CSCI102 An Introduction to Programming using Java

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CSCI102 An Introduction to Programming using Java

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Title: CSCI102 An Introduction to Programming using Java

1
CSCI102 An Introduction to Programming using
Java
2
Chapter 4

Defining Classes and Methods

3
Objects

An object is a variable of a class type.
Objects have data but they also can perform
actions.
Actions performed by objects are known as
methods. (We have already used objects of the
String class which is supplied with the Java
language).

This week and next week you will learn how to
define your own classes, and how to use objects
of those classes.

5
Chapter 4

4.1 Class and Method Definitions
4.2 Information Hiding and Encapsulation
4.3 Objects and Reference

6
Chapter 4

4.1 Class and Method Definitions
4.2 Information Hiding and Encapsulation
4.3 Objects and Reference

7
Class and Method Definitions

Class Files and Separate Compilation
Instance Variables
Using Methods and void- Method Definitions
Methods that Return a Value
The this Parameter
Local Variables and Blocks
Parameters of a Primitive Type

8
Class and Method Definitions

Class Files and Separate Compilation
Instance Variables
Using Methods and void- Method Definitions
Methods that Return a Value
The this Parameter
Local Variables and Blocks
Parameters of a Primitive Type

9
Class Files and Separate Compilation

Each Java class definition has to be in a file by
itself.
The name of the file must match the name of the
class, and have the extension .java
After successfully compiling the class, the byte
code is stored in a file with the same name, but
with the extension .class

To use a class in a Java application program,
there must be a .java file containing the main
program part. I will call this the main program
file.
The main program file also has a class name at
the start of the file, and has a file name to
match, with the extension .java.

11
A Class Definition SpeciesFirstTry.java

http//www.annedawson.net/SpeciesFirstTry.java
page 225 of 4th Ed textbook

12
Using Classes and Methods SpeciesFirstTryDemo.jav
a

http//www.annedawson.net/SpeciesFirstTryDemo.java
page 227 of 4th Ed textbook

13
Class and Method Definitions

Class Files and Separate Compilation
Instance Variables
Using Methods and void- Method Definitions
Methods that Return a Value
The this Parameter
Local Variables and Blocks
Parameters of a Primitive Type

14
Instance Variables

Objects have data items and methods.

15
an instance variable is a data item
16
Examples of instance variables

public String employee
public double salary
public int languages

17
the keyword public

public String employee
The keyword public means that the variable
employee can be used anywhere.

18
Declaring Instance VariablesSpeciesFirstTryDemo.
java

http//www.annedawson.net/SpeciesFirstTryDemo.java
page 227 of 4th Ed textbook

19
To create a new object

Car fastcar new Car()
Assigns memory for the object called fastcar

20
To access the value of an instance variable of an
object

fastcar.color
object (dot) instance variable

21
To invoke (call) a method of an object, the dot
is used again

fastcar.move()
object (dot) method

22
Class and Method Definitions

Class Files and Separate Compilation
Instance Variables
Using Methods and void- Method Definitions
Methods that Return a Value
The this Parameter
Local Variables and Blocks
Parameters of a Primitive Type

23
To invoke (call) a method of an object

fastcar.move()
object (dot) method

24
Another way to call a method

For some special methods (called static or
class methods - see text book) you can call the
method using
Classname.methodname( )
e.g.
int result Math.max(n1,n2)

25
Static Methods

The SavitchIn class uses static methods for
data input. . .

26
Calling a static method

SavitchIn.readLineInt()
Class (dot)
method

27
There are only two kinds of methods

those that return a value....
...and those that don't

28
A method that does not return a value looks like
this

public void writeOutput()
System.out.println("Name " name)
System.out.println("Pop " pop)

29
Calling a method that does not return a value

speciesOfTheMonth.writeOutput()
object (dot)
method

30
Calling a method that returns a value

data flow
number_of_cars SavitchIn.readLineInt()

31
Class and Method Definitions

Class Files and Separate Compilation
Instance Variables
Using Methods and void- Method Definitions
Methods that Return a Value
The this Parameter
Local Variables and Blocks
Parameters of a Primitive Type

