Classes and Objects
EECS2030 B: Advanced Object Oriented Programming Fall 2018
CHEN-WEI WANG
Object Orientation:
Observe, Model, and Execute
○ Study this tutorial video that walks you through the idea of .
Real World: Entities
Entities:
jim, jonathan, …
Entities:
p1(2, 3), p2(-1, -2), …
Compile-Time: Classes (definitions of templates)
Run-Time: Objects (instantiations of templates)
class Person { String name; double weight; double height;
}
class Potint { double x; double y;
}
Model
Execute
○ We how real-world entities behave.
○ We the common attributes and behaviour of a set of entities in a single class.
○ We execute the program by creating instances of classes, which
Person
Person
name “Jonathan”
weight 80 height 1.80
Point
jim
p1
name weight height
“Jim” 80 1.80
jonathan
Point
2
y3 y-2
x
p2
x
-1
………
object orientation
observe
model
interact in a way analogous to that of real-world entities. 3 of 73
Separation of Concerns: App/Tester vs. Model
● In EECS1022:
○ Model Component: One or More Java Classes
e.g., Person vs. SMS, Student, CourseRecord
○ Another Java class that “manipulates” the model class
(by creating instances and calling methods):
● Controller (e.g., BMIActivity, BankActivity). Effects?
Visualized (via a GUI) at connected tablet
● Testerwithmain(e.g.,PersonTester,BankTester).Effects?
Seen (as textual outputs) at console ● In Java:
○ We may define more than one classes.
○ Each class may contain more than one methods. object-oriented programming in Java:
○ Use to define templates
○ Use to instantiate classes
○ At runtime, create objects and call methods on objects, to simulate
classes
objects
2 of 73 interactions between real-life entities.
Object-Oriented Programming (OOP)
● In real life, lots of entities exist and interact with each other. e.g., People gain/lose weight, marry/divorce, or get older.
e.g., Cars move from one point to another. e.g., Clients initiate transactions with banks.
● Entities:
○ Possess attributes;
○ Exhibit bebaviour; and ○ Interact with each other.
● Goals: Solve problems programmatically by ○ Classifying entities of interest
Entities in the same class share common attributes and bebaviour. ○ Manipulating data that represent these entities
Each entity is represented by specific values. 4 of 73
OO Thinking: Templates vs. Instances (1.1)
A person is a being, such as a human, that has certain attributes and behaviour constituting personhood: a person ages and grows on their heights and weights.
● A template called Person defines the common
○ (e.g., age, weight, height) [≈ nouns] ○ (e.g., get older, gain weight) [≈ verbs]
attributes
behaviour
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OO Thinking: Templates vs. Instances (2.1)
Points on a two-dimensional plane are identified by their signed distances from the X- and Y-axises. A point may move arbitrarily towards any direction on the plane. Given two points, we are often interested in knowing the distance between them.
● A template called Point defines the common
○ (e.g., x, y) [≈ nouns] ○ (e.g., move up, get distance from) [≈ verbs]
attributes
behaviour
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OO Thinking: Templates vs. Instances (1.2)
● Persons share these common attributes and behaviour. ○ Each person possesses an age, a weight, and a height.
○ Each person’s age, weight, and height might be distinct
e.g., jim is 50-years old, 1.8-meters tall and 80-kg heavy
e.g., jonathan is 65-years old, 1.73-meters tall and 90-kg heavy
● Each person, depending on the specific values of their attributes, might exhibit distinct behaviour:
○ When jim gets older, he becomes 51
○ When jonathan gets older, he becomes 66.
○ jim’s BMI is based on his own height and weight
○ jonathan’s BMI is based on his own height and weight
[ 802 ] [ 1.8 ]
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90 1.732
OO Thinking: Templates vs. Instances (2.2)
● Points share these common attributes and behaviour.
○ Each point possesses an x-coordinate and a y-coordinate. ○ Each point’s location might be distinct
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e.g., p1 is located at (3, 4) e.g., p2 is located at (−4, −3)
● Each point, depending on the specific values of their attributes (i.e., locations), might exhibit distinct behaviour:
○ When p1 moves up for 1 unit, it will end up being at (3, 5)
○ When p2 moves up for 1 unit, it will end up being at (−4, −2)
○ Then, p1’s distance from origin: [√32 + 52] ○ Then, p2’s distance from origin: [ (−4)2 + (−2)2]
OO Thinking: Templates vs. Instances (3)
● A template defines what’s shared by a set of related entities. ○ Common attributes (age in Person, x in Point)
○ Common behaviour (get older for Person, move up for Point)
● Each template may be instantiated into multiple instances. ○ Person instances: jim and jonathan
○ Point instances: p1 and p2
● Each instance may have specific values for the attributes. ○ Each Person instance has an age:
jim is 50-years old, jonathan is 65-years old ○ Each Point instance has a location:
p1 is at (3,4), p2 is at (−3,−4)
● Therefore, instances of the same template may exhibit distinct
behaviour .
