); System.out.println("The course grade is " + getcoursegrade()); // End of displaystudent And public int getnumberoftests() return NUM_OF_TESTS; Note that the abstract method and the method that implements it on the subclasses should be such that all of them return the same type, and the type and number of arguments should also be the same. Also note that a class that has any abstract method can t be implemented, and we must define the class as an abstract class. We only will have instances of the subclasses. So our program for working with the students will look now like this: /** * Student is an abstract superclass that encloses both graduate and undergraduate students * A. Gutierrez */

4 public abstract class Student private String name; private int test[]; private String coursegrade; //constructor public Student(String studentname, int howmanytests) name = studentname; test = new int[howmanytests]; coursegrade = "*******"; // Setting a non valid grade on the student tests for(int i= 0; i < howmanytests ; i++) test[i] = -1; // accessors public String getcoursegrade() return coursegrade; public String getname() return name; public int gettestscore(int testnumber) return test[testnumber]; // mutators public void setname(string newname) name = newname; public void settestscore(int testnumber, int testscore) test[testnumber- 1] = testscore; public void setcoursegrade(string sgrade) coursegrade = sgrade; // Two new abstract methods abstract public void displaystudent();

5 abstract public int getnumberoftests(); // End Class Student /** * Undergraduate is a subclass that extends the Student class. Implements two abstract methods * A. Gutierrez */ import java.util.*; public class Undergraduate extends Student private static final int NUM_OF_TESTS = 3; private static Scanner myinputs = new Scanner(System.in); public Undergraduate(String sname) super(sname, NUM_OF_TESTS); public void computecoursegrade( ) int total = 0; for (int i = 0; i < NUM_OF_TESTS; i++) total += gettestscore(i); if (total/num_of_tests >= 70) setcoursegrade( "Pass"); setcoursegrade( "No Pass"); public void displaystudent() System.out.println("The name of the student is: " + getname()); System.out.println("The student is an Undergraduate student"); boolean fin = true; for( int i=0; i < NUM_OF_TESTS; i++) int score = gettestscore(i); if (score == -1) fin = false; System.out.println("The score for test " + (i+1) + " is " + score); if (fin) computecoursegrade();

6 System.out.println("The course grade is " + getcoursegrade()); // End of displaystudent public void settestscore(int testnumber, int testscore) String answer= ""; if (gettestscore(testnumber -1)!= -1) System.out.println("This student has already grade for this test"); System.out.println("Do you want to enter a new grade for test # "+ testnumber +"?"); System.out.println("Please enter y/n."); System.out.print(">"); answer = myinputs.nextline(); answer = new String(answer.trim().toUpperCase()); answer = "Y"; if (answer.equals("y")) super.settestscore(testnumber, testscore); // End of settestscore public int getnumberoftests() return NUM_OF_TESTS; // End of Undergraduate Class /** * Graduate is a subclass that extends the Student class. It implements two abstract methods * A. Gutierrez */ import java.util.*; public class Graduate extends Student private static final int NUM_OF_TESTS = 2; private static Scanner myinputs = new Scanner(System.in); public Graduate(String sname) super(sname, NUM_OF_TESTS); public void computecoursegrade( )

7 int total = 0; for (int i = 0; i < NUM_OF_TESTS; i++) total += gettestscore(i); if (total/num_of_tests >= 80) setcoursegrade( "Pass"); setcoursegrade( "No Pass"); public void displaystudent() System.out.println("The name of the student is: " + getname()); System.out.println("The student is a Graduate student"); boolean fin = true; for( int i=0; i < NUM_OF_TESTS; i++) int score = gettestscore(i); if (score == -1) fin = false; System.out.println("The score for test " + (i+1) + " is " + score); if (fin) computecoursegrade(); System.out.println("The course grade is " + getcoursegrade()); // End of displaystudent public void settestscore(int testnumber, int testscore) String answer= ""; if (gettestscore(testnumber -1)!= -1) System.out.println("This student has already grade for this test"); System.out.println("Do you want to enter a new grade for test # "+ testnumber +"?"); System.out.println("Please enter y/n."); System.out.print(">"); answer = myinputs.nextline(); answer = new String(answer.trim().toUpperCase()); answer = "Y"; if (answer.equals("y")) super.settestscore(testnumber, testscore); // End of settestscore public int getnumberoftests()

8 return NUM_OF_TESTS; // End of Graduate Class import java.util.*; /** * AllStudents is a collection of Student objects. * It uses the HashMap class, so be sure to import the java.util.hashmap, or * the java.util.* package * A. Gutierrez */ public class AllStudents // Storage for an arbitrary number of Undergraduate objects private HashMap<String, Student > alstudent; public Scanner myscanner = new Scanner(System.in); // Constructor public AllStudents () alstudent = new HashMap<String, Student >(); // Mutators public void storestudent(student nstudent) /* Check that the student is not already in the system, before inserting it into the * HashMap. */ // We could omit the checking because it was done in Working with students String name = nstudent.getname(); boolean exists = alstudent.containskey(name); if (exists) System.out.println("This student is already in the system"); alstudent.put(name, nstudent); // end of storestudent

9 public void modifystudent(student nstudent) String name = nstudent.getname(); alstudent.remove(name); alstudent.put(name, nstudent); // end of modifystudent public void removestudent(string studname) Student studenttodelete = alstudent.get(studname); System.out.println("Is the student " + studname +" the one you want to remove?"); System.out.println("Please enter y/n."); System.out.print(">"); //Gets the next String of data from the keyboard up to a "\n" and stores it in name String answer = myscanner.nextline(); // Convert the answer to upper case to establish a canonical form for the answer answer = new String(answer.trim().toUpperCase()); if (answer.equals("y")) alstudent.remove(studname); System.out.println("You should verify the student you want to remove"); // end of removesudent // Accessors // For the students public int numberofstudents() return alstudent.size(); public void showstudent(student nstudent) nstudent.displaystudent(); // End of showstudent public Student getstudent(string studname) boolean exists = alstudent.containskey(studname);

