Class 2 variables, classes methods...

Computer Skills and
Programming Update
Institute of Informatics - Tallinn University
Fernando Loizides, Sonia Sousa, Romil Rõbtšenkov
2015

Outline
Expressions
Data Conversion
Interactive Programs
Creating Objects
The String Class
The Random and Math Classes
2

Expressions
• An expression is a combination of one or more
operators and operands
• Arithmetic expressions compute numeric results
and make use of the arithmetic operators:
Addition
Subtraction
Multiplication
Division
Remainder
+
-
*
/
%
• If either or both operands are floating point values,
then the result is a floating point value
3

Division and Remainder
• If both operands to the division operator (/)
are integers, the result is an integer (the
fractional part is discarded)
14 / 3 equals 4
8 / 12 equals 0
• The remainder operator (%) returns the remainder after
dividing the first operand by the second
14 % 3 equals 2
8 % 12 equals 8
4

Quick Check
What are the results of the following expressions?
12 / 2
12.0 / 2.0
10 / 4
10 / 4.0
4 / 10
4.0 / 10
12 % 3
10 % 3
3 % 10
5

Quick Check
What are the results of the following expressions?
12 / 2
12.0 / 2.0
10 / 4
10 / 4.0
4 / 10
4.0 / 10
12 % 3
10 % 3
3 % 10
= 6
= 6.0
= 2
= 2.5
= 0
= 0.4
= 0
= 1
= 0
6

Operator Precedence
• Operators can be combined into larger
expressions
result = total + count / max - offset;
• Operators have a well-defined precedence
which determines the order in which they are
evaluated
• Multiplication, division, and remainder are
evaluated before addition, subtraction, and string
concatenation
• Arithmetic operators with the same precedence
are evaluated from left to right, but parentheses
can be used to force the evaluation order
7

Quick Check
a + b + c + d + e a + b * c - d / e
a / (b + c) - d % e
a / (b * (c + (d - e)))
In what order are the operators evaluated in the
following expressions?
8

Quick Check
a + b + c + d + e a + b * c - d / e
a / (b + c) - d % e
a / (b * (c + (d - e)))
1 432 3 241
2 341
4 123
In what order are the operators evaluated in the
following expressions?
9

Assignment Revisited
• The assignment operator has a lower
precedence than the arithmetic operators
First the expression on the right hand
side of the = operator is evaluated
Then the result is stored in the
variable on the left hand side
answer = sum / 4 + MAX * lowest;
14 3 2
10

• The right and left hand sides of an assignment
statement can contain the same variable
First, one is added to the
original value of count
Then the result is stored back into count
(overwriting the original value)
count = count + 1;
11

Increment and Decrement
• The increment (++) and decrement (--)
operators use only one operand
• The statement
count++;
is functionally equivalent to
count = count + 1;
12

Increment and Decrement
• The increment and decrement operators can
be applied in postfix form:
count++
• or prefix form:
++count
• When used as part of a larger expression, the
two forms can have different effects
• Because of their subtleties, the increment and
decrement operators should be used with care
13

Assignment Operators
• Often we perform an operation on a variable,
and then store the result back into that variable
• Java provides assignment operators to simplify
that process
• For example, the statement
num += count;
is equivalent to
num = num + count;
14

• There are many assignment operators in
Java, including the following:
Operator
+=
-=
*=
/=
%=
Example
x += y
x -= y
x *= y
x /= y
x %= y
Equivalent To
x = x + y
x = x - y
x = x * y
x = x / y
x = x % y
15

• The right hand side of an assignment
operator can be a complex expression
• The entire right-hand expression is evaluated
first, then the result is combined with the
original variable
• Therefore
result /= (total-MIN) % num;
is equivalent to
result = result / ((total-MIN) % num);
16

• The behavior of some assignment operators
depends on the types of the operands
• If the operands to the += operator are
strings, the assignment operator performs
string concatenation
• The behavior of an assignment operator (+=)
is always consistent with the behavior of the
corresponding operator (+)
17