○ Each Person instance can get older: jim getting older from 50 to
51; jonathan getting older from 65 to 66.
○ Each Point instance can move up: p1 moving up from (3, 3)
9 of 73 results in (3, 4); p1 moving up from (−3, −4) results in (−3, −3).
OOP:
Define Constructors for Creating Objects (1.1)
● Within class Point, you define constructors , specifying how instances of the Point template may be created.
public class Point {
… /* attributes: x, y */ Point(double newX, double newY) {
x = newX;
y = newY; } }
● In the corresponding tester class, each to the Point constructor creates an instance of the Point template.
public class PointTester {
public static void main(String[] args) {
Point p1 = new Point (2, 4); println(p1.x + ” ” + p1.y);
Point p2 = new Point (-4, -3); 11of73 println(p2.x + ” ” + p2.y); } }
call
OOP: Classes ≈ Templates
In Java, you use a class to define a template that enumerates
attributes that are common to a set of entities of interest.
public class Person { int age;
String nationality; double weight; double height;
}
public class Point { double x;
double y;
}
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OOP:
Define Constructors for Creating Objects (1.2)
1. RHS (Source) of Assignment: new Point(2, 4) creates a new Point object in memory.
2. LHS (Target) of Assignment: Point p1 declares a variable that is meant to store the address of some Point object.
3. Assignment: Executing stores new object’s address in p1.
Point p1 = new Point(2, 4);
Point
x
2.0
y
4.0
=
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Point
p1
x
2.0
y
4.0
OOP:
Define Constructors for Creating Objects (2.1)
● Within class Person, you define constructors , specifying how instances of the Person template may be created.
● In the corresponding tester class, each to the Person constructor creates an instance of the Person template.
public class PersonTester {
public static void main(String[] args) {
Person jim = new Person (50, “British”); println(jim.nationlaity + ” ” + jim.age);
Person jonathan = new Person (60, “Canadian”); println(jonathan.nationlaity + ” ” + jonathan.age); } }
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public class Person {
… /* attributes: age, nationality, weight, height */ Person(int newAge, String newNationality) {
age = newAge;
nationality = newNationality; } }
call
Visualizing Objects at Runtime (1)
● To trace a program with sophisticated manipulations of objects, it’s critical for you to visualize how objects are:
○ Created using constructors
Person jim = new Person(50, “British”, 80, 1.8);
○ Inquired using accessor methods
double bmi = jim.getBMI();
○ Modified using mutator methods ● To visualize an object:
jim.gainWeightBy(10);
○ Draw a to represent contents of that object:
● ● ●
○ Draw 15 of 73
indicates the name of class from which the object is instantiated. enumerates names of attributes of the instantiated class.
fills in values of the corresponding attributes.
for variable(s) that store the object’s address .
rectangle box
Title
Left column
Right column
arrow(s)
OOP:
Define Constructors for Creating Objects (2.2)
1. RHS (Source) of Assignment:
creates a new Person object in memory.
2. LHS (Target) of Assignment: declares a variable that is meant to store the address of some Person object.
3. Assignment: Executing stores new object’s address in jim.
Person jim = new Person(50, “British”);
Person
new Person(50, “British”)
age
50 “British” 0.0 0.0
nationality
weight
height
=
Point jim
Person
age
jim
nationality
50
“British”
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weight
0.0
height
0.0
Visualizing Objects at Runtime (2.1)
After calling a constructor to create an object:
Person jim = new Person(50, “British”, 80, 1.8);
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age
nationality
weight
height
Person
50
jim
“British”
80
1.8
Visualizing Objects at Runtime (2.2)
After calling an accessor to inquire about context object jim:
● Contents of the object pointed to by jim remain intact.
● Retuned value 80 of jim.getBMI() stored in variable bmi.
double bmi = jim.getBMI();
(1.8)2
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Person
jim
age
nationality
weight
height
50
“British”
80
1.8
Visualizing Objects at Runtime (2.4)
After calling the same accessor to inquire the modified state of context object jim:
● Contents of the object pointed to by jim remain intact.
