DCS/100: Procedural Programming

Transcription

1 DCS/100: wk 12 p.1/23 DCS/100: Procedural Programming Week 12: Classes Queen Mary, University of London

2 DCS/100: wk 12 p.2/23 Last Week: Learning Outcomes From last week you should be able to: explain several different ways to sort arrays of data into order write programs to sort arrays write programs containing matrices explain how to declare and use matrices

3 DCS/100: wk 12 p.3/23 This Week: Learning Outcomes By the end of the week you should be able to: explain in outline the object model of programming

4 DCS/100: wk 12 p.4/23 Objects Next term we will move on a stage. You will spend a lot of time learning how to use objects and classes. These are used for mid-level structure. You will move on to using an IDE: a nice one called BlueJ. Today I want to give you something of a taste...

5 DCS/100: wk 12 p.5/23 Systems and state The programs we ve been writing consist of: main auxiliary methods The data state of a program is given by the values in its variables, and the control state is given by the location of the point of control in its code.

6 DCS/100: wk 12 p.6/23 Systems and state This global picture is OK for small programs. For larger programs the question is how to break the program up into smaller bits or subsystems.

7 DCS/100: wk 12 p.7/23 The object model Objects are the current standard way of breaking a system up into small chunks (it s just the standard one, there are others which are as good, better for some purposes). These things change. In a few years objects will probably have been replaced, possibly by agents.

8 DCS/100: wk 12 p.8/23 The object model An object is a small system. It has variables, whose values give its state. It has methods, which allow other programs to access its state.

9 DCS/100: wk 12 p.9/23 The object model Perhaps the simplest example is a kind of push-button counter. It has a variable count giving the current count, and methods : press: increment the counter reset: reset the counter view: view its current value

10 DCS/100: wk 12 p.10/23 Interfaces The variable gives the state of the object. The methods give its interface. An interface specifies the way in which the outside world can communicate with a system.

11 DCS/100: wk 12 p.11/23 Interfaces Interfaces are important because: we want to break systems down into chunks we want to use other peoples parts the interfaces are where things fit together we want to be able to change the rest of a subsystem provided we keep its interface fixed

12 DCS/100: wk 12 p.12/23 Interfaces The interface for a method is given by its header: static void printarray(output out, int[]a) throws Exception The header specifies how the method can be called by the rest of the program. Provided we keep the header, we can change the method body without affecting the rest of the program. (At least without affecting whether it compiles or not!)

13 DCS/100: wk 12 p.13/23 Interfaces In object-oriented programming another part of the program communicates with an object by calling its methods. The methods form the interface for the object. If we want to do something with an object there has to be a method that does it.

14 DCS/100: wk 12 p.14/23 The Object Model In object-oriented languages like Java it s possible to write a little piece of stand-alone code that generates counters. The code has to: specify a variable for the current state show how to make a single counter (give initial values for any variables) give implementations of the methods other code can use to interact with the counter it does not have to have a main

15 DCS/100: wk 12 p.15/23 The Object Model Since this code does not have a main we cannot run it directly. We have to write a program that uses it. This program has to create a counter: counter mycounter = new counter(); This syntax creates a new counter called mycounter. We can use it later in the program by calling its methods... mycounter.press() mycounter.reset() mycounter.view() If this looks familiar, it should...

16 DCS/100: wk 12 p.16/23 The Object Model it s how we ve used input and output streams. input keybd = new input(); keybd.readint() keybd.next() keybd.readln() Input and output streams are objects. So are Strings. You have been able to use these without knowing how they are implemented. The interface is all you have needed to know.

17 DCS/100: wk 12 p.17/23 class counter_test extends basic { public static void main (String param[]) throws Exception { output screen = new output(); counter mycounter = new counter(); screen.writeln(mycounter.view()); mycounter.press(); mycounter.press(); screen.writeln(mycounter.view()); mycounter.reset(); screen.writeln(mycounter.view()); } } screen.close();

18 DCS/100: wk 12 p.18/23 New kinds of objects The above uses the type counter But Java does not have such a type How do we create types? We write a class definition It defines what we mean by our new type...how it is implemented and what its interface is.

19 DCS/100: wk 12 p.19/23 class counter { private int count; public counter() { count = 0; } public void press() { count = count + 1; } public void reset() { count = 0; } } public int view() { return count; }

20 Objects and Classes Systems can have many thousands of objects. It would be very wasteful to write code for each of them individually. We need a mechanism for generating lots of similar objects. So objects are grouped in classes. A class is a group of similar objects: counters or input streams or Strings or... The code we ve seen is not code for an object: it s code for a class. It tells us how to generate objects of that class and how they behave. It can be re-used to generate lots of counters. DCS/100: wk 12 p.20/23

21 DCS/100: wk 12 p.21/23 Objects and Classes Classes categorise objects in the same kind of way that types categorise basic values. Basic values have types. The type tells you what sort of thing it is, what you can do with it, and roughly how it will behave. Objects have classes. The class tells you what sort of thing it is, what you can do with it, and roughly how it will behave. Classes are what give structure to your system.

22 DCS/100: wk 12 p.22/23 Objects and Classes Parts of an object need to be kept private: internal methods, most variables. In Java you can declare variables and methods: public: anyone can see and use them private: only members of the same class can some other categories. main has to be declared public

23 DCS/100: wk 12 p.23/23 Objects and Classes In the counter example, count has to be declared private. Otherwise programs could simply set it to whatever they want.

24 DCS/100: wk 12 p.24/23 Objects and Classes Next semester you will spend a lot of time on objects and classes. You will move to using an IDE called BlueJ, which is better for managing them. But for now you may like to look at Brinch Hansen s classes... Reading: Computing Without Computers Chapter 9