32
A method that returns a value

number_of_cars SavitchIn.readLineInt()

33
A sample definition of a method that returns a
value . . .
34

public int populationIn10()
double populationAmount population
int count 10
while ((count gt 0) (populationAmount gt 0))
populationAmount (populationAmount
(growthRate/100) populationAmount)
count--
if (populationAmount gt 0)
return (int)populationAmount // (type cast)
else
return 0

public int populationIn10()
double populationAmount population
int count 10
while ((count gt 0) (populationAmount gt 0))
populationAmount (populationAmount
(growthRate/100) populationAmount)
count--
if (populationAmount gt 0)
return (int)populationAmount // (type cast)
else
return 0

36
Please Note!

Although void methods cannot return a value,
you can still use the return statement to exit
the method . See page 237 (4th Ed) of the text
book for a description of this.

37
Class and Method Definitions

Class Files and Separate Compilation
Instance Variables
Using Methods and void- Method Definitions
Methods that Return a Value
The this Parameter
Local Variables and Blocks
Parameters of a Primitive Type

Outside of a method definition an instance
variable is normally accessed with reference to
the object which owns it, e.g.
fastcar.color

On the other hand, inside of a method
definition instance variables can be referred to
by just their name, e.g.
color
after all, the name of the calling object is
not known within the method definition, but it is
understood that an object will be involved at
some point (i.e. when the method is called).

Inside of a method definition, instance
variables can be referred to by just their name,
e.g. color
Should you wish to explicitly refer to the
calling object from inside the method definition,
the keyword this is used as a placeholder for
that calling object
this.color

41
The reserved word this

The reserved word (keyword) this stands for
the name of the calling object. It is used
inside a method definition.
Most of the time you don't need to use it.
Later you will see examples of when you do need
to use this.

42
Class and Method Definitions

Class Files and Separate Compilation
Instance Variables
Using Methods and void- Method Definitions
Methods that Return a Value
The this Parameter
Local Variables and Blocks
Parameters of a Primitive Type

43
Local Variables

A variable declared within a method definition
is called a local variable.

44
Local Variables

A local variable can only be used in the method
where it is declared.

45
Local Variables

If two methods each have a local variable of
the same name, then these are two different
variables even though they have the same name.
They occupy different areas in memory.

46
Java has no global variables! (which is a good
thing)
47
Blocks

A block is a compound statement with variable
declarations. . .

48
A block example

String name "Anne"
int age 21
System.out.println(name age)
// a compound statement is a
// series of statements enclosed
// in curly braces

49
Blocks

A block is a compound statement with variable
declarations. . .

50
Blocks

Variables declared within a block are local to
that block. . .

51
Warning!

In Java you cannot use the same name twice
within the same method - even when they are in
different blocks!

52
Class and Method Definitions

Class Files and Separate Compilation
Instance Variables
Using Methods and void- Method Definitions
Methods that Return a Value
The this Parameter
Local Variables and Blocks
Parameters of a Primitive Type

53
Using a Method with a Parameter
SpeciesSecondTry.java

http//www.annedawson.net/SpeciesSecondTry.java
page 244 of 4th Ed textbook

54
Call-By-Value

If the parameter is a primitive type (char,
int
etc) then the parameter is passed
call-by-value.
This means that the value of the variable is
passed, not the variable itself. Any changes
made to that value within the method do not
affect the variable which was passed to the
method.

55
The formal parameter

is a local variable in the method

56
Note!

Formal parameters have a type, and you should
aim to ensure that the data passed to the method
matches this type.
If you do not pass the matching type, Java
will attempt to perform a type cast into the
required type.

57
Note!

Multiple arguments are matched to multiple
formal parameters by position. . .

