1. Using Classes and Objects
Chapter 3
Using Classes and Objects

2. Conditional Statements
A conditional statement lets us choose which statement will be executed next
Therefore they are sometimes called selection statements
Conditional statements give us the power to make basic decisions
Java's conditional statements are the if statement, the if-else statement, and the switch statement

3. The if Statement
The condition must be a boolean expression that evaluates to T or F.
The boolean expression:
If( grade > 60) System.out.println (“Passed”);
If the grade is greater than 60, then Passed is printed on the screen.
The boolean condition must be within parenthesis.

4. Logic of an if statement
condition
evaluated
false
statement
true

5. The if Statement The if statement has the following syntax:
The condition must be a boolean expression.
It must evaluate to either true or false.
if is a Java
reserved word
if ( condition )
statement;
If the condition is true, the statement is executed.
If it is false, the statement is skipped.

6. greater than or equal to
Boolean Expressions
The condition of an if statement must evaluate to a true or false result
Java has several equality and relational operators:
Operator == !=
<
<=
>
>=
Meaning
equal to
not equal to
less than
less than or equal to
greater than
greater than or equal to
Relational operators always evaluate to a Boolean.

7. Boolean Expressions The boolean expression:
boolean result = (total = = sum)
compares two integer variable, total and sum, for equality. If they contain the same value the result is true.
Notice that this equality operator is not the same as the = operator.

8. Booleans
Relational and equality operators have precedence relationships similar to and less than arithmetic operators.
Be careful when programming to allow for this.
The use of booleans provide basic decision-making capabilities.

9. The if Statement An example of an if statement: if (sum > MAX)
delta = sum - MAX;
System.out.println ("The sum is " + sum);
First, the condition is evaluated. The value of sum
is either greater than the value of MAX, or it is not.
If the condition is true, the assignment statement is executed. If it is not, the assignment statement is skipped.
Either way, the call to println is executed next.

10. Block Statements if (temperature < Threshold) {
Several statements can be grouped together into a block statement. Blocks are delimited by braces.
A block statement can be used wherever a statement is called for in the Java syntax.
In Temperature2.java
if (temperature < Threshold) {
System.out.println (“It is cold”);
System.out.println (“But we will live”);
} // what is the output?

11. Indentation
The statement controlled by the if statement is indented to indicate that relationship
The use of a consistent indentation style makes a program easier to read and understand
Although it makes no difference to the compiler, proper indentation is crucial
"Always code as if the person who ends up maintaining your code will be a violent psychopath who knows where you live."
-- Martin Golding

12. The if Statement What do the following statements do?
if (top >= MAXIMUM)
top = 0;
Sets top to zero if the current value of top is greater than or equal to the value of MAXIMUM
if (total != stock + warehouse)
inventoryError = true;
Sets a flag to true if the value of total is not equal to the sum of stock and warehouse
The precedence of the arithmetic operators is higher than the precedence of the equality and relational operators

13. The if-else Statement
An else clause can be added to an if statement to make it an if-else statement:
if (condition)
statement1;
else
statement2;
If the condition is true, statement1 is executed; if the condition is false, statement2 is executed.
In Temperature3.java, the following statement:

14. If-else Statements If (temperature <= FREEZING_POINT)
System.out.print (“Its freezing in here”);
else
System.out.print (“Above Freezing”);
If the value 90 is entered for temperature what is printed?
(FREEZING_POINT set to 32).
What data type is FREEZING_POINT?
What is the output?

15. Nested if Statements
The statement executed as a result of an if statement or else clause could be another if statement
These are called nested if statements
An else clause is matched to the last unmatched if (no matter what the indentation implies)

16. Nested if Statements
The body of an if statement or else clause can be another if statement
if ( coin == HEADS)
if (choice == receive)
System.out.println(“You won and will receive”)
else
System.out println (“You won the coin toss and will not receive.”);
System.out.println (“You lost the coin toss”)
An else is matched with the closest unmatched if. In this case, the second else matches to the first if. Note that no braces were needed. The whole program is one statement.

17. The if-else Statement
statement1
condition
false
true
statement2

18. Object-Oriented Programming
Java is an object-oriented language.
Programs are made from software components called objects which represent objects in the real world. A student is represented in a program by a class Student
An object contains data and methods - it groups related methods and the data those methods use. The student class contains data about a student.
An object is defined by its class. We create student objects based on the student class.
Multiple objects can be created from the same class
Objects are meant to be re-usable in other programs.