● Retuned value 90 of jim.getBMI() stored in variable bmi.
bmi = p.getBMI();
(1.8)2
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age
nationality
weight
Person
50
jim
“British”
80 90
height
1.8
Visualizing Objects at Runtime (2.3)
After calling a mutator to modify the state of context object jim:
jim.gainWeightBy(10);
● Contents of the object pointed to by jim change. ● Address of the object remains unchanged.
⇒ jim points to the same object!
Person
age
50
nationality
“British”
weight
80 90
height
1.8
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jim
The this Reference (1)
● Each class may be instantiated to multiple objects at runtime.
● Each time when we call a method of some class, using the dot notation, there is a specific target/context object.
1 2 3 4
class Point {
double x; double y;
void moveUp(double units) { y += units; }
}
Point p1 = new Point(2, 3); Point p2 = new Point(4, 6); p1.moveUp(3.5); p2.moveUp(4.7);
○ p1andp2arecalledthe or .
○ Lines 3 and 4 apply the same definition of the moveUp method. ○ But how does Java distinguish the change to p1.y versus the
change to p2.y? 20 of 73
call targets
context objects
The this Reference (2)
● In the method definition, each attribute has an implicit this
which refers to the in a call to that method.
context object
class Point {
double x;
double y;
Point(double newX, double newY) {
this.x = newX;
this.y = newY; }
void moveUp(double units) { this.y = this.y + units;
} }
● Each time when the class definition is used to create a new
Point object, the this reference is substituted by the name of
the new object.
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The this Reference (4)
● After we create p2 as an instance of Point
● When invoking p2.moveUp(4.7), a version of moveUp that is specific to p2 will be used:
Point p2 = new Point(4, 6);
class Point {
double x;
double y;
Point(double newX, double newY) {
.x = newX;
.y = newY; }
void moveUp(double units) { p2.y = p2.y + units;
} }
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p2 p2
The this Reference (3)
● After we create p1 as an instance of Point
● When invoking p1.moveUp(3.5), a version of moveUp that is specific to p1 will be used:
Point p1 = new Point(2, 3);
class Point {
double x;
double y;
Point(double newX, double newY) {
.x = newX;
.y = newY; }
void moveUp(double units) { p1.y = p1.y + units;
} }
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p1 p1
The this Reference (5)
The this reference can be used to disambiguate when the names of input parameters clash with the names of class attributes.
class Point {
double x;
double y;
Point(double x, double y) {
this.x = x;
this.y = y; }
void setX(double x) { this.x = x;
}
void setY(double y) {
this.y = y; }
}
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The this Reference (6.1): Common Error The following code fragment compiles but is problematic:
class Person {
String name;
int age;
Person(String name, int age) {
name = name;
age = age; }
void setAge(int age) { age = age;
} }
Why? Fix?
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OOP: Methods (1.1)
● A is a named block of code, reusable via its name.
method
T1 p1 T2 p2
…
Tn pn
RT
● The header of a method consists of: ○ Return type
○ Nameofmethod
○ Zero or more parameter names
[see here] [ RT (which can be void) ] [m] [ p1, p2, …, pn ] [ T1, T2, . . . , Tn ]
m
{
…
/* implementation of method m */
}
○ The corresponding parameter types
● A call to method m has the form: m(a1,a2,…,an)
Types of argument values a1, a2, . . . , an must match the the
corresponding parameter types T1, T2, …, Tn. 27 of 73
The this Reference (6.2): Common Error Always remember to use this when input parameter names
clash with class attribute names.
class Person {
String name;
int age;
Person(String name, int age) {
this.name = name;
this.age = age; }
void setAge(int age) { this.age = age;
} }
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OOP: Methods (1.2)
● In the body of the method, you may ○ Declare and use new local variables
Scope of local variables is only within that method. ○ Use or change values of attributes.
○ Use values of parameters, if any.
class Person {
String nationality;
void changeNationality(String newNationality) {
nationality = newNationality; } }
● Call a method, with a , by passing arguments.
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context object
class PersonTester {
public static void main(String[] args) {
Person jim = new Person(50, “British”); Person jonathan = new Person(60, “Canadian”); jim.changeNationality(“Korean”); jonathan.changeNationality(“Korean”); } }
OOP: Methods (2)
● Each C defines a list of methods. ○ A misanamedblockofcode.
● We reuse the code of method m by calling it on an object obj of class C.
Foreach methodcall obj.m(…):
○ obj is the context object of type C
○ m is a method defined in class C
○ We intend to apply the code effect of method m to object obj.
class
method
e.g., jim.getOlder() vs. jonathan.getOlder() e.g., p1.moveUp(3) vs. p2.moveUp(3)
● All objects of class C share the same definition of method m.
