Chapter 2: Objects and Chapter 2: Objects and
Primitive Data Primitive Data
Presentation slides for Presentation slides for
Java Software Solutions Java Software Solutions
Foundations of Program Design Foundations of Program Design
Second Edition Second Edition
by John Lewis and William Loftus by John Lewis and William Loftus
Java Software Solutions is published by Addison-Wesley Java Software Solutions is published by Addison-Wesley
Presentation slides are copyright 2000 by John Lewis and William Loftus. All rights reserved.
Presentation slides are copyright 2000 by John Lewis and William Loftus. All rights reserved.
Instructors using the textbook may use and modify these slides for pedagogical purposes.
Instructors using the textbook may use and modify these slides for pedagogical purposes.
2
Objects and Primitive Data Objects and Primitive Data
We can now explore some more fundamental programming We can now explore some more fundamental programming concepts
concepts
Chapter 2 focuses on: Chapter 2 focuses on:
• predefined objectspredefined objects
• primitive dataprimitive data
• the declaration and use of variablesthe declaration and use of variables
• expressions and operator precedenceexpressions and operator precedence
• class librariesclass libraries
• Java appletsJava applets
• drawing shapesdrawing shapes
Introduction to Objects Introduction to Objects
Initially, we can think of an Initially, we can think of an object object as a collection of services as a collection of services that we can tell it to perform for us
that we can tell it to perform for us
The services are defined by methods in a class that defines The services are defined by methods in a class that defines the object
the object
In the Lincoln program, we invoked the In the Lincoln program, we invoked the println println method method of the
of the System.out System.out object: object:
System.out.println ("Whatever you are, be a good one.");
object
object methodmethod
Information provided to the method Information provided to the method
(parameters) (parameters)
4
The println and print Methods The println and print Methods
The The System.out System.out object provides another service as well object provides another service as well
The The print print method is similar to the method is similar to the println println method, method, except that it does not advance to the next line
except that it does not advance to the next line
Therefore anything printed after a Therefore anything printed after a print statement will print statement will appear on the same line
appear on the same line
See See Countdown.java Countdown.java (page 53) (page 53)
Abstraction Abstraction
An An abstraction abstraction hides (or ignores) the right details at the hides (or ignores) the right details at the right time
right time
An object is abstract in that we don't really have to think An object is abstract in that we don't really have to think about its internal details in order to use it
about its internal details in order to use it
We don't have to know how the We don't have to know how the println println method works in method works in order to invoke it
order to invoke it
A human being can only manage seven (plus or minus 2) A human being can only manage seven (plus or minus 2) pieces of information at one time
pieces of information at one time
But if we group information into chunks (such as objects) But if we group information into chunks (such as objects) we can manage many complicated pieces at once
we can manage many complicated pieces at once
Therefore, we can write complex software by organizing it Therefore, we can write complex software by organizing it
6
The String Class The String Class
Every character string is an object in Java, defined by the Every character string is an object in Java, defined by the String
String class class
Every string literal, delimited by double quotation marks, Every string literal, delimited by double quotation marks, represents a
represents a String String object object
The The string concatenation operator string concatenation operator (+) is used to append one (+) is used to append one string to the end of another
string to the end of another
It can also be used to append a number to a string It can also be used to append a number to a string
A string literal cannot be broken across two lines in a A string literal cannot be broken across two lines in a program
program
See Facts.java (page 56) See Facts.java (page 56)
String Concatenation String Concatenation
The plus operator (+) is also used for arithmetic addition The plus operator (+) is also used for arithmetic addition
The function that the + operator performs depends on the The function that the + operator performs depends on the type of the information on which it operates
type of the information on which it operates
If both operands are strings, or if one is a string and one is If both operands are strings, or if one is a string and one is a number, it performs string concatenation
a number, it performs string concatenation
If both operands are numeric, it adds them If both operands are numeric, it adds them
The + operator is evaluated left to right The + operator is evaluated left to right
Parentheses can be used to force the operation order Parentheses can be used to force the operation order
See Addition.java (page 58) See Addition.java (page 58)
8
Escape Sequences Escape Sequences
What if we wanted to print a double quote character? What if we wanted to print a double quote character?