// the header of the method definition
public void doThis(int n1, int n2, double cost,
char code)
anObject.doThis(42,100,9.99,'A')
// calling the method

59
Chapter 4

4.1 Class and Method Definitions
4.2 Information Hiding and Encapsulation
4.3 Objects and Reference

60
Information Hiding and Encapsulation

Information Hiding and Abstraction
Precondition and Postcondition Comments
Public and Private Modifiers
Private Methods and Accessor Methods
Abstract Data Types and User Interface
Encapsulation
Automatic Documentation with javadoc

61
Information Hiding and Encapsulation

Information Hiding and Abstraction
Precondition and Postcondition Comments
Public and Private Modifiers
Private Methods and Accessor Methods
Abstract Data Types and User Interface
Encapsulation
Automatic Documentation with javadoc

62
Abstraction

abstraction is the result of converting a
complex thing into a simple thing. . .

63
Abstraction Example

What has four walls, a door, windows and a
roof?

64
Abstraction

means listing the essentials and hiding the
details
the opposite of abstract means to show all the
details of an object

65
Abstraction IS Information Hiding
66
Information Hiding ISAbstraction
67
Information Hiding ISEncapsulation
68
Why is abstraction used in Java programs?

Abstraction is used in a programming language
to make it easier for the programmer to focus on
the essential parts of a program, and not get
bogged down by the details.

69
Why is abstraction used in Java programs?

Hiding information simplifies programs by
grouping together data relevant to an object and
hiding the data from other objects (or
programmers). . .

A programmer can use the methods of a class
which was written by another programmer without
even seeing the source code.
All a programmer needs to know is what data is
required as input to the method, and what data
(if any) is returned.

71
The Black Box Analogy
known inputs
known outputs
72
Abstraction in Programming

To use a method of a class, all a programmer
needs to know is what data is required as input
to the method, and what data (if any) is returned.

The information about the input and output
data of a method are known as the method's
preconditions and postconditions, respectively.

74
The Black Box Analogy
postconditions
preconditions
known inputs
known outputs
75
Information Hiding and Encapsulation

Information Hiding and Abstraction
Precondition and Postcondition Comments
Public and Private Modifiers
Private Methods and Accessor Methods
Abstract Data Types and User Interface
Encapsulation
Automatic Documentation with javadoc

76
Documentation of Methods

Preconditions and postconditions of methods
are usually documented as comments just above the
method definition in the source code.

77
Precondition and Postcondition Comments

/
Precondition years is a nonnegative number.
Returns the projected population of the calling
object
after the specified number of years.
/

By commenting pre- and postconditions in this
manner, documentation for the method can be
automatically generated using the Sun
Microsystems application javadoc
javadoc is part of the Java SDK, and hence
should be located on your hard drive.

79
javadoc

See
http//www.annedawson.net/javadoc.htm

80
Documentation produced by javadoc. . .

. . . is all a programmer needs to study in
order to use a class (and its methods) written by
somebody else.

If you give a .class file to another
programmer, with the documentation produced by
javadoc, the programmer can use the class without
having to study the source code. Indeed, they do
not need the .java source code in order to use
the class.

82
You do not need the source code to be able to use
a class

This is like saying that you don't need to
know the details of how a calculator works to be
able to use it. It's what you put in and what
comes out that matters. How the calculator does
it is irrelevant.

83
Buying classes from a software vendor

You can buy classes from software vendors, to
use with your program, saving you the job of
writing them yourself. The vendor gives you the
bytecode (.class) file (which is a binary file,
and hence does not contain any source code), and
documentation on how to use it.

84
Information Hiding and Encapsulation

Information Hiding and Abstraction
Precondition and Postcondition Comments
Public and Private Modifiers
Private Methods and Accessor Methods
Abstract Data Types and User Interface
Encapsulation
Automatic Documentation with javadoc

85
public and private Modifiers

The public and private modifiers specify
where the variable being declared, or function
being defined, can be accessed. . .

86
The Public Modifier

Variables declared (or methods defined) using
the Public modifier, can be accessed anywhere in
a program or class.

87
The Private Modifier

Variables declared (or methods defined) using
the Private modifier, can only be accessed by a
method defined in the same class.

88
Using Public and Private

In general, all instance variables are Private
and all methods are Public.

89
private variables - public methods

There are a few exceptions. For example
'helper' methods are usually private. See page
267 (4th Ed) of the text book.