● However:
∵ Each object may have distinct attribute values.
∴ Applying the same definition of method m has distinct effects. 29 of 73
OOP: The Dot Notation (1.1)
A binary operator:
○ LHS stores an address (which denotes an object)
○ RHS the name of an attribute or a method ○ LHS . RHS means:
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Locate the context object whose address is stored in LHS,
then apply RHS.
What if LHS stores null? [ NullPointerException ]
OOP: Methods (3)
1. Constructor
○ Same name as the class. No return type. Initializes attributes. ○ Called with the new keyword.
○ e.g., Person jim = new Person(50, “British”);
2. Mutator
○ Changes (re-assigns) attributes
○ void return type
○ Cannot be used when a value is expected
○ e.g., double h = jim.setHeight(78.5) is illegal!
3. Accessor
○ Uses attributes for computations (without changing their values) ○ Any return type other than void
○ An explicit return statement (typically at the end of the method)
returns the computation result to where the method is being used.
e.g., double bmi = jim.getBMI();
e.g., println(p1.getDistanceFromOrigin()); 30 of 73
OOP: The Dot Notation (1.2)
● Given a variable of some reference type that is not null:
○ We use a dot to retrieve any of its attributes . Analogous to ’s in English
e.g., jim.nationality means jim’s nationality
○ We use a dot to invoke any of its mutator methods , in order to change values of its attributes.
e.g., jim.changeNationality(“CAN”) changes the nationality attribute of jim
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○ We use a dot to invoke any of its accessor methods , in order to use the result of some computation on its attribute values.
e.g., jim.getBMI() computes and returns the BMI calculated based on jim’s weight and height
○ Return value of an accessor method must be stored in a variable. e.g., double jimBMI = jim.getBMI()
OOP: Method Calls
1 2 3 4 5 6 7 8
● Lines 1 and 2 create two different instances of Point
● Lines 3 and 4: invoking the same accessor method on two
different instances returns distinct values
● Lines 5 and 6: invoking the same mutator method on two
different instances results in independent changes
● Lines 3 and 7: invoking the same accessor method on the
same instance may return distinct values, why? Line 5 33 of 73
Point p1 = new (3, 4);
Point p2 = new (-6, -8); System.out.println(p1. ); System.out.println(p2. );
p1. ;
p2. ;
System.out.println(p1. ); System.out.println(p2. );
Point
Point
getDistanceFromOrigin()
getDistanceFromOrigin()
moveUp(2)
moveUp(2)
getDistanceFromOrigin()
getDistanceFromOrigin()
OOP: Class Constructors (2)
public class Person {
int age;
String nationality;
double weight;
double height;
Person(int initAge, String initNat) {
age = initAge;
nationality = initNat; }
Person (double initW, double initH) { weight = initW;
height = initH;
}
Person(int initAge, String initNat,
double initW, double initH) {
… /* initialize all attributes using the parameters */
} }
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OOP: Class Constructors (1)
● The purpose of defining a class is to be able to create instances out of it.
● To instantiate a class, we use one of its constructors . ● A constructor
○ declares input parameters
○ uses input parameters to initialize some or all of its attributes
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OOP: Class Constructors (3)
public class Point { double x;
double y;
Point(double initX, double initY) { x = initX;
y = initY;
}
Point(char axis, double distance) {
if (axis == ’x’) { x = distance; }
else if (axis == ’y’) { y = distance; }
else { System.out.println(“Error: invalid axis.”) }
} }
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OOP: Class Constructors (4)
● For each class, you may define one or more constructors : ○ Names of all constructors must match the class name.
○ No return types need to be specified for constructors.
○ Each constructor must have a distinct list of input parameter types. ○ Each parameter that is used to initialize an attribute must have a
matching type.
○ The body of each constructor specifies how some or all
attributes may be initialized.
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OOP: Object Creation (2)
A constructor may only initialize some attributes and leave others uninitialized .