The following line would confuse the compiler because it The following line would confuse the compiler because it would interpret the second quote as the end of the string would interpret the second quote as the end of the string
System.out.println ("I said "Hello" to you.");
An An escape sequence escape sequence is a series of characters that represents is a series of characters that represents a special character
a special character
An escape sequence begins with a backslash character ( An escape sequence begins with a backslash character ( \ \ ), ), which indicates that the character(s) that follow should be which indicates that the character(s) that follow should be
treated in a special way treated in a special way
System.out.println ("I said \"Hello\" to you.");
Escape Sequences Escape Sequences
Some Java escape sequences: Some Java escape sequences:
See Roses.java (page 59) See Roses.java (page 59)
Escape Sequence
\b
\t
\n
\r
\"
\'
\\
Meaning backspace
tab newline carriage return
double quote single quote
backslash
10
Variables Variables
A A variable variable is a name for a location in memory is a name for a location in memory
A variable must be A variable must be declared declared , specifying the variable's name , specifying the variable's name and the type of information that will be held in it
and the type of information that will be held in it
int total;
int count, temp, result;
Multiple variables can be created in one declaration Multiple variables can be created in one declaration data type
data type variable namevariable name
Variables Variables
A variable can be given an initial value in the declaration A variable can be given an initial value in the declaration
When a variable is referenced in a program, its current When a variable is referenced in a program, its current value is used
value is used
See PianoKeys.java (page 60) See PianoKeys.java (page 60)
int sum = 0;
int base = 32, max = 149;
12
Assignment Assignment
An An assignment statement assignment statement changes the value of a variable changes the value of a variable
The assignment operator is the The assignment operator is the = sign = sign
total = 55;
You can only assign a value to a variable that is consistent You can only assign a value to a variable that is consistent with the variable's declared type
with the variable's declared type
The expression on the right is evaluated and the result is The expression on the right is evaluated and the result is stored in the variable on the left
stored in the variable on the left
The value that was in The value that was in total is overwritten total is overwritten
See Geometry.java (page 62) See Geometry.java (page 62)
Constants Constants
A constant is an identifier that is similar to a variable A constant is an identifier that is similar to a variable except that it holds one value for its entire existence except that it holds one value for its entire existence
The compiler will issue an error if you try to change a The compiler will issue an error if you try to change a constant
constant
In Java, we use the In Java, we use the final final modifier to declare a constant modifier to declare a constant
final int MIN_HEIGHT = 69;
Constants: Constants:
• give names to otherwise unclear literal valuesgive names to otherwise unclear literal values
• facilitate changes to the codefacilitate changes to the code
• prevent inadvertent errorsprevent inadvertent errors
Primitive Data Primitive Data
There are exactly eight primitive data types in Java There are exactly eight primitive data types in Java
Four of them represent integers: Four of them represent integers:
• bytebyte, , shortshort, , intint, , longlong
Two of them represent floating point numbers: Two of them represent floating point numbers:
• float, float, doubledouble
One of them represents characters: One of them represents characters:
• charchar
And one of them represents boolean values: And one of them represents boolean values:
• booleanboolean
Numeric Primitive Data Numeric Primitive Data
The difference between the various numeric primitive types The difference between the various numeric primitive types is their size, and therefore the values they can store:
is their size, and therefore the values they can store:
Type byte short int long float double
Storage 8 bits 16 bits 32 bits 64 bits 32 bits 64 bits
Min Value -128
-32,768
-2,147,483,648
< -9 x 1018
+/- 3.4 x 1038 with 7 significant digits +/- 1.7 x 10308 with 15 significant digits
Max Value 127
32,767
2,147,483,647
> 9 x 1018
16
Characters Characters
A A char char variable stores a single character from the variable stores a single character from the Unicode character set
Unicode character set
A A character set character set is an ordered list of characters, and each is an ordered list of characters, and each character corresponds to a unique number
character corresponds to a unique number
The Unicode character set uses sixteen bits per character, The Unicode character set uses sixteen bits per character, allowing for 65,536 unique characters
allowing for 65,536 unique characters
It is an international character set, containing symbols and It is an international character set, containing symbols and characters from many world languages
characters from many world languages
Character literals are delimited by single quotes: Character literals are delimited by single quotes:
'a' 'X' '7' '$' ',' '\n'
'a' 'X' '7' '$' ',' '\n'
Characters Characters
The The ASCII character set ASCII character set is older and smaller than Unicode, is older and smaller than Unicode, but is still quite popular
but is still quite popular
The ASCII characters are a subset of the Unicode The ASCII characters are a subset of the Unicode character set, including:
character set, including:
uppercase letters lowercase letters punctuation
digits
special symbols control characters
A, B, C, … a, b, c, …
period, semi-colon, … 0, 1, 2, …
&, |, \, …
carriage return, tab, ...