90
Information Hiding and Encapsulation

Information Hiding and Abstraction
Precondition and Postcondition Comments
Public and Private Modifiers
Private Methods and Accessor Methods
Abstract Data Types and User Interface
Encapsulation
Automatic Documentation with javadoc

91
Accessor Methods

An accessor method is a method that allows you
to read or to set one or more instance variables.

92
A Class With Accessor Methods SpeciesFourthTry.ja
va

http//www.annedawson.net/SpeciesFourthTry.java
page 258 of 4th Ed textbook

93
Using an Accessor Method SpeciesFourthTryDemo.jav
a

http//www.annedawson.net/SpeciesFourthTryDemo.jav
a
page 259 of 4th Ed textbook

94
Information Hiding and Encapsulation

Information Hiding and Abstraction
Precondition and Postcondition Comments
Public and Private Modifiers
Private Methods and Accessor Methods
Abstract Data Types and User Interface
Encapsulation
Automatic Documentation with javadoc

95
Abstract Data Types

An abstract data type (ADT) is a data type (a
class) which exhibits information hiding and
encapsulation.

96
When you write your own classes

make sure that each class is an abstract data
type!

97
Implementing an ADT

1. summarize the purpose of the class in
comments at the top of the .java file
2. make instance variables private
3. provide public accessor and I/O methods.
4. write pre- and post-condition comments for
every method
5. make helping methods private

98
A class which is an ADT

has two parts
interface
and
implementation

99
The ADT Interface

The interface is simply the set of method
headers and associated comments.
These define the 'behaviour' of the class.
Put another way, the interface specifies how to
interact with objects of this class.

100
A class which is an ADT

has two parts
interface
and
implementation

101
Implementation

The implementation is simply the code.

102
Information Hiding and Encapsulation

Information Hiding and Abstraction
Precondition and Postcondition Comments
Public and Private Modifiers
Private Methods and Accessor Methods
Abstract Data Types and User Interface
Encapsulation
Automatic Documentation with javadoc

103
Encapsulation

Encapsulation means that the data and
functions relevant to an object, are stored
together as a single item, and the details are
hidden.

104
Information Hiding and Encapsulation

Information Hiding and Abstraction
Precondition and Postcondition Comments
Public and Private Modifiers
Private Methods and Accessor Methods
Abstract Data Types and User Interface
Encapsulation
Automatic Documentation with javadoc

105
Automatic Documentation with javadoc

javadoc is a program provided free with Sun
Microsystem's implementation of java. When you
run javadoc, a document is generated which
describes the user interface to your classes.

106
javadoc

See
http//www.annedawson.net/javadoc.htm

107
Chapter 4

4.1 Class and Method Definitions
4.2 Information Hiding and Encapsulation
4.3 Objects and Reference

108
Objects and Reference

Variables of a Class Type and Objects
the new keyword
Boolean valued methods
Parameters of a class type
Comparing Class and Primitive type parameters

109
Objects and Reference

Variables of a Class Type and Objects
the new keyword
Boolean valued methods
Parameters of a class type
Comparing Class and Primitive type parameters

110
Variables of a Class Type and Objects

A variable of a class type
is an object.

111
To create an object

Syntax
Classname objectname new Classname()
Examples
Animal dog new Animal()
Animal cat new Animal()

112
To create an object

Classname objectname new Classname( )
is the short version of
Classname objectname // declares a name to
// hold the
// address of the object
objectname new Classname( ) //object
//created

113

The name of any object is the address of the
first member.

114

If you remember the preceding statement
whenever you're programming with objects, you'll
save yourself a lot of grief.

115
To create an object

Classname objectname new Classname()
This is the default constructor, (explained
in the next chapter) which is a special
initializing method which, amongst other
things,
returns the start address in memory
of the new object.

116
When an object is referred to in a program, for
example. . .

objectname.set(100)
you are actually
referring to an address,
not any value within the object.

117
Objects and Reference

Variables of a Class Type and Objects
the new keyword
Boolean valued methods
Parameters of a class type
Comparing Class and Primitive type parameters

118
The new keyword

The new keyword allocates memory for the
object, placing the start address in the object's
variable name. . .

