Java and OOP. Part 5 More

Transcription

1 Java and OOP Part 5 More 1

2 More OOP More OOP concepts beyond the introduction: constants this clone and equals inheritance exceptions reflection interfaces 2

3 Constants final is used for classes which cannot be subclassed Data members can also be declared final Which means they are constants for example public final double π = ; 3

4 object = object? Two distinct questions - do two references refer to the same object? do two objects contain equal data? 4

5 Example.equals Integer i1 = new Integer(4); Integer i2=new Integer(4); if (i1.equals(i2)) YES System.out.println("Contain same data"); if (i1 == i2) NO System.out.println("Same object"); 5

6 Example = Integer i1 = new Integer(4); Integer i2=i1; if (i1.equals(i2)) YES System.out.println("Contain same data"); if (i1 == i2) YES System.out.println("Same object"); 6

7 Exceptions exception = unusual situation here program cannot execute 'normally' with a more-or-less external cause not a program bug eg file not found connection with server lost out of memory broken Java machine 7

8 Exceptions Some methods are declared to 'throw' an exception Compiler will insist code 'catches' checked exception Code form - try {.. call of method that may throw ExceptionType } catch ( ExceptionType e) {.. code to deal with exception } 8

9 Example try catch FileReader fr = null; try { fr= new FileReader("test.dat"); } catch (FileNotFoundException fe) { System.out.println("Cant find file"); } FileReader is in package java.io Note declare fr outside try and initialise Else compiler complains fr may not be initialised 9

10 Exercise A FileReader inherits a method called read read takes a char[] as argument Look at the documentation Extend the code on the last slide to display the file 10

11 More on inheritance Suppose we have a base class called BaseClass, and that SubClass extends this. What do we get if we say: BaseClass s = new SubClass(); 11

12 More on inheritance Suppose we have a base class called BaseClass, and that SubClass extends this. What do we get if we say: BaseClass s = new SubClass(); Then s is an object which is 'really' a SubClass But its been declared to be a BaseClass What happens? We get the subclass methods, but The base class fields For example 12

13 public class BaseClass { public void method() { System.out.println("Base class method"); } public int field=1;} Example public class SubClass extends BaseClass { public void method() { System.out.println("Subclass method"); } public int field = 2; } BaseClass s = new SubClass(); s.method(); System.out.println(s.field); outputs: Subclass method 1 13

14 Reflection Sometimes your code must deal with objects when you do not know what type they are Reflection enables you to ask an object what class it is A Class object carries info about a class The Object class has a method called getclass So all classes have this - eg BaseClass s = new SubClass(); Class theclass= s.getclass(); System.out.println(theClass);.. outputs class test.subclass (SubClass was defined in package test) 14

15 This example shows how we can trace up a class hierarchy See the doc of Class for more info public static void main(string[] args) { } BaseClass s = new SubClass(); Class theclass= s.getclass(); uptree(theclass); Using Class objects public static void uptree(class someclass) { if (someclass==null) return; System.out.println(someClass); uptree(someclass.getsuperclass()); } output: class test.subclass class test.baseclass class java.lang.object 15

16 Reflection exercise Look at the doc of Class Use Class.forName and.getmethods to output the methods of a given class Note.forName needs package eg theclass= Class.forName("java.lang.Class"); and need to catch exception 16

17 No multiple inheritance In some languages (C++) a class can inherit from more than one super class Called multiple inheritance Cannot do it in Java Idea of interface kind of substitute Enables a class to 'be like' more than one class interface Interfaces heavily used in Swing - simpler than what follows.. 17

18 Like a ghostly class What is an interface An interface has a set of methods (names, arguments, return types) The interface declares but does not define the methods A class can be declared to implement the interface It must then define the methods in the interface Or the compiler will complain Classes can implement several interfaces 18

19 Example interface Comparable just one method - int compareto(object obj) should return negative, 0 or positive depending on whether obj comes before, is =, or comes after 19

20 Example implementation public class Order implements Comparable { public Order(double amount, int id, int day, int month, int year) { value=amount; key = id; date=new GregorianCalendar(); date.set(year, month, day); } public int compareto(object obj) { Order otherone = (Order) obj; return date.compareto(otherone.date); } private GregorianCalendar date; private double value; private int key; } Calendar implements Comparable 20

21 Using a Comparable class Order order1 = new Order(34.50, 1, 2, 1, 2004); Order order2 = new Order(44.50, 2, 1, 1, 2004); System.out.println(order1.compareTo(order2)); get 1 Exercise Copy the Order class Change it so it compares on order value not order date Test it 21

22 Variables of interface type You can declare variables of a type which is interface not a class eg Comparable someobj; But you can't refer to any method other than that in the interface Enables you to write routines which work on any object which implements the interface So you could write a Sorter class which would sort arrays of any Comparable type (like a template in C ++): 22

23 Example : the Sorter class public class Sorter { public static void sort(comparable[] things) {.... the sort method.. } } in use.. String[] array = {"ppp", "a", "aaa", "ccc"}; Sorter.sort(array); for (int i = 0; i<array.length; i++) System.out.println(array[i]); output - a aaa ccc ppp 23

24 Exercise Copy the Sorter class Fill in the sort method eg a bubble sort Check it works Check it works with arrays of other Comparable types eg Integer 24