1005ICT Object Oriented Programming Lecture Notes

Transcription

1 1005ICT Object Oriented Programming Lecture Notes School of Information and Communication Technology Griffith University Semester 2,

2 16 Generics This section introduces generics, or parameterised classes. The main use for this is to define container classes, which define useful abstract data types for storing collections of objects. It does this by way of example code for a container class, a double ended queue, and a program that uses it, with and without generics. 1005ICT Object Oriented Programming

3 16.1 Before parameterised classes Early versions of Java provided classes that implemented collections such as linked lists and hash tables. They all declared the type Object for their contained values. They did this so that the container classes could contain any object type, because they are all subclasses of Object. Putting any object into a container was easy. Getting an object out of a container was error prone, because it was up to the programmer to ensure that the object that came out was actually the type they thought they had put in. The compiler could not help by doing its job type checking. 1005ICT Object Oriented Programming

4 Example Deque This class defines a double ended queue (without generics). It is implemented with a doubly linked list with a dummy head. Deque0 -value : Object -prev : DequeO -next : DequeO -length : int +DequeO() -DequeO(value : Object) +addtoback(value : Object) +addtofront(value : Object) +front() : Object +back() : Object +dropback() : void +dropfront() : void +isempty() : boolean +length() : int 1005ICT Object Oriented Programming

5 front back dummy 1005ICT Object Oriented Programming

6 /* ** file: DequeO.java ** author: Andrew Rock ** purpose: A double ended queue implementation ** using a circular doubly linked list ** with a dummy head node. This version ** does not use generics, and saves only ** references to Object. */ import java.util.*; public class DequeO { // the value stored at each node private Object value; 1005ICT Object Oriented Programming

7 private DequeO prev, // previous node, // previous of head is the front next; // next node, // next of the head is the back // length of the deque, // only used in the dummy head private int length = 0; // new DequeO() makes a new empty deque with // a dummy head node. public DequeO() { value = null; prev = next = this; 1005ICT Object Oriented Programming

8 // new DequeO(value) makes a real node to // save the value. private DequeO(Object value) { this.value = value; // addtoback(value) adds this value to the back. public void addtoback(object value) { DequeO n = new DequeO(value); this.next.prev = n; n.next = this.next; n.prev = this; this.next = n; this.length++; 1005ICT Object Oriented Programming

9 // addtofront(value) adds this value to the // front. public void addtofront(object value) { DequeO n = new DequeO(value); this.prev.next = n; n.prev = this.prev; n.next = this; this.prev = n; this.length++; 1005ICT Object Oriented Programming

10 // front() returns the value at the front. public Object front() { if (length > 0) { return prev.value; else { throw new NoSuchElementException(); // back() returns the value at the back. public Object back() { if (length > 0) { return next.value; else { throw new NoSuchElementException(); 1005ICT Object Oriented Programming

11 // dropfront() detaches the front value. public void dropfront() { if (length > 0) { this.prev = prev.prev; prev.next = this; this.length--; else { throw new NoSuchElementException(); 1005ICT Object Oriented Programming

12 // dropback() detaches the back value. public void dropback() { if (length > 0) { this.next = next.next; next.prev = this; this.length--; else { throw new NoSuchElementException(); // isempty() returns true iff the deque is // empty. public boolean isempty() { return length == 0; 1005ICT Object Oriented Programming

13 // length() returns the number of values in the // deque currently. public int length() { return length; 1005ICT Object Oriented Programming

14 Example problem requiring a deque An organisation has a strict human resources policy for staff. The only person that can be fired, is the most recently hired. The only person that can retire, is the least recently hired. Write a staff management program that saves the names of new hires and reports the names of the person to fire or retire. The program should be interactive, with the following commands: h name hire a person with this name. f fire the most recent hire. r retire the least recent hire. q save the staff list for the next session and quit. 1005ICT Object Oriented Programming