Outline
Expressions
Data Conversion
Creating Objects
The String Class
18

Data Conversion
• Sometimes it is convenient to convert data
from one type to another
• For example, in a particular situation we may
want to treat an integer as a floating point
value
• These conversions do not change the type
of a variable or the value that's stored in it –
they only convert a value as part of a
computation
19

Data Conversion
• Widening conversions are safest because they
tend to go from a small data type to a larger one
(such as a short to an int)
• Narrowing conversions can lose information
because they tend to go from a large data type
to a smaller one (such as an int to a short)
• In Java, data conversions can occur in three
ways:
– assignment conversion
– promotion
– casting
20

Data Conversion
Widening Conversions Narrowing Conversions
21

Assignment Conversion
• Assignment conversion occurs when a value
of one type is assigned to a variable of
another
• Example:
int dollars = 20;
double money = dollars;
• Only widening conversions can happen via
assignment
• Note that the value or type of dollars did
not change
22

Promotion
• Promotion happens automatically when
operators in expressions convert their
operands
• Example:
int count = 12;
double sum = 490.27;
result = sum / count;
• The value of count is converted to a
floating point value to perform the division
calculation
23

Casting
• Casting is the most powerful, and dangerous,
technique for conversion
• Both widening and narrowing conversions can
be accomplished by explicitly casting a value
• To cast, the type is put in parentheses in front
of the value being converted
int total = 50;
float result = (float) total / 6;
• Without the cast, the fractional part of the
answer would be lost
24

Outline
Expressions
Data Conversion
Creating Objects
The String Class
25

• Programs generally need input on which to
operate
• The Scanner class provides convenient
methods for reading input values of various
types
• A Scanner object can be set up to read input
from various sources, including the user typing
values on the keyboard
• Keyboard input is represented by the
System.in object
26

Reading Input
• The following line creates a Scanner object
that reads from the keyboard:
Scanner scan = new Scanner (System.in);
• The new operator creates the Scanner object
• Once created, the Scanner object can be
used to invoke various input methods, such as:
answer = scan.nextLine();
27

Reading Input
• The Scanner class is part of the
java.util class library, and must be
imported into a program to be used
• The nextLine method reads all of the input
until the end of the line is found
• See Echo.java
28

//********************************************************************
// Echo.java Author: Lewis/Loftus
//
// Demonstrates the use of the nextLine method of the Scanner class
// to read a string from the user.
//********************************************************************
import java.util.Scanner;
public class Echo
{
//-----------------------------------------------------------------
// Reads a character string from the user and prints it.
//-----------------------------------------------------------------
public static void main (String[] args)
{
String message;
System.out.println ("Enter a line of text:");
message = scan.nextLine();
System.out.println ("You entered: "" + message + """);
}
}
29

//********************************************************************
// Echo.java Author: Lewis/Loftus
//
// Demonstrates the use of the nextLine method of the Scanner class
// to read a string from the user.
//********************************************************************
public class Echo
{
//-----------------------------------------------------------------
// Reads a character string from the user and prints it.
//-----------------------------------------------------------------
{
String message;
System.out.println ("Enter a line of text:");
message = scan.nextLine();
System.out.println ("You entered: "" + message + """);
}
}
Sample Run
Enter a line of text:
You want fries with that?
You entered: "You want fries with that?"
30

Input Tokens
• Unless specified otherwise, white space is
used to separate the elements (called tokens)
of the input
• White space includes space characters, tabs,
new line characters
• The next method of the Scanner class reads
the next input token and returns it as a string
• Methods such as nextInt and nextDouble
read data of particular types
• See GasMileage.java
31