public class PersonTester {
public static void main(String[] args) {
/* initialize age and nationality only */
Person jim = new Person(50, “BRI”);
/* initialize age and nationality only */ Person jonathan = new Person(65, “CAN”);
/* initialize weight and height only */
Person alan = new Person(75, 1.80);
/* initialize all attributes of a person */ Person mark = new Person(40, “CAN”, 69, 1.78);
} }
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OOP: Object Creation (1)
Point p1 = new Point(2, 4); System.out.println(p1);
Point@677327b6
By default, the address stored in p1 gets printed. Instead, print out attributes separately:
System.out.println(“(” + p1.x + “, ” + p1.y + “)”);
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(2.0, 4.0)
OOP: Object Creation (3)
Person jim = new Person(50, “BRI”)
Pe age
nationality
Pe age
nationality
jim
“BRI”
weight
height
rson
50
Person alan = new Person(75, 1.80)
alan null
0.0
0.0
Person jonathan = new Person(65, “CAN”)
jonathan “CAN”
weight
height
rson
Pe age
nationality
0
weight
75.0
height
1.80
Pe age
rson
65
0.0
0.0
Person mark = new Person(40, “CAN”, 69, 1.78) rson
mark
40
69.0 1.78
“CAN”
nationality
weight
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height
OOP: Object Creation (4)
A constructor may only initialize some attributes and leave others uninitialized .
public class PointTester {
public static void main(String[] args) {
Point p1 = new Point(3, 4); Point p2 = new Point(-3 -2); Point p3 = new Point(’x’, 5); Point p4 = new Point(’y’, -7);
} }
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OOP: Object Creation (6)
● When using the constructor, pass valid argument values:
○ The type of each argument value must match the corresponding
parameter type.
○ e.g., Person(50, “BRI”) matches
Person(int initAge, String initNationality) ○ e.g., Point(3, 4) matches
● When creating an instance, uninitialized attributes implicitly get assigned the default values .
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Point(double initX, double initY)
○ Set uninitialized attributes properly later using mutator methods
Person jim = new Person(50, “British”); jim.setWeight(85);
jim.setHeight(1.81);
OOP: Object Creation (5)
Point p1 = new Point(3, 4)
p1
x
y
x
y
Person
Point p3 = new Point(‘x’, 5)
Person
3.0
4.0
p2
x
Person
p3
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5.0
0
Point p2 = new Point(-3, -2)
y
Point p4 = new Point(‘y’, -7)
-3.0
-2.0
p4
x
y
Person
0
-7.0
OOP: Mutator Methods
● These methods change values of attributes.
● We call such methods mutators (with void return type).
public class Person { …
void gainWeight(double units) { weight = weight + units;
} }
public class Point { …
void moveUp() { y = y + 1;
} }
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OOP: Accessor Methods
● These methods return the result of computation based on attribute values.
● We call such methods (with non-void return type).
accessors
public class Person { …
double getBMI() {
double bmi = height / (weight * weight); return bmi;
} }
public class Point { …
double getDistanceFromOrigin() { double dist = Math.sqrt(x*x + y*y); return dist;
}
}
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OOP: Method Parameters
● Principle 1: A constructor needs an input parameter for every attribute that you wish to initialize.
e.g., Person(double w, double h) vs. Person(String fName, String lName)
● Principle 2: A mutator method needs an input parameter for every attribute that you wish to modify.
e.g., In Point, void moveToXAxis() vs. void moveUpBy(double unit)
● Principle 3: An accessor method needs input parameters if the attributes alone are not sufficient for the intended computation to complete.
e.g., In Point, double getDistFromOrigin() vs.
double getDistFrom(Point other)
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OOP: Use of Mutator vs. Accessor Methods
● Calls to mutator methods cannot be used as values.
○ e.g., System.out.println(jim.setWeight(78.5)); × ○ e.g., double w = jim.setWeight(78.5); × ○ e.g., jim.setWeight(78.5); ✓
● Calls to accessor methods should be used as values.
○ e.g., jim.getBMI(); × ○ e.g., System.out.println(jim.getBMI()); ✓ ○ e.g., double w = jim.getBMI(); ✓
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OOP: Object Alias (1)
1 inti=3;
2 int j = i; System.out.println(i == j); /* true */
3 int k = 3; System.out.println(k == i && k == j); /* true */
○ Line 2 copies the number stored in i to j.
○ After Line 4, i, j, k refer to three separate integer placeholder,
which happen to store the same value 3.
1 Point p1 = new Point(2, 3);
2 Point p2 = p1; System.out.println(p1 == p2); /* true */
3 Point p3 = new Point(2, 3);
4 Systme.out.println(p3 == p1 || p3 == p2); /* false */
5 Systme.out.println(p3.x == p1.x && p3.y == p1.y); /* true */ 6 Systme.out.println(p3.x == p2.x && p3.y == p2.y); /* true */
○ Line 2 copies the address stored in p1 to p2.
○ Both p1 and p2 refer to the same object in memory!
○ p3, whose contents are same as p1 and p2, refer to a different
object in memory.