119

Because variables of a class type hold
addresses, they behave very differently to
variables of a primitive type (which hold values
of their own type). . .

120
Differences between class objects and primitive
objects

int num1 6
int num2 9
num1 num2 // puts 9 into num1
Animal cat new Animal()
// puts an address into cat
Animal dog new Animal()
// puts another address into dog
cat dog // they both hold the address
// of the dog object, hence now cat
// and dog refer to the dog object
cat dog // returns true

121

The name of any object is the
address of the first member.

122

The name of any primitive is its value.

num1
9
123
Differences between class objects and primitive
objects

int num1 6
int num2 9
num1 num2 // puts 9 into num1
Animal cat new Animal()
// puts an address into cat
Animal dog new Animal()
// puts another address into dog
cat dog // they both hold the address
// of the dog object, hence now cat
// and dog refer to the dog object
cat dog // true

124
cat dog

The above line compares addresses. If you
want to compare objects of the same class, you
should define your own equals method for the
class, with a usage
obj1.equals(obj2)
See page 275 (4th Ed) of the text book.

125
The equals method

If you do not define an equals method for your
class, then Java will automatically create a
default definition of equals, but it is unlikely
to behave the way you want it to behave.

126
The equals method

Always define your own equals method.

127
Objects and Reference

Variables of a Class Type and Objects
the new keyword
Boolean valued methods
Parameters of a class type
Comparing Class and Primitive type parameters

128
Defining an equals Method Species.java

http//www.annedawson.net/Species.java
page 277 of 4th Ed textbook

129
Demonstrating an equals Method
SpeciesEqualsDemo.java

http//www.annedawson.net/SpeciesEqualsDemo.java
page 278 of 4th Ed textbook

130
Objects and Reference

Variables of a Class Type and Objects
the new keyword
Boolean valued methods
Parameters of a class type
Comparing Class and Primitive type parameters

131
Parameters of a Class Type

Firstly, let's just remind ourselves of
parameters of a primitive type...

132
Parameters of a Primitive Type

public int multiply_by_two (int number)
int double_it
double_it number number
return double_it
to call the method n multiply_by_two(num1)

133
The value of num1 is not changed by the method.

public int multiply_by_two (int number)
int double_it
double_it number number
return double_it
to call the method n multiply_by_two(num1)

134
The name of any primitive variable is the value
it contains!

public int multiply_by_two (int number)
int double_it
double_it number number
return double_it
to call the method n multiply_by_two(num1)

135
Inside the function, number is a local variable
which takes a copy of the value of num1.

public int multiply_by_two (int number)
int double_it
double_it number number
return double_it
// end of method definition
to call the method n multiply_by_two(num1)

136
Call-By-Value

When primitive types are used as arguments to
methods, it is known as.....

137
Parameters of a Class Type

Parameters of a class type behave very
differently to parameters of a primitive type....

138
Parameters of a Class Type
public void do_something_to(Animal thing) //
whatever happens to thing within // this
method actually // happens to cat. These
are // permanent changes to cat //
because the method receives cat's //
address. It is as if the method // receives
the cat object itself. // end of method
definitionmethod call do_something_to(cat)

139

The name of any object is the address of the
first member.

140
Call-By-Reference?

Some people call passing a class object to a
method as call-by-reference, but this is not
strictly true. (Aside for C programmer's -
this is like saying an array parameter is not
exactly call-by-reference.)

There are similarities, but there are also
important differences. Hence for correctness, we
will not refer to class parameters as
call-by-reference.
141
Objects and Reference

Variables of a Class Type and Objects
the new keyword
Boolean valued methods
Parameters of a class type
Comparing Class and Primitive type parameters

142
Comparing Class and Primitive-Type Parameters

A method cannot change the value of a
primitive type that is an argument to the method.
On the other hand, a method can change the
values of the instance variables of an argument
of a class type.

143
And finally...

The constant value null is a special constant
that can be used to give a value to any variable
of any class type.
null is a sort of placeholder for an address,
but it is not an address itself. You can use
and ! to compare objects with null,
eg (cat ! null)

144
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