//********************************************************************
// GasMileage.java Author: Lewis/Loftus
//
// Demonstrates the use of the Scanner class to read numeric data.
//********************************************************************
public class GasMileage
{
//-----------------------------------------------------------------
// Calculates fuel efficiency based on values entered by the
// user.
//-----------------------------------------------------------------
{
int miles;
double gallons, mpg;
continue
32

continue
System.out.print ("Enter the number of miles: ");
miles = scan.nextInt();
System.out.print ("Enter the gallons of fuel used: ");
gallons = scan.nextDouble();
mpg = miles / gallons;
System.out.println ("Miles Per Gallon: " + mpg);
}
}
33

continue
System.out.print ("Enter the number of miles: ");
miles = scan.nextInt();
System.out.print ("Enter the gallons of fuel used: ");
gallons = scan.nextDouble();
mpg = miles / gallons;
System.out.println ("Miles Per Gallon: " + mpg);
}
}
Sample Run
Enter the number of miles: 328
Enter the gallons of fuel used: 11.2
Miles Per Gallon: 29.28571428571429
34

Outline
Expressions
Data Conversion
Creating Objects
The String Class
35

Creating Objects
• Primitive data type vs objects
• Objects can be used effectively to represent
real-world entities
• For instance, an object might represent a
particular employee in a company
• Each employee object handles the
processing and data management related to
that employee
• It can be seen as an autonomous and closed
box which performs operations
36

Creating Objects
• An object has:
– state (attributes) - descriptive
characteristics
– behaviors - what it can do (or what
can be done to it)
• The state of a bank account includes
its account number and its current
balance
• The behaviors associated with a
bank account include the ability to
make deposits and withdrawals
• Note that the behavior of an object
might change its state
37
BankAcount
AccounNumber
Name
Balance
Deposit
Withrow
Transfer

Classes
• An object is defined by a class
• A class is the blueprint of an object
• The class uses methods to define the behaviors
of the object
• The class that contains the main method of a
Java program represents the entire program
• A class represents a concept, and an object
represents the embodiment of that concept
• Multiple objects can be created from the same
class
38

Class = Blueprint
• One blueprint to create several similar, but
different, houses:
39

Objects and Classes
Bank
Account
A class
(the concept)
John’s Bank Account
Balance: $5,257
An object
(the realization)
Bill’s Bank Account
Balance: $1,245,069
Mary’s Bank Account
Balance: $16,833
Multiple objects
from the same class
40

Creating Objects
• A variable holds either a primitive value or a
reference to an object
• A class name can be used as a type to
declare an object reference variable
String title;
• No object is created with this declaration
• An object reference variable holds the
address of an object
• The object itself must be created separately
41

Creating Objects
• Generally, we use the new operator to create
an object
• Creating an object is called instantiation
• An object is an instance of a particular class
title = new String ("Java Software Solutions");
This calls the String constructor, which is
a special method that sets up the object
42

Invoking Methods
• We've seen that once an object has been
instantiated, we can use the dot operator to
invoke its methods
numChars = title.length()
• A method may return a value, which can be
used in an assignment or expression
• A method invocation can be thought of as
asking an object to perform a service
43

References
• Note that a primitive variable contains the
value itself, but an object variable contains
the address of the object
• An object reference can be thought of as a
pointer to the location of the object
• Rather than dealing with arbitrary addresses,
we often depict a reference graphically
"Steve Jobs"name1
num1 38
44

• The act of assignment takes a copy of a
value and stores it in a variable
• For primitive types:
num1 38
num2 96
Before:
num2 = num1;
num1 38
num2 38
After:
45

Reference Assignment
• For object references, assignment copies
the address:
name2 = name1;
name1
name2
Before:
"Steve Jobs"
"Steve Wozniak"
name1
name2
After:
"Steve Jobs"
46

Aliases
• Two or more references that refer to the
same object are called aliases of each other
• That creates an interesting situation: one
object can be accessed using multiple
reference variables
• Aliases can be useful, but should be
managed carefully
• Changing an object through one reference
changes it for all of its aliases, because
there is really only one object
47

Garbage Collection
• When an object no longer has any valid
references to it, it can no longer be accessed by
the program
• The object is useless, and therefore is called
garbage
• Java performs automatic garbage collection
periodically, returning an object's memory to the
system for future use
• In other languages, the programmer is
responsible for performing garbage collection
48

Outline
Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Creating Objects
The String Class
49

The String Class
• Because strings are so common, we don't have
to use the new operator to create a String
object
title = "Java Software Solutions";
• This is special syntax that works only for strings
• Each string literal (enclosed in double quotes)
represents a String object
50

