1. Classes
Lecture 7 from Chapter 6
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2. Review Class definition Declaring Objects
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Class definition
Declaring Objects
Assigning object reference variables
Introducing methods
Constructors
The this keyword
Finalized method

3. Introducing to Class The class is the core of Java.
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The class is the core of Java.
The class is the logical construct upon which the entire Java language is built.
Class defines the shape and nature of an object.
It is formed as the basics of object-oriented programming

4. The General Form of a class
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Class classname {
Type instance-variable1; //
Type methodname1(parameter-list) {
//body of method }
Type methodname2 (parameter-list {
//body of method } ….
Class person {
Double height;
Double weight; }

5. Example - The General Form of a class
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Class classname {
Type instance-variable1; //
Type methodname1(parameter-list) {
//body of method }
Type methodname2 (parameter-list {
//body of method } ….
class Pair {
int x, y;
Pair(int u, int v) {
x = u; //
y = v; }
Pair(int x) {
this.x = x; //
y = 0;
Pair() {
x = 0;

6. Creating the object box
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Person theperson = new person();
Here, theperson will be an instance of person.
To assign a value, use
theperson.weight = 70;
theperson.height = 170;

7. Declaring the object – page 135
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Statement Effect
Person theperson;
theperson = new Person();
Null
theperson
Weight
theperson
Height
theperson.height

8. Example of creating and assigning value
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9. Example of two objects, area and area1
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10. Assigning object reference variables
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Area a1 = new area();
Area a2 = a1;
In this case, both a1 and a2 will refer to the same object
Height a1
Width a2

11. Assigning object reference variables
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Area a1 = new area();
Area a2 = a1;
// a1 = null;
In this case, a2 will point to object, but a1 will set to null. a1
null
Height a2
Width

12. Introducing Methods Classes usually consist of two things:
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Classes usually consist of two things:
Instance variables (int x, y) and methods (pair())

13. Passing the values x = 1 and y = 2 will pass to Pair
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x = 1 and y = 2
will pass to Pair

14. Returning a value – from myarea.area()
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15. Example – with three methods
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16. Constructor
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It is tedious to initialise all variables in a class each time an instance is created.
Constructors are a special sort of method that initialise freshly minted objects.
They are like static methods in that we don't need an existing object to use them, just new.

17. Example
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the values
are fixed

18. Constructor In the previous example, the values are fixed
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In the previous example, the values are fixed
In order to create different values, we use a parameterised constructor.

19. Example of parameterised constructor
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20. This Keyword This refers to the current object.
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This refers to the current object.
Or it is used instead of the class name as the constructor name.
Example
this.name = name;

21. Example of this
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22. Finalise()
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It is invoked before an object is discarded by the garbage collector, allowing it to clean up its state.
The purpose is to clean up an object before the object is garbage collected.
Protected void finalise() {
//finalise code }

23. More examples of class
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24. More examples of class 2019/1/112019/1/11 import java.io.*;
class D2Class {
String name;
int birth;
int num = 0;
static int nextNum = 0;
D2Class()
num = nextNum; ; }
D2Class(String s, int i)
this();
name = s;
birth = i;
public void show()
System.out.println("nextnum = " + D2Class.nextNum);
System.out.println("num = " + this.num);
System.out.println("Name = " + this.name);
System.out.println("birth = " + this.birth);
class a0502
public static void main(String args[]) throws IOException
System.out.println("nextNum = " + D2Class.nextNum);
D2Class d = new D2Class("Chan", );
D2Class d1 = new D2Class("Tai", );
d.show();
d1.show();
System.out.println("num = " + d1.num);

25. Summary Class definition – the core of java
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Class definition – the core of java
Declaring Objects – Person thisperson = new Person()
Assigning object reference variables – point to object
Introducing methods – class consists of variables and methods
Constructors – initialise the variables
The this keyword – this class
Finalized method – perform some actions when it is destroyed