15 An employee This class represents one employee. Each employee is to be assigned a unique id number. The static members support this. Employee -next_id : int -id : int -name : String +Employee(name : String) +Employee(id : int, name : String) +getnextid() : int +setnextid(next_id : int) : void +getid() : int +getname() : String The two different constructors are for a genuinely new employee to be assigned the next id, and for an employee with an existing id. 1005ICT Object Oriented Programming

16 /* ** file: Employee.java ** author: Andrew Rock ** purpose: A personnel record. */ import java.util.*; import java.io.*; public class Employee { // the next available id number private static int next_id = 1; private int id; // employee id number 1005ICT Object Oriented Programming

17 private String name; // employee name // new Employee(name) makes a new Employee with // this name and the next available id number. public Employee(String name) { this.id = next_id; next_id++; this.name = name.trim(); // new Employee(id, name) makes a new Employee // with this id and name. public Employee(int id, String name) { this.id = id; this.name = name.trim(); 1005ICT Object Oriented Programming

18 // getnextid() returns the next available // id number. public static int getnextid() { return next_id; // setnextid(next_id) sets the next available // id number. public static void setnextid(int next_id) { Employee.next_id = next_id; // getid() returns this employee s id. public int getid() { return id; 1005ICT Object Oriented Programming

19 // getname() returns this employee s name. public String getname() { return name; 1005ICT Object Oriented Programming

20 The program This class is the main program. The private methods represent the major actions: CampbellO -d : DequeO +main(args : String[]) : void -load() : void -save() : void -interact() : void 1. Load the employee list from a file if there is one. 2. Interact with the user, modifying the employee list. 3. Save the employee list to a file. 1005ICT Object Oriented Programming

21 /* ** file: CampbellO.java ** author: Andrew Rock ** purpose: A dumb HRM system, without generics. */ import java.util.*; import java.io.*; public class CampbellO { // A deque contains the employees. private static DequeO d = new DequeO(); 1005ICT Object Oriented Programming

22 // saved data file name private static final String FILE_NAME = "Campbell.txt"; public static void main(string[] args) { load(); interact(); save(); 1005ICT Object Oriented Programming

23 // save() saves the next_id and employees // for the next session. private static void save() { try { PrintStream out = new PrintStream(FILE_NAME); out.println(employee.getnextid()); while (!d.isempty()) { Employee f = (Employee) d.front(); out.println(f.getid() + " " + f.getname()); d.dropfront(); out.close(); Note the required type cast. 1005ICT Object Oriented Programming

24 catch (Exception e) { System.err.println("Could not save " + FILE_NAME); System.err.println(e); 1005ICT Object Oriented Programming

25 // load() loads the next_id and employees // left from the previous session. private static void load() { try { Scanner in = new Scanner(new File(FILE_NAME)); Employee.setNextId(in.nextInt()); while (in.hasnextint()) { d.addtoback (new Employee(in.nextInt(), in.nextline())); in.close(); catch (Exception e) { // skip (there was no data file) 1005ICT Object Oriented Programming

26 // interact() prompts for and processes user // commands. private static void interact() { Scanner sc = new Scanner(System.in); System.out.print("? "); String command = sc.next(); while (!command.equals("q")) { if (command.equals("h")) { Employee e = new Employee(sc.nextLine()); d.addtoback(e); System.out.println(e.getName() + " is hired as employee number " + e.getid() + "."); 1005ICT Object Oriented Programming

27 else if (command.equals("f")) { try { Employee b = (Employee) d.back(); System.out.println("Employee " + b.getid() + ", " + b.getname() + ", is fired."); d.dropback(); catch (NoSuchElementException e) { System.err.println( "No-one to fire."); 1005ICT Object Oriented Programming

28 else if (command.equals("r")) { try { Employee f = (Employee) d.front(); System.out.println("Employee " + f.getid() + ", " + f.getname() + ", has retired."); d.dropfront(); catch (NoSuchElementException e) { System.err.println( "No-one to retire."); 1005ICT Object Oriented Programming

29 else { System.out.println("Bad command."); System.out.print("? "); command = sc.next(); In this program, putting Employees into the deque is easy, but taking them out requires a type cast back to Employee. The compiler can t tell whether what was put in was really an Employee. Mistakes won t be discovered until run time. 1005ICT Object Oriented Programming