10 if (!exists) return null; // If the student is in the system, retrieve it. Student getstudent = alstudent.get(studname); return getstudent; // End of getstudent // End of AllStudents class /* WorkingWithStudents is a program to insert, remove, modify and display students in the system * They could be graduate or undergraduate students * A. Gutierrez */ import java.util.*; public class WorkingWithStudents private static Scanner myinputs = new Scanner(System.in); // Creating HashMap private static AllStudents unstudent = new AllStudents(); public static void main(string args[]) // Menu int choice = 0; String name = ""; String type = ""; Student studenttowork = null; while (choice!= 5) System.out.println("Choose the number for your option:"); System.out.println("To insert a student:...1"); System.out.println("To enter grades:...2"); System.out.println("To remove a student:...3");

11 System.out.println("To display a student:...4"); System.out.println("To finish:...5"); System.out.println("Please enter your choice."); System.out.print(">"); choice = myinputs.nextint(); myinputs.nextline(); if (choice <=4 && choice >= 1) name = getstudentname(); studenttowork= unstudent.getstudent(name); if (studenttowork == null) if (choice > 1) System.out.println("We could not find the student with name " + name); for(int i = 0; i < 64536; i++); choice = -1; type = getstudenttype(); if (choice == 1) System.out.println("This student is already in the system"); choice = -1 ; switch (choice) case 1: insertstudent(name, type); break; case 2: modifystudent(studenttowork); break; case 3: unstudent.removestudent(name); break; case 4: unstudent.showstudent(studenttowork); break; case 5: break; default: if (choice!= -1) System.out.print("Your choice should be a number between"); System.out.println(" 1 and 5. You entered " + choice); // End switch // End while // End main public static void insertstudent(string sname,string stype)

12 Student newstudent; if (stype.equals("gr")) newstudent = new Graduate(sName); newstudent = new Undergraduate(sName); unstudent.storestudent(newstudent); // end of insertstudent public static void modifystudent(student studenttomodify) String answer= ""; do int testchoice = 0; // Select the test to enter grades for System.out.println("Choose the number of the test you want to enter grades"); System.out.println("Please enter the number."); System.out.print(">"); testchoice = myinputs.nextint(); myinputs.nextline(); if ((testchoice > studenttomodify.getnumberoftests()) (testchoice < 1)) System.out.println("Wrong test number"); System.out.println("The number should be between 1 and " + studenttomodify.getnumberoftests()); System.out.println("You entered " + testchoice ); // Enter the grades for that test int ntestscore = 0; System.out.println("Enter the score for the test # "+ testchoice ); System.out.print(">"); ntestscore = myinputs.nextint(); myinputs.nextline(); studenttomodify.settestscore(testchoice, ntestscore); unstudent.modifystudent(studenttomodify); // Check if more grades want to be entered for this student System.out.println("Do you want to enter more grades for this student?"); System.out.println("Please enter y/n."); System.out.print(">"); answer = myinputs.nextline(); // Convert the answer to upper case to establish a canonical form for the answer

13 answer = new String(answer.trim().toUpperCase()); // end of do while (answer.equals("y")); // End of modifystudent public static String getstudentname() System.out.print("Enter the name of the student"); System.out.print(" you want to work with "); String name; do name = myinputs.nextline(); name = new String(name.trim().toUpperCase()); while (name.equals("")); return name; // End of getstudentname public static String getstudenttype() System.out.println("Enter the type of the student Graduate (GR) or Undergraduate (UG) "); System.out.println(" you want to work with. Please don't enter the brackets before and after"); System.out.print("> "); String type; do type = myinputs.nextline(); type = new String(type.trim().toUpperCase()); while (type.equals("") &&!type.equals("gr") &&!type.equals("ug")); return type; // End of getstudenttype // End of WorkingWithStudents Java does not allow extending more than one superclass. In other word, every class will have, at most, one superclass. But Java allows to define something besides an abstract class and a concrete class. We are talking about an interface, which is like a class, in which all the methods are abstract methods, and all the fields, if any, are public static and final. So instead of defining a class we define an interface. By just declaring that something is an interface, you don t have to explicitly define the fields as public,

14 static and final (it is understood), neither you have to specify that the methods are abstract methods (again, it is understood). A class can inherit from an interface in a similar way to the way it inherits from a class. But now we must use the word implements instead of extends. Contrary to the extension of superclasses, a class may implement several interfaces. It has to implement all the methods in them. The advantage of the interfaces is that allows different classes of objects (the classes that implement the interface) to be treated uniformly, allowing, at the same time, the existence of polymorphic variables and methods. Note that if a class extends another one and implement several interface, the extends part should always go in the definition of the class before the implements part(s). Example: public class Polar extends Component implements MouseListener, MouseMotionListener. It is clear from the heading of the class Polar that this class will serve as its own MouseListener and MouseMotionListener implementing the methods found in both interfaces. In our case the interface MouseListener is defined as: public interface MouseListener extends EventListener public void mouseclicked(mouseevent e); // Invoked when the mouse button has been clicked (pressed and released) on a component. public void void mousepressed(mouseevent e); // Invoked when a mouse button has been released on a component. public void void mouseentered(mouseevent e); // Invoked when the mouse enters a component. public void mousereleased(mouseevent e); // Invoked when a mouse button has been released on a component. public void mouseexited(mouseevent e); // Invoked when the mouse exits a component.

15 Note that the interface MouseListener extends the interface EventListener, that all event listener interfaces must extend.