String Methods
• Once a String object has been created,
neither its value nor its length can be
changed
• Therefore we say that an object of the
String class is immutable
• However, several methods of the String
class return new String objects that are
modified versions of the original
51

String Indexes
• It is occasionally helpful to refer to a particular
character within a string
• This can be done by specifying the character's
numeric index
• The indexes begin at zero in each string
• In the string "Hello", the character 'H' is at
index 0 and the 'o' is at index 4
• See StringMutation.java
52

//********************************************************************
// StringMutation.java Author: Lewis/Loftus
//
// Demonstrates the use of the String class and its methods.
//********************************************************************
public class StringMutation
{
//-----------------------------------------------------------------
// Prints a string and various mutations of it.
//-----------------------------------------------------------------
{
String phrase = "Change is inevitable";
String mutation1, mutation2, mutation3, mutation4;
System.out.println ("Original string: "" + phrase + """);
System.out.println ("Length of string: " + phrase.length());
mutation1 = phrase.concat (", except from vending machines.");
mutation2 = mutation1.toUpperCase();
mutation3 = mutation2.replace ('E', 'X');
mutation4 = mutation3.substring (3, 30);
continued
53

continued
// Print each mutated string
System.out.println ("Mutation #1: " + mutation1);
System.out.println ("Mutated length: " + mutation4.length());
}
}
54

continued
// Print each mutated string
System.out.println ("Mutated length: " + mutation4.length());
}
}
Output
Original string: "Change is inevitable"
Length of string: 20
Mutation #1: Change is inevitable, except from vending machines.
Mutation #2: CHANGE IS INEVITABLE, EXCEPT FROM VENDING MACHINES.
Mutation #3: CHANGX IS INXVITABLX, XXCXPT FROM VXNDING MACHINXS.
Mutation #4: NGX IS INXVITABLX, XXCXPT F
Mutated length: 27
55

Quick Check
What output is produced by the following?
String str = "Space, the final frontier.";
System.out.println (str.length());
System.out.println (str.substring(7));
System.out.println (str.toUpperCase());
56

Quick Check
What output is produced by the following?
String str = "Space, the final frontier.";
System.out.println (str.substring(7));
System.out.println (str.toUpperCase());
26
the final frontier.
SPACE, THE FINAL FRONTIER.
26
57

Outline
Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Creating Objects
The String Class
58

Class Libraries
• A class library is a collection of classes that
we can use when developing programs
• The Java standard class library is part of any
Java development environment
• Its classes are not part of the Java language
per se, but we rely on them heavily
• Various classes we've already used (System
, Scanner, String) are part of the Java
standard class library
59

The Java API
• The Java class library is sometimes referred
to as the Java API
• API stands for Application Programming
Interface
• Clusters of related classes are sometimes
referred to as specific APIs:
– The Swing API
– The Database API
60

The Java API
• Get comfortable navigating the online Java
API documentation
http://docs.oracle.com/javase/6/docs/api/
61

Packages
• For purposes of accessing them, classes in
the Java API are organized into packages
• These often overlap with specific APIs
• Examples:
Package
java.lang
java.applet
java.awt
javax.swing
java.net
java.util
javax.xml.parsers
Purpose
General support
Creating applets for the web
Graphics and graphical user interfaces
Additional graphics capabilities
Network communication
Utilities
XML document processing
62

The import Declaration
• When you want to use a class from a package,
you could use its fully qualified name
java.util.Scanner
• Or you can import the class, and then use just
the class name
• To import all classes in a particular package,
you can use the * wildcard character
import java.util.*;
63

The import Declaration
• All classes of the java.lang package are
imported automatically into all programs
• It's as if all programs contain the following line:
import java.lang.*;
• That's why we didn't have to import the System
or String classes explicitly in earlier programs
• The Scanner class, on the other hand, is part
of the java.util package, and therefore must
be imported
64

The Random Class
• The Random class is part of the java.util
package
• It provides methods that generate
pseudorandom numbers
• A Random object performs complicated
calculations based on a seed value to produce a
stream of seemingly random values
• See RandomNumbers.java
65

