英文java 讲义Lecture21 ClassesObjects1

Exercise: Draw a triangle
********** ********* ******** .. ** *
public static void drawTriangle (char symbol, int size) { }
Nested loops
********** ********* ******** .. ** *
What are the components -> objects
• Software usually models some real-world object • OO programming uses abstraction.
– To abstract is to extract or define the essential, relevant characteristics of an object.
Examples of objects in a soccer game
• model the program accordingly to the "real things" appear in the soccer games. • Player:
– Knows name, number, position, locaห้องสมุดไป่ตู้ion in the field, and etc; – Can run, jump, kick-the-ball, block ,… and etc.
Structured programming
• • • • Naming conventions Block structures – control structures (if / while) divide and conquer into modules Start with top-down design: design of the toplevel of a system and breaking tasks down into multiple, more-detailed modules at lower and lower levels • Dividing into methods is not enough – we will also start dividing into objects
Idea: Define your own type which is composed of other data!
• When you define a class, you are actually defining a new type. • So after defining HelloWorld you actually defined a type HelloWorld. • This means that in another .java file, you actually could declare a variable of type HelloWorld. • This doesn't really make sense though because the new type doesn't store anything.
Object-oriented programming
• Design “components” • that perform certain tasks • And provide an interface, so it can be connected to other components to make a program
Comparison – Building Computer
• You buy components: e.g.: motherboard, a processor, some RAMs, a hard disk, a casing, a power supply • You need not worry about the motherboard internals (e.g., number of copper-clad laminate layers), the hard disk internals (number of platters) etc.. • You care about:
Programmers’ Goals
• Build good software
• that solves the problems / meets requirements • AND can be maintained and modified to address the inevitable changes
}
Example: Cat
public class Cat { //store all of the values related to a cat into a new object public String name; public int age; public boolean isGiantCat; } We are saying that every Cat object, will consist of 3 things: -A String for the name, an int for the age, and a boolean for the sort of cat they are. -public has to do with the sort of property we will have. We are going to see that we almost NEVER will make a property public. For now we will start with it, because it's simpler
• Ball:
– Knows: position, size – Can: move, bounce
• Reusable objects:
– Ball – Audience – Can be used in a basketball game with little/no changes – Doesn’t need to be retested
– – The specifications: what it does The interface: how to connect it / use it
• You don’t care about how it does what it says it does, as long as it does it correctly
public static void drawTriangle (char symbol, int size) { int row = 1, col = 1; while (row <= size) { while (col <= size) { System.out.print (symbol); col++; } System.out.println (); row++; col = row; } }
Managing your data
• In a large scale application, there may be millions or even billions of pieces of data that need to be stored. • If we had to create 1 variable for each of them, it'd be impossible to ever keep track of the variables. • Just like how we divided our millions of instructions into several methods, each of which may be further subdivided, we will also divide our data into subdata.
• • • •
Referee: Field: Audience: Weather:
.sg/home/ehchua/programming/java/J3a_OOPBasics.html 11
OO Application
• Only 1 class has public static void main method • The main method starts the application
Specifying the data
public class NameOfClass { //At the top, a list of attributes or properties // This is what your new data type consists of modifier1 type1 name1; modifier2 type2 name2; modifier3 type3 name3; ….
-> reusable in multiple programs that need that same “component”
Benefits of OOP
• Easier to work in large team:
– Objects have well defined capabilities and interfaces, so different team members can focus on different aspects
• For example, a student object would have the ID number as an attribute but not hair colour • A student object would have the ability to calculate its rScore but not the number of years driving.