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OO Program Programming: Object Alias (2.1)
Problem: Consider assignments to variables:
1 2 3 4 5 6 7 8 9
10 11
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primitive
int i1 = 1;
int i2 = 2;
int i3 = 3;
int[] numbers1 = {i1, i2, i3};
int[] numbers2 = new int[numbers1.length]; for(int i = 0; i < numbers1.length; i ++) {
numbers2[i] = numbers1[i]; }
numbers1[0] = 4; System.out.println(numbers1[0]); System.out.println(numbers2[0]);
OO Program Programming: Object Alias (3)
Person tom = new Person("TomCruise");
Person ethanHunt = tom;
Person spy = ethanHunt;
tom.setWeight(77); print(tom.weight); /* 77 */ ethanHunt.gainWeight(10); print(tom.weight); /* 87 */ spy.loseWeight(10); print(tom.weight); /* 77 */ Person prof = new Person("Jackie"); prof.setWeight(80);
;;;
print(prof.name+" teaches 2030");/*TomCruise teaches 2030*/
print("EthanHunt is "+ethanHunt.name);/*EthanHunt is TomCruise* print("EthanHunt is "+spy.name);/*EthanHunt is Jackie*/ print("TomCruise is "+tom.name);/*TomCruise is Jackie*/ print("Jackie is "+prof.name);/*Jackie is TomCruise*/
effect as calling the same method on any of its other identities. 51 of 73
spy = prof prof = tom tom = spy
● An object at runtime may have more than one identities. Its address may be stored in multiple reference variables.
● Calling a method on one of an object’s identities has the same
/
OO Program Programming: Object Alias (2.2)
Problem: Consider assignments to reference variables:
1 Person alan = new Person("Alan");
2 Person mark = new Person("Mark");
3 Person tom = new Person("Tom");
4 Person jim = new Person("Jim");
5 Person[] persons1 = {alan, mark, tom};
6 Person[] persons2 = new Person[persons1.length];
7 for(int i = 0; i < persons1.length; i ++) {
8 persons2[i] = persons1[(i + 1) % persons1.length]; }
9 persons1[0].setAge(70);
10 System.out.println(jim.age); /* 0 */
11 System.out.println(alan.age); /* 70 */
12 System.out.println(persons2[0].age); /* 0 */
13 persons1[0] = jim;
14 persons1[0].setAge(75);
15 System.out.println(jim.age); /* 75 */
16 System.out.println(alan.age); /* 70 */
17 System.out.println(persons2[0].age); /* 0 */ 50 of 73
Anonymous Objects (1)
1● What’s the difference between these two fragments of code? 21
After L2, the result of x * x:
○ LHS: it can be reused (without recalculating) via the name sqr. ○ RHS: it is not stored anywhere and returned right away.
double square(double x) { double sqr = x * x; return sqr; }
32
double square(double x) { return x * x; }
1● Same principles applies to objects:
2 1 Person getP(String n) {
Person getP(String n) { Person p =
return p; }
new Person(n) ;
3
2 return new Person(n) ; }
new Person(n) denotes an object without a name reference. ○ LHS: L2 stores the address of this anonymous object in p.
○ RHS: L2 returns the address of this anonymous object directly. 52 of 73
Anonymous Objects (2.1)
Anonymous objects can also be used as assignment sources or argument values:
class Member {
Order[] orders;
int noo;
/* constructor ommitted */ void addOrder(Order o) {
orders[noo] = o;
noo ++; }
void addOrder(String n, double p, double q) { addOrder( new Order(n, p, q) );
/* Equivalent implementation:
* orders[noo] = new Order(n, p, q);
noo ++; */
} }
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Java Data Types (1)
A (data) type denotes a set of related runtime values. 1. Primitive Types
○ Integer Type ● int
[setof32-bitintegers] [setof64-bitintegers]
[setof64-bitFPnumbers]
[setofsinglecharacters]
● long
○ Floating-Point Number Type
● double
○ Character Type
● char
○ Boolean Type
● boolean [setoftrueandfalse]
2. Reference Type : Complex Type with Attributes and Methods
○ String ○ Person ○ Point
[set of references to character sequences] [set of references to Person objects] [set of references to Point objects] [set of references to Scanner objects]
○ Scanner 55 of 73
Anonymous Objects (2.2)
One more example on using anonymous objects:
class MemberTester {
public static void main(String[] args) {
Member m = new Member("Alan");
Order o = new Order("Americano", 4.7, 3); m.addOrder(o);
m.addOrder( new Order("Cafe Latte", 5.1, 4) );
} }
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Java Data Types (2)
● A variable that is declared with a type but uninitialized is
implicitly assigned with its default value .