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Boolean Boolean
A A boolean boolean value represents a true or false condition value represents a true or false condition
A boolean can also be used to represent any two states, such A boolean can also be used to represent any two states, such as a light bulb being on or off
as a light bulb being on or off
The reserved words The reserved words true true and and false false are the only valid are the only valid values for a boolean type
values for a boolean type
boolean done = false;
boolean done = false;
Arithmetic Expressions Arithmetic Expressions
An An expression expression is a combination of operators and operands is a combination of operators and operands
Arithmetic expressions Arithmetic expressions compute numeric results and make compute numeric results and make use of the arithmetic operators:
use of the arithmetic operators:
Addition +
Subtraction -
Multiplication *
Division /
Remainder %
If either or both operands to an arithmetic operator are If either or both operands to an arithmetic operator are floating point, the result is a floating point
floating point, the result is a floating point
Division and Remainder Division and Remainder
If both operands to the division operator ( If both operands to the division operator ( /) are integers, / ) are integers, the result is an integer (the fractional part is discarded) the result is an integer (the fractional part is discarded)
The remainder operator (%) returns the remainder after The remainder operator (%) returns the remainder after dividing the second operand into the first
dividing the second operand into the first
14 / 3 equals?
8 / 12 equals?
4 0
14 % 3 equals?
8 % 12 equals?
2 8
Operator Precedence Operator Precedence
Operators can be combined into complex expressions Operators can be combined into complex expressions
result = total + count / max - offset;
Operators have a well-defined precedence which Operators have a well-defined precedence which determines the order in which they are evaluated determines the order in which they are evaluated
Multiplication, division, and remainder are evaluated prior Multiplication, division, and remainder are evaluated prior to addition, subtraction, and string concatenation
to addition, subtraction, and string concatenation
Arithmetic operators with the same precedence are Arithmetic operators with the same precedence are evaluated from left to right
evaluated from left to right
Parentheses can always be used to force the evaluation Parentheses can always be used to force the evaluation order
order
Operator Precedence Operator Precedence
What is the order of evaluation in the following What is the order of evaluation in the following expressions?
expressions?
a + b + c + d + e
1 2 3 4
a + b * c - d / e
3 1 4 2
a / (b + c) - d % e
2 1 4 3
a / (b * (c + (d - e)))
4 3 2 1
Assignment Revisited Assignment Revisited
The assignment operator has a lower precedence than the The assignment operator has a lower precedence than the arithmetic operators
arithmetic operators
First the expression on the right hand First the expression on the right hand
side of the = operator is evaluated side of the = operator is evaluated
Then the result is stored in the Then the result is stored in the variable on the left hand side variable on the left hand side
answer = sum / 4 + MAX * lowest;
1
4 3 2
Assignment Revisited Assignment Revisited
The right and left hand sides of an assignment statement The right and left hand sides of an assignment statement can contain the same variable
can contain the same variable
First, one is added to the First, one is added to the
original value of
original value of countcount
Then the result is stored back into
Then the result is stored back into countcount (overwriting the original value)
(overwriting the original value) count = count + 1;
Data Conversions Data Conversions
Sometimes it is convenient to convert data from one type to Sometimes it is convenient to convert data from one type to another
another
For example, we may want to treat an integer as a floating For example, we may want to treat an integer as a floating point value during a computation
point value during a computation
Conversions must be handled carefully to avoid losing Conversions must be handled carefully to avoid losing information
information
Widening conversions Widening conversions are safest because they tend to go are safest because they tend to go from a small data type to a larger one (such as a
from a small data type to a larger one (such as a short short to to an an int int ) )
Narrowing conversions Narrowing conversions can lose information because they can lose information because they
tend to go from a large data type to a smaller one (such as
tend to go from a large data type to a smaller one (such as
an an int to a to a short ) )
Data Conversions Data Conversions
In Java, data conversions can occur in three ways: In Java, data conversions can occur in three ways:
• assignment conversionassignment conversion
• arithmetic promotionarithmetic promotion
• castingcasting
Assignment conversion Assignment conversion occurs when a value of one type is occurs when a value of one type is assigned to a variable of another
assigned to a variable of another
Only widening conversions can happen via assignment Only widening conversions can happen via assignment
Arithmetic promotion Arithmetic promotion happens automatically when happens automatically when
operators in expressions convert their operands
operators in expressions convert their operands
Data Conversions Data Conversions
Casting Casting is the most powerful, and dangerous, technique for is the most powerful, and dangerous, technique for conversion
conversion
Both widening and narrowing conversions can be Both widening and narrowing conversions can be accomplished by explicitly casting a value
accomplished by explicitly casting a value
To cast, the type is put in parentheses in front of the value To cast, the type is put in parentheses in front of the value being converted
