WEBVTT

1
00:00:02.110 --> 00:00:04.114
In a previous demo I should you how to create

2
00:00:04.116 --> 00:00:06.001
two objects of the same type

3
00:00:06.005 --> 00:00:07.225
two Bicycle objects.

4
00:00:08.003 --> 00:00:10.001
In this demo I'm going to show you how to create

5
00:00:10.002 --> 00:00:11.116
two different kinds of objects

6
00:00:11.220 --> 00:00:13.119
a Bicycle and Rider object

7
00:00:13.229 --> 00:00:16.009
and how to make those work together.

8
00:00:17.119 --> 00:00:20.116
First I'm going to create a Bicycle object as I did before

9
00:00:21.005 --> 00:00:23.117
not that these menu items called constructors of the

10
00:00:23.118 --> 00:00:25.006
Bicycle class, a constructor

11
00:00:25.009 --> 00:00:28.003
is rather like a method, except that it runs only once

12
00:00:28.005 --> 00:00:30.119
at a point when an object is being created

13
00:00:30.227 --> 00:00:35.000
and it belongs to the class rather than to a particular object.

14
00:00:35.223 --> 00:00:37.221
So as before I'll enter

15
00:00:37.222 --> 00:00:39.228
parameters which provide values for

16
00:00:39.229 --> 00:00:42.227
the wheelDiameter and the numberOfGears

17
00:00:43.007 --> 00:00:45.115
click Ok and the Bicycle object

18
00:00:45.116 --> 00:00:47.226
appears in the object bench.

19
00:00:48.112 --> 00:00:50.007
now I'm going to create a Rider object

20
00:00:50.112 --> 00:00:53.000
I right click on the class and choose a constructor

21
00:00:53.009 --> 00:00:55.112
note that the Rider class has two constructors

22
00:00:55.112 --> 00:00:57.116
and the menu tells us, the first one

23
00:00:57.221 --> 00:00:59.113
has one parameter which means one 

24
00:00:59.114 --> 00:01:01.116
piece of information needs to be entered

25
00:01:01.222 --> 00:01:03.229
the second one has two parameters

26
00:01:04.113 --> 00:01:07.005
so I'm going to click on the second constructor

27
00:01:07.112 --> 00:01:09.114
and the dialog box

28
00:01:09.117 --> 00:01:13.006
then prompts me to enter two pieces of information.

29
00:01:14.007 --> 00:01:17.002
The first parameter is a name for this Rider object

30
00:01:17.220 --> 00:01:19.223
and that parameter is of type string

31
00:01:19.224 --> 00:01:22.002
so I need to enter a string in this box

32
00:01:22.009 --> 00:01:23.117
when you enter a string you need 

33
00:01:23.118 --> 00:01:26.110
to wrap that in double quotes

34
00:01:27.000 --> 00:01:30.224
so lets give this rider a name

35
00:01:31.007 --> 00:01:33.002
so whats they type of the other parameter

36
00:01:33.007 --> 00:01:35.001
what type is Bicycle

37
00:01:35.113 --> 00:01:38.000
so that means it needs to be a Bicycle object

38
00:01:38.007 --> 00:01:40.008
so I need to put an object in here

39
00:01:40.222 --> 00:01:43.000
well actually I need to put in something that

40
00:01:43.001 --> 00:01:46.007
identifies the Bicycle object that i want this Rider

41
00:01:46.111 --> 00:01:48.114
to be riding.

42
00:01:48.229 --> 00:01:50.222
So I have one Bicycle object here

43
00:01:50.226 --> 00:01:52.007
the one I created earlier

44
00:01:52.115 --> 00:01:53.228
called bicycle one

45
00:01:54.008 --> 00:01:55.227
so I could just type in

46
00:01:56.002 --> 00:01:59.000
the name, the instance name bicycle one

47
00:01:59.117 --> 00:02:01.227
even simplier I can just click 

48
00:02:01.228 --> 00:02:04.228
on that object in the object bench in BlueJ

49
00:02:05.004 --> 00:02:08.006
pops that name into the parameter box

50
00:02:08.116 --> 00:02:10.224
and now I can click Ok

51
00:02:11.003 --> 00:02:12.009
and a Rider object

52
00:02:12.113 --> 00:02:15.007
is created and appears in the object bench

53
00:02:15.113 --> 00:02:17.114
I now have two objects in the object bench

54
00:02:17.115 --> 00:02:19.118
that are somehow related to each other

55
00:02:19.228 --> 00:02:23.006
that Rider object is riding that Bicycle object.