30 16.2 Parameterised classes Arrays are an example of a parameterized type. We always declare an array by specifying its element type. Arrays with different element types are different types themselves. The compiler can check that all uses of the elements are correct with respect to their types. The deque example above is useful for elements of any type, but the compiler can t check that the elements are used properly, causing type errors at run time. A safe solution would be to write a new implementation of deque for every element type, but that would be labourious. 1005ICT Object Oriented Programming

31 Parameterised classes, generics, allow the compiler to do this for us. In C++, this is done by the compiler literally creating new source files with the actual types substituted. C++ parameterised classes are called templates. 1005ICT Object Oriented Programming

32 Generics syntax Creating a parameterised class is similar to creating a method. A method has parameters, that are passed actual values at run time. A parameterised class has a type parameter that is substituted for an actual type at compile time. These type parameters are written between angle brackets < > after the class name. Separate multiple type parameters with commas. The term generics, comes from the idea that the type parameter is just some unspecified, generic class. 1005ICT Object Oriented Programming

33 Deque with generics This is how a parameterised class is represented in UML. The dummy parameter type, E (for element), is declared in the dashed box at top-right. The members are declared in terms of that type. E has replaced all mentions of Object. E DequeG -value : E -prev : DequeG<E> -next : DequeG<E> -length : int +DequeG() -DequeG(value : E) +addtoback(value : E) +addtofront(value : E) +front() : E +back() : E +dropback() : void +dropfront() : void +isempty() : boolean +length() : int 1005ICT Object Oriented Programming

34 /* ** file: DequeG.java ** author: Andrew Rock ** purpose: A double ended queue implementation ** using a circular doubly linked list ** with a dummy head node. This version ** uses generics. */ import java.util.*; public class DequeG<E> { The class name is parameterised in the class header. This declares the dummy type, which can now be used throughout the rest of the class. 1005ICT Object Oriented Programming

35 // the value stored at each node private E value; The value field is now an E, rather than an Object. private DequeG<E> prev, // previous node, // previous of head is the front next; // next node, // next of the head is the back The references to the previous and next nodes must be declared with their full type, parameter and all. 1005ICT Object Oriented Programming

36 // new DequeG() makes a new empty deque with // a dummy head node. public DequeG() { value = null; prev = next = this; // new DequeG(value) makes a real node to // save the value. private DequeG(E value) { this.value = value; The names of the constructors should not be parameterised. 1005ICT Object Oriented Programming

37 // addtoback(value) adds this value to the back. public void addtoback(e value) { DequeG<E> n = new DequeG<E>(value); this.next.prev = n; n.next = this.next; n.prev = this; this.next = n; this.length++; The methods are all modified in this manner, and the rest are omitted from these notes for brevity. 1005ICT Object Oriented Programming

38 The client program with generics Now, the HRM program with the differences indicated. CampbellG -d : DequeG<Employee> +main(args : String[]) : void -load() : void -save() : void -interact() : void /* ** file: CampbellG.java ** author: Andrew Rock ** purpose: A dumb HRM system, using generics. */ 1005ICT Object Oriented Programming

39 // A deque contains the employees. private static DequeG<Employee> d = new DequeG<Employee>(); Now the deque is declared with the actual type parameter, Employee. The rest of the program is the same, except that no type casts are required when elements are retrieved from the deque: Employee f = d.front(); and the compiler is able to verify that the types are compatible. 1005ICT Object Oriented Programming

40 16.3 Section summary This section covered: the problem of type checking as elements are retrieved from a container class; a doubly linked list deque implementation, with a sample client program; generics, parameterised classes; and an example container class and client with generics. 1005ICT Object Oriented Programming

41 16.4 End of section feedback questions Send us your answers to these questions any time you like by clicking on them. What was the most useful topic in this section? What was the least useful topic in this section? What was the least clear topic in this section? What topic in this section would you like to like to know more about? Did you find an error in this section? 1005ICT Object Oriented Programming