being converted
For example, if For example, if total total and and count count are integers, but we are integers, but we want a floating point result when dividing them, we can want a floating point result when dividing them, we can
cast
cast total total : :
result = (float) total / count;
Creating Objects Creating Objects
A variable either holds a primitive type, or it holds a A variable either holds a primitive type, or it holds a reference
reference to an object to an object
A class name can be used as a type to declare an A class name can be used as a type to declare an object object reference variable
reference variable
String title;
No object has been created with this declaration No object has been created with this declaration
An object reference variable holds the address of an object An object reference variable holds the address of an object
The object itself must be created separately The object itself must be created separately
Creating Objects Creating Objects
We use the We use the new new operator to create an object operator to create an object
title = new String ("Java Software Solutions");
This calls the
This calls the StringString constructorconstructor, which is, which is a special method that sets up the object a special method that sets up the object
Creating an object is called Creating an object is called instantiation instantiation
An object is an An object is an instance instance of a particular class of a particular class
Creating Objects Creating Objects
Because strings are so common, we don't have to use the Because strings are so common, we don't have to use the new new operator to create a operator to create a String String object object
title = "Java Software Solutions";
This is special syntax that only works for strings This is special syntax that only works for strings
Once an object has been instantiated, we can use the Once an object has been instantiated, we can use the dot dot operator
operator to invoke its methods to invoke its methods
title.length()
String Methods String Methods
The The String class has several methods that are useful for String class has several methods that are useful for manipulating strings
manipulating strings
Many of the methods Many of the methods return a value return a value , such as an integer or a , such as an integer or a new new String String object object
See the list of See the list of String methods on page 75 and in Appendix String methods on page 75 and in Appendix M M
See StringMutation.java (page 77) See StringMutation.java (page 77)
Class Libraries Class Libraries
A A class library class library is a collection of classes that we can use when is a collection of classes that we can use when developing programs
developing programs
There is a There is a Java standard class library Java standard class library that is part of any that is part of any Java development environment
Java development environment
These classes are not part of the Java language per se, but These classes are not part of the Java language per se, but we rely on them heavily
we rely on them heavily
The The System class and the System class and the String class are part of the String class are part of the Java standard class library
Java standard class library
Other class libraries can be obtained through third party Other class libraries can be obtained through third party vendors, or you can create them yourself
vendors, or you can create them yourself
Packages Packages
The classes of the Java standard class library are organized The classes of the Java standard class library are organized into packages
into packages
Some of the packages in the standard class library are: Some of the packages in the standard class library are:
Package
java.lang java.applet java.awt
javax.swing java.net
java.util
Purpose
General support
Creating applets for the web
Graphics and graphical user interfaces
Additional graphics capabilities and components Network communication
Utilities
The import Declaration The import Declaration
When you want to use a class from a package, you could When you want to use a class from a package, you could use its
use its fully qualified name fully qualified name
java.util.Random
Or you can Or you can import import the class, then just use the class name the class, then just use the class name
import java.util.Random;
To import all classes in a particular package, you can use To import all classes in a particular package, you can use the * wildcard character
the * wildcard character
import java.util.*;
The import Declaration The import Declaration
All classes of the All classes of the java.lang java.lang package are automatically package are automatically imported into all programs
imported into all programs
That's why we didn't have to explicitly import the That's why we didn't have to explicitly import the System System or or String String classes in earlier programs classes in earlier programs
The The Random class is part of the Random class is part of the java.util java.util package package
It provides methods that generate pseudo-random numbers It provides methods that generate pseudo-random numbers
We often have to We often have to scale scale and and shift shift a number into an a number into an appropriate range for a particular purpose
appropriate range for a particular purpose
See RandomNumbers.java (page 82) See RandomNumbers.java (page 82)
Class Methods Class Methods
Some methods can be invoked through the class name, Some methods can be invoked through the class name, instead of through an object of the class
instead of through an object of the class
These methods are called These methods are called class methods class methods or or static methods static methods
The The Math Math class contains many static methods, providing class contains many static methods, providing various mathematical functions, such as absolute value, various mathematical functions, such as absolute value,
trigonometry functions, square root, etc.
trigonometry functions, square root, etc.