○ Primitive Type ● int i;
[
is implicitly assigned to i] is implicitly assigned to d] is implicitly assigned to b]
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● double d;
[
● boolean b; [ ○ Reference Type
● String s;
● Person jim; [ ● Point p1;
● Scanner input; [
[
is implicitly assigned to s] is implicitly assigned to jim] is implicitly assigned to p1] is implicitly assigned to input]
Make sure the default value is what you want!
● Calling a method on a uninitialized reference variable crashes
[
● You can use a primitive variable that is uninitialized.
0.0
false
0
null
null
null
null
your program. [ NullPointerException ]
Always initialize reference variables!
Java Data Types (3.1)
● An attribute may store the reference to some object.
● Methods may take as references to other objects.
● Return values from methods may be references to other objects.
class Point {
void moveUpBy(int i) { y = y + i; } Point movedUpBy(int i) {
Point np = new Point(x, y); np.moveUp(i);
return np;
}
class Person { Person spouse; }
parameters
class Person {
void marry(Person other) { ... } }
}
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Java Data Types (3.2.2)
1 class PointCollectorTester {
2 public static void main(String[] args) {
3 PointCollector pc = new PointCollector();
4 System.out.println(pc.nop); /* 0 */
5 pc.addPoint(3, 4);
6 System.out.println(pc.nop); /* 1 */
7 pc.addPoint(-3, 4);
8 System.out.println(pc.nop); /* 2 */
9 pc.addPoint(-3, -4);
10 System.out.println(pc.nop); /* 3 */
11 pc.addPoint(3, -4);
12 System.out.println(pc.nop); /* 4 */
13 Point[] ps = pc.getPointsInQuadrantI();
14 System.out.println(ps.length); /* 1 */
15 System.out.println("(" + ps[0].x + ", " + ps[0].y + ")");
16 /* (3, 4) */
17 }
18 }
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Java Data Types (3.2.1)
An attribute may be of type Point[] , storing references to
Point objects.
1 2 3 4 5 6 7 8 9
10
11
12
13
14
15
16
class PointCollector {
Point[] points; int nop; /* number of points */ PointCollector() { points = new Point[100]; } void addPoint(double x, double y) {
points[nop] = new Point(x, y); nop++; } Point[] getPointsInQuadrantI() {
Point[] ps = new Point[nop];
int count = 0; /* number of points in Quadrant I */ for(int i = 0; i < nop; i ++) {
Point p = points[i];
if(p.x > 0 && p.y > 0) { ps[count] = p; count ++; } } Point[] = new Point[count];
for(int i = 0; i < count; i ++) { q1Points[i] = ps[i] }
return q1Points ; }}
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Required Reading: Point and PointCollector
q1Points
/* ps contains null if count < nop */
Static Variables (1)
class Account {
int id;
String owner;
Account(int id, String owner) {
this.id = id;
this.owner = owner; }
}
class AccountTester {
Account acc1 = new Account(1, "Jim"); Account acc2 = new Account(2, "Jeremy"); System.out.println(acc1.id != acc2.id);
}
But, managing the unique id’s manually is error-prone ! 60 of 73
Static Variables (2)
class Account {
static int globalCounter = 1;
int id; String owner; Account(String owner) {
this.id = globalCounter ; globalCounter ++; this.owner = owner; } }
class AccountTester {
Account acc1 = new Account("Jim"); Account acc2 = new Account("Jeremy"); System.out.println(acc1.id != acc2.id); }
● Each instance of a class (e.g., acc1, acc2) has a local copy of each attribute or instance variable (e.g., id).
○ Changing acc1.id does not affect acc2.id.
● A static variable (e.g., globalCounter) belongs to the class.
○ All instances of the class share a single copy of the static variable.
○ Change to globalCounter via c1 is also visible to c2. 61 of 73
Static Variables (4.1): Common Error
class Client { Account[] accounts;
static int numberOfAccounts = 0; void addAccount(Account acc) {
accounts[numberOfAccounts] = acc;
numberOfAccounts ++; }}
class ClientTester {
Client bill = new Client("Bill"); Client steve = new Client("Steve"); Account acc1 = new Account(); Account acc2 = new Account(); bill.addAccount(acc1);
/* correctly added to bill.accounts[0] */ steve.addAccount(acc2);
/* mistakenly added to steve.accounts[1]! */
}
63 of 73
Static Variables (3)
class Account {
static int globalCounter = 1;
int id; String owner; Account(String owner) {
this.id = ;
this.owner = owner; }}
globalCounter
globalCounter ++;
● Static variable globalCounter is not instance-specific like instance variable (i.e., attribute) id is.
