Agenda
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•
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Re-cap object-oriented design
Static attribute and static method
Difference between Array and Vector
Prepare problem set 3
– Walk through Problem 1
– Explain Problem 2
A 5-step guideline for object-oriented
design
• Step 1: read the problem statement and identify
classes
– Classes are nouns
– Each class contains a set of attributes and methods
• Step 2: identify the attributes of each class
–
–
–
–
Attributes are nouns as well
Is an attribute private or public?
Should it be static?
If an attribute is private, should we provide its access
methods (getX() and setX())?
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A 5-step guideline for object-oriented
design (continue)
• Step 3: Write the constructor
– The purpose of a constructor is to create an instance of the class
using the values provided by the user (through the constructor
arguments)
– It must be public with NO return type
– A class can have more than one constructors
– A class decides which constructor to call by the by the number of
arguments passed to the constructor
• Step 4: Define special methods
–
–
–
–
Scope
Return type
Name
Arguments and their data types
A 5-step guideline for object-oriented
design (continue)
• Step 5: Create the user class
– Only the user class contains the main() function
– The primary purpose of the user class is to interface with the user (e.g.
take input) and manage the interactions among classes
public static void main(String[] args) {
String text= JOptionPane.showInputDialog("Enter highest floor: ");
int maxFloor = Integer.parseInt(text);
text= JOptionPane.showInputDialog("Enter lowest floor: ");
int minFloor = Integer.parseInt(text);
Elevator one= new Elevator(maxFloor, minFloor , "One");
Elevator two= new Elevator(maxFloor, minFloor , "Two");
Elevator three= new Elevator(maxFloor, minFloor, "Three");
Controller master= new Controller(one, two, three);
…………………
}
2
Static Attribute and Static Method
• Every time you instantiate an instance of a class, the
computer creates a copy of all the variables
• However, regardless of how many objects you have
created, there is only 1 copy of the static variables in
the memory
int A;
int B;
Static int C;
class Dummy
Constructors
Methods
A=1
B=1
A=2
B=2
d1
Constructors
Methods
d2
Constructors
Methods
A=3
B=3
d3
Constructors
Methods
Array vs. Vecor
Array
Vector
Length
Fixed
Flexible
Data type of
elements
Must be the same
Can be different
Create
int[] Arr = new int[4]
Vector Vr = new Vector();
Assign value
Arr[0] = 4;
Vr.addElement(p0);
Access element
Arr[2]
Vr.elementAt(2)
Length
Arr.length
Vr.size()
3
Static Attribute and Static Method
(continue)
• Since each class only has 1 copy of static variables, they
should be referred to by the class name , NOT the object
name
System.out.println(Dummy.C);
• The same is true for static method
class Pipe {
public static void setRoughness (double r)
{ …….}
}
class Test {
public static void main (string[] args) {
Pipe.setRoughness (9.8);
}
}
Problem set 3, Problem 1
• Two classes: Fluid and Pipe
Fluid
Attributes:
String name;
double density;
Constructors
Two arguments
Methods
printData ()
Pipe
Attributes:
String name;
double length;
double diameter;
static double roughness;
final static double gravity = 9.8;
Constructors
Three arguments
Methods
printData ()
static setRoughness(…)
static printStatistics()
• We are ready to create them
4
Problem set 3, Problem 1
• Functionality of the main() function of the user class
– Create an array of 3 Fluids
Fluid[] substance = new Fluid[3];
substance[0] = new Fluid(“water”, 1.0E3);
– Create a Vector of 3 Pipes
Vector network = new Vector ();
Pipe p0 = new Pipe(“AB”, 50.0, 0.5);
network.addElement(p0);
Is there a better way to do it?
– Set the roughness of the Pipe class
Pipe.setRoughness(0.0001);
– Output the data
Problem set 3, Problem 2
• Pipe class
Attribute
Regular
D
X
L
X
z
X
r
X
v
q
Static
Final
X(1.0)
X
R
X
f
X
g = 9.8
X
h
X(.00001)
Pb
Calculated
X
X
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Problem set 3, Problem 2
• Output (“water”, 100000, 1, 0.00001)
Pipe AB
Reynolds number: 500000.0
Friction: 0.013030745525640063
Pressure at inlet: 100000.0
Pressure at outlet: 100328.462723718
Pressure drop: 328.4627237179957
Volume flow rate: 0.19634954084936207
Pipe BC
……………………..
Problem set 3, Problem 2: one solution
• Design challenge: pressure calculation
• Pipes and Fluids are separate classes; however, the
pressure calculation depends on both Pipes, Fluids,
and the input pressure from the upstream pipe.
This means that pressures in the pipes must be
calculated in a particular order.
• Suggestion: have pipes include a references to their
output pipes (using a Vector, for example)
• Think about pipes as being “empty” (no flow of
fluid yet), or “full” (with fluid flowing)
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Problem set 3, Problem 2: one solution
class Pipe {
private Vector outflows; /* set via addOutflow() */
public void startFlow(Fluid f, double v, double
inputPressure) {
/* calculate pressure here */
...
/* calculate pressure of all outflows */
for (int i=0; i<outflows.size(); i++) {
Pipe outPipe = (Pipe)outflows.elementAt(i);
outPipe.startFlow(f, v, outputPressure);
}
}
...
}
Call startFlow() on first pipe of network only to calculate the pressures at all pipes
in the network.
Problem set 3, Problem 2: one solution
• This is just one particular design, many others are
possible
• This solution delegate all pressure calculations to
the startFlow() method, which just needs to be
called once
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