56
00:02:23.119 --> 00:02:25.111
Lets see if we can convince ourselves

57
00:02:25.112 --> 00:02:27.111
that this is the case

58
00:02:27.228 --> 00:02:29.223
if I insect the Bicycle object

59
00:02:29.224 --> 00:02:32.229
we can see its properties and see that pedalRpm

60
00:02:33.002 --> 00:02:36.115
for this Bicycle object is currently zero.

61
00:02:37.004 --> 00:02:41.221
I can now call one of the methods of the Rider object

62
00:02:42.001 --> 00:02:45.223
here's its methods Rider has a method called speedUp

63
00:02:46.111 --> 00:02:49.007
which should make the Bicycle that it is riding

64
00:02:49.116 --> 00:02:51.224
speed up, so lets try that

65
00:02:51.229 --> 00:02:54.009
nothing much obviously happens there

66
00:02:54.224 --> 00:02:58.110
lets go back to the Bicycle object and inspect it again

67
00:02:59.113 --> 00:03:02.224
and we can see that pedalRpm has increased to fifty

68
00:03:03.001 --> 00:03:05.226
so calling a method of that Rider object

69
00:03:06.003 --> 00:03:08.008
has had an effect, its changed one of the 

70
00:03:08.009 --> 00:03:11.110
properties of that Bicycle object.

71
00:03:12.008 --> 00:03:14.009
That demonstrates the relationship between these 

72
00:03:14.110 --> 00:03:16.227
two objects the rider wants to speed up

73
00:03:17.005 --> 00:03:19.000
so how can this happen, well the rider

74
00:03:19.003 --> 00:03:21.002
can pedal the bicycle faster

75
00:03:21.111 --> 00:03:26.005
which in turn changes the pedalRpm property of the bicycle.

76
00:03:27.006 --> 00:03:30.004
Now I'm going to create another Bicycle object

77
00:03:31.223 --> 00:03:34.223
as before call a constructor and enter some

78
00:03:34.226 --> 00:03:36.226
values for the parameters

79
00:03:37.110 --> 00:03:41.224
and that creates a second Bicycle object in the object bench.

80
00:03:43.004 --> 00:03:44.220
Now I'm going to make this rider 

81
00:03:44.221 --> 00:03:46.222
change from riding bicycle one

82
00:03:47.111 --> 00:03:49.005
to riding bicycle two

83
00:03:49.228 --> 00:03:53.223
the Rider object has a method called changeBicycle

84
00:03:54.118 --> 00:03:56.110
so lets call that

85
00:03:57.002 --> 00:03:59.112
and the method call dialog

86
00:03:59.116 --> 00:04:02.221
asks for a parameter of type Bicycle

87
00:04:03.005 --> 00:04:05.117
and as before that needs to be something that

88
00:04:05.118 --> 00:04:07.220
identifies the Bicycle object

89
00:04:07.224 --> 00:04:09.227
that I want to change to

90
00:04:10.116 --> 00:04:12.000
and I can just click on the 

91
00:04:12.001 --> 00:04:14.004
new Bicycle object in the object bench

92
00:04:14.009 --> 00:04:17.221
and that puts the instance name into this dialog.

93
00:04:18.002 --> 00:04:21.221
I can click Ok and now rider one

94
00:04:21.228 --> 00:04:24.004
is riding bicycle two

95
00:04:24.112 --> 00:04:25.227
to demonstrate this

96
00:04:26.111 --> 00:04:28.227
inspect bicycle two

97
00:04:29.229 --> 00:04:33.115
note pedalRpm is currently zero

98
00:04:34.113 --> 00:04:37.002
close that then call

99
00:04:37.006 --> 00:04:39.114
speedUp and Rider

100
00:04:39.228 --> 00:04:43.115
and inspect bicycle two and bicycle two

101
00:04:43.225 --> 00:04:46.114
has now had its pedalRpm property change

102
00:04:46.117 --> 00:04:48.009
so that relationship now

103
00:04:48.222 --> 00:04:51.118
is between rider one and bicycle two.