//********************************************************************
// RandomNumbers.java Author: Lewis/Loftus
//
// Demonstrates the creation of pseudo-random numbers using the
// Random class.
//********************************************************************
import java.util.Random;
public class RandomNumbers
{
//-----------------------------------------------------------------
// Generates random numbers in various ranges.
//-----------------------------------------------------------------
{
Random generator = new Random();
int num1;
float num2;
num1 = generator.nextInt();
System.out.println ("A random integer: " + num1);
num1 = generator.nextInt(10);
System.out.println ("From 0 to 9: " + num1);
continued
66

continued
num1 = generator.nextInt(10) + 1;
num1 = generator.nextInt(20) - 10;
System.out.println ("From -10 to 9: " + num1);
num2 = generator.nextFloat();
System.out.println ("A random float (between 0-1): " + num2);
num2 = generator.nextFloat() * 6; // 0.0 to 5.999999
num1 = (int)num2 + 1;
}
}
67

continued
num1 = generator.nextInt(20) - 10;
System.out.println ("From -10 to 9: " + num1);
num2 = generator.nextFloat();
System.out.println ("A random float (between 0-1): " + num2);
num2 = generator.nextFloat() * 6; // 0.0 to 5.999999
num1 = (int)num2 + 1;
}
}
Sample Run
A random integer: 672981683
From 0 to 9: 0
From 1 to 10: 3
From 20 to 34: 30
From -10 to 9: -4
A random float (between 0-1): 0.18538326
From 1 to 6: 3
68

Quick Check
Given a Random object named gen, what range of
values are produced by the following expressions?
gen.nextInt(25)
gen.nextInt(6) + 1
gen.nextInt(100) + 10
gen.nextInt(10) – 5
69

Quick Check
Given a Random object named gen, what range of
values are produced by the following expressions?
gen.nextInt(25)
gen.nextInt(6) + 1
Range
0 to 24
1 to 6
10 to 109
100 to 149
-5 to 4
12 to 33
70

Quick Check
Write an expression that produces a random integer
in the following ranges:
Range
0 to 12
1 to 20
15 to 20
-10 to 0
71

Quick Check
Write an expression that produces a random integer
in the following ranges:
gen.nextInt(13)
gen.nextInt(20) + 1
gen.nextInt(6) + 15
Range
0 to 12
1 to 20
15 to 20
-10 to 0
72

The Math Class
• The Math class is part of the java.lang
package
• The Math class contains methods that
perform various mathematical functions
• These include:
– absolute value
– square root
– exponentiation
– trigonometric functions
73

The Math Class
• The methods of the Math class are static
methods (also called class methods)
• Static methods are invoked through the class
name – no object of the Math class is
needed
value = Math.cos(90) + Math.sqrt(delta);
• See squarertandpower.java
74

75
//********************************************************************
// squarertandpower.java Author: Isaias Barreto da Rosa
//
// Demonstrates the use of Math Class methods
//********************************************************************
package squarertandpower;
public class SquarertAndPower {
public static void main(String[] args) {
double x;
Scanner scan = new Scanner(System.in);
System.out.print(”Enter a number ");
x = scan.nextDouble();
System.out.println("The square route of "+ x +" is "+Math.sqrt(x));
System.out.println(x+” raised to the power of 2 is ”+Math.pow(x, 2));
}
}

//********************************************************************
// squarertandpower.java Author: Isaias Barreto da rosa
//
// Demonstrates the use of Math Class methods
//********************************************************************
package squarertandpower;
public class SquarertAndPower {
public static void main(String[] args) {
double x;
Scanner scan = new Scanner(System.in);
System.out.print(”Enter a number ");
x = scan.nextDouble();
System.out.println("The square route of "+ x +" is "+Math.sqrt(x));
System.out.println(x+” raised to the power of 2 is ”+Math.pow(x, 2));
}
}
76
Sample Run
Enter a number: 4
The square route of 4.0 is 2.0
4.0 raised to the power of 2 is 16.0

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