temp = Math.cos(90) + Math.sqrt(delta);
The Keyboard Class The Keyboard Class
The The Keyboard class is NOT part of the Java standard Keyboard class is NOT part of the Java standard class library
class library
It is provided by the authors of the textbook to make It is provided by the authors of the textbook to make reading input from the keyboard easy
reading input from the keyboard easy
Details of the Details of the Keyboard Keyboard class are explored in Chapter 8 class are explored in Chapter 8
For now we will simply make use of it For now we will simply make use of it
The The Keyboard class is part of a package called Keyboard class is part of a package called cs1 cs1 , and , and contains several static methods for reading particular types contains several static methods for reading particular types
of data of data
See Echo.java (page 86) See Echo.java (page 86)
See Quadratic.java (page 87) See Quadratic.java (page 87)
Formatting Output Formatting Output
The The NumberFormat NumberFormat class has static methods that return a class has static methods that return a formatter object
formatter object
getCurrencyInstance() getPercentInstance()
Each formatter object has a method called Each formatter object has a method called format format that that returns a string with the specified information in the
returns a string with the specified information in the appropriate format
appropriate format
See Price.java (page 89) See Price.java (page 89)
Formatting Output Formatting Output
The The DecimalFormat class can be used to format a DecimalFormat class can be used to format a floating point value in generic ways
floating point value in generic ways
For example, you can specify that the number be printed to For example, you can specify that the number be printed to three decimal places
three decimal places
The constructor of the The constructor of the DecimalFormat DecimalFormat class takes a class takes a string that represents a pattern for the formatted number string that represents a pattern for the formatted number
See CircleStats.java (page 91) See CircleStats.java (page 91)
Applets Applets
A Java application is a stand-alone program with a A Java application is a stand-alone program with a main main method (like the ones we've seen so far)
method (like the ones we've seen so far)
An An applet applet is a Java program that is intended to transported is a Java program that is intended to transported over the web and executed using a web browser
over the web and executed using a web browser
An applet can also be executed using the appletviewer tool An applet can also be executed using the appletviewer tool of the Java Software Development Kit
of the Java Software Development Kit
An applet doesn't have a An applet doesn't have a main main method method
Instead, there are several special methods that serve Instead, there are several special methods that serve specific purposes
specific purposes
The The paint paint method, for instance, is automatically executed method, for instance, is automatically executed and is used to draw the applets contents
and is used to draw the applets contents
Applets Applets
The paint method accepts a parameter that is an object of The paint method accepts a parameter that is an object of the the Graphics Graphics class class
A A Graphics Graphics object defines a object defines a graphics context graphics context on which we on which we can draw shapes and text
can draw shapes and text
The The Graphics Graphics class has several methods for drawing class has several methods for drawing shapes
shapes
The class that defines the applet The class that defines the applet extends extends the Applet class the Applet class
This makes use of This makes use of inheritance inheritance , an object-oriented concept , an object-oriented concept explored in more detail in Chapter 7
explored in more detail in Chapter 7
See Einstein.java (page 93)
Applets Applets
An applet is embedded into an HTML file using a tag that An applet is embedded into an HTML file using a tag that references the bytecode file of the applet class
references the bytecode file of the applet class
It is actually the bytecode version of the program that is It is actually the bytecode version of the program that is transported across the web
transported across the web
The applet is executed by a Java interpreter that is part of The applet is executed by a Java interpreter that is part of the browser
the browser
Drawing Shapes Drawing Shapes
Let's explore some of the methods of the Let's explore some of the methods of the Graphics class Graphics class that draw shapes in more detail
that draw shapes in more detail
A shape can be filled or unfilled, depending on which A shape can be filled or unfilled, depending on which method is invoked
method is invoked
The method parameters specify coordinates and sizes The method parameters specify coordinates and sizes
Recall from Chapter 1 that the Java coordinate system has Recall from Chapter 1 that the Java coordinate system has the origin in the upper left corner
the origin in the upper left corner
Many shapes with curves, like an oval, are drawn by Many shapes with curves, like an oval, are drawn by specifying its
specifying its bounding rectangle bounding rectangle
An arc can be thought of as a section of an oval An arc can be thought of as a section of an oval
Drawing a Line Drawing a Line
X
Y
10 20
150
45
page.drawLine (10, 20, 150, 45);
page.drawLine (150, 45, 10, 20);
oror
Drawing a Rectangle Drawing a Rectangle
X
Y
page.drawRect (50, 20, 100, 40);
50 20
100
40
Drawing an Oval Drawing an Oval
X
Y
page.drawOval (175, 20, 50, 80);
175 20
50
80
bounding bounding rectangle rectangle
The Color Class The Color Class
A color is defined in a Java program using an object A color is defined in a Java program using an object created from the
created from the Color Color class class
The The Color Color class also contains several static predefined class also contains several static predefined colors
colors
Every graphics context has a current foreground color Every graphics context has a current foreground color
Every drawing surface has a background color Every drawing surface has a background color