● To access a static variable:
○ No context object is needed.
○ Use of the class name suffices, e.g., Account.globalCounter.
● Each time Account’s constructor is called to create a new
instance, the increment effect is visible to all existing objects
of Account. 62 of 73
Static Variables (4.2): Common Error
● Attribute numberOfAccounts should not be declared as static as its value should be specific to the client object.
● If it were declared as static, then every time the addAccount method is called, although on different objects, the increment effect of numberOfAccounts will be visible to all Client objects.
● Here is the correct version:
class Client {
Account[] accounts;
int numberOfAccounts = 0;
void addAccount(Account acc) {
accounts[numberOfAccounts] = acc;
numberOfAccounts ++; }
}
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Static Variables (5.1): Common Error
1 2 3 4 5 6 7 8
public class Bank {
public string branchName;
public static int nextAccountNumber = 1; public static void useAccountNumber() {
System.out.println (branchName + ...);
nextAccountNumber ++; }
}
● Non-static method cannot be referenced from a static context
● Line 4 declares that we can call the method userAccountNumber without instantiating an object of the class Bank.
● However, in Lined 5, the static method references a non-static attribute, for which we must instantiate a Bank object.
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Static Variables (5.3): Common Error
There are two possible ways to fix:
1. Remove all uses of non-static variables (i.e., branchName) in the static method (i.e., useAccountNumber).
2. Declare branchName as a static variable. ○ This does not make sense.
∵ branchName should be a value specific to each Bank instance. 67 of 73
Static Variables (5.2): Common Error
1 2 3 4 5 6 7 8
public class Bank {
public string branchName;
public static int nextAccountNumber = 1; public static void useAccountNumber() {
System.out.println (branchName + ...);
nextAccountNumber ++; }
}
● To call useAccountNumber(), no instances of Bank are required:
● Contradictorily, to access branchName, a context object is required:
Bank
.useAccountNumber();
Bank b1 = new Bank(); b1.setBranch("Songdo IBK"); System.out.println( .branchName);
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b1
Index (1)
Separation of Concerns: App/Tester vs. Model Object Orientation:
Observe, Model, and Execute
Object-Oriented Programming (OOP)
OO Thinking: Templates vs. Instances (1.1) OO Thinking: Templates vs. Instances (1.2) OO Thinking: Templates vs. Instances (2.1) OO Thinking: Templates vs. Instances (2.2) OO Thinking: Templates vs. Instances (3)
OOP: Classes ≈ Templates
OOP:
Define Constructors for Creating Objects (1.1) OOP:
Define Constructors for Creating Objects (1.2)
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Index (2) OOP:
Define Constructors for Creating Objects (2.1) OOP:
Define Constructors for Creating Objects (2.2) Visualizing Objects at Runtime (1)
Visualizing Objects at Runtime (2.1) Visualizing Objects at Runtime (2.2) Visualizing Objects at Runtime (2.3) Visualizing Objects at Runtime (2.4) The this Reference (1)
The this Reference (2) The this Reference (3) The this Reference (4)
The this Reference (5) 69 of 73
Index (4)
OOP: Object Creation (2)
OOP: Object Creation (3)
OOP: Object Creation (4)
OOP: Object Creation (5)
OOP: Object Creation (6)
OOP: Mutator Methods
OOP: Accessor Methods
OOP: Use of Mutator vs. Accessor Methods OOP: Method Parameters
OOP: Object Alias (1) OOP: Object Alias (2.1) OOP: Object Alias (2.2) OOP: Object Alias (3)
Anonymous Objects (1)
71 of 73
Index (3)
The this Reference (6.1): Common Error The this Reference (6.2): Common Error OOP: Methods (1.1)
OOP: Methods (1.2)
OOP: Methods (2)
OOP: Methods (3)
OOP: The Dot Notation (1.1) OOP: The Dot Notation (1.2) OOP: Method Calls
OOP: Class Constructors (1) OOP: Class Constructors (2) OOP: Class Constructors (3) OOP: Class Constructors (4)
OOP: Object Creation (1)
70 of 73
Index (5)
Anonymous Objects (2.1) Anonymous Objects (2.2) Java Data Types (1)
Java Data Types (2)
Java Data Types (3.1)
Java Data Types (3.2.1)
Java Data Types (3.2.2)
Static Variables (1)
Static Variables (2)
Static Variables (3)
Static Variables (4.1): Common Error Static Variables (4.2): Common Error Static Variables (5.1): Common Error
Static Variables (5.2): Common Error
72 of 73
Index (6)
Static Variables (5.3): Common Error
73 of 73