19. Object-Oriented Programming
A class represents a real world concept and an object represents the realization of that concept.
It is description or a blueprint of a category. Objects are specific examples within the category and are described by their class.
Objects
Class
My first car
John's car
Dad's car
Car

20. Objects and Classes Classes can be used to define other classes.
If we had a class vehicle, it may include the definition of other subcategories , such as planes, trains, and automobiles.
It would contain all the generic characteristics of a vehicle with each sub- category defining specific individual characteristics within its own class.

21. Variable types
We have discussed primitive types. Variables can also be reference types.
Reference types are so-called because the value of a reference variable is a reference to (a pointer to the location in memory) of the actual value.
When used in a statement or expression, a reference variable is equal to the address of the memory location where the value you are trying to access is located.

22. Creating Objects
A variable holds either a primitive type 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

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

24. title = "Java Software Solutions";
Creating Objects
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 only works for strings.
Once an object has been instantiated, we can use the dot operator to invoke its methods.
title.length()

25. int count = title.length()
Invoking Methods
We've seen that once an object has been instantiated, we can use the dot operator to invoke its methods
int count = 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

26. 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

27. Assignment Revisited
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
After:

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

29. 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

30. 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

31. Outline
Creating Objects
The String Class

32. The String Class
Recall that 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

33. String Methods
Once a String object has been created, neither its value nor its length can be changed
Thus 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
See the list of String methods on page 119 and in Appendix M

34. 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

35. The String Class
The String class is defined in the java.lang package (and is therefore automatically imported).
Many helpful methods are defined in the String class. They mostly are used to manipulate strings that input as data to a program.
See StringMutation.java (page 120)

36. String Methods: Some example string methods using String name:
name.charAt (int index) - returns the character at location index
name. startsWith (String prefix) - True if name begins with prefix.
name.toLowercase () - returns name converted to lowercase.

37. String Methods:
name.length() - returns the length of the string name.(# of characters)
name.equals (Object obj) - return true if the string name and the parameter obj contain the same characters.
name.toUppercase - returns the string name in uppercase.

38. Strings public static void main(String[] args) {
String str1 = “Seize the day”;
String str2 = “Day of the seize”
String str3 = new string(str1);
System.out.println (“Length of str1: “ + str1.length());
//output: Length of str1 : 13
System.out.println(“Str1 starts with \”Seize\”: “ + str1.startsWith(“Seize”) );
//output: str1 starts with “Seize”: true

39. System. out. println(“The character at position3 in str1: “ + str1
System.out.println(“The character at position3 in str1: “ str1.charAt(3));
//output : The character at position 3 in str1: z
if(str1.equals(str3))
System.out.println(“str1 and str3 contain the same characters”);
//output : str1 and str3 contain the same characters.

40. String methods
String str1 , str2; int i, n = 1; boolean t; char c; str2 = “ok”;
Method Result
str1 = str2.toUppercase(); str1= “OK”; str1 = str1.toLowerCase() str1 = “ok”; str1 = str2.replace(‘k’,’h’); str1 = “Oh”;
// k replaces h t = str2.equalsIgnoreCase(“ok”); t = true;

41. String Methods
String str1 , str2 = “ ok”; int i ; boolean t; char c; Result t = str2.endsWith(“K”); t = false;
t = str2.startsWith(“o”) t = true; c = str.charAt(1); c = ‘k’; i = str2.indexOf(‘j’); i = -1; i = str2.length(); i = 2; str1 = str1.toString(i); str1 = “2”; i =str2.compareTo(“Oh”); i = 1;
// 0 if the strings are equal, -1 if str1 is less than, 1 if str1 is greater than str2

42. String Comparison
When we compare primitive data types, such as two ints, two chars, two doubles, etc., we can use the = = operator.
We can also use the = = operator to compare two objects.
However, when used with an object, the = = operator will only check to see if they are the same objects, not if they hold the same contents.
Two strings could contain the same data and not be the same object.

43. String Comparison
This means that code like the following will compare the strings address but not content:
if ( string1 = = string2 )
{ System.out.println ("Match found"); }
This code will only evaluate to true if string1 and string2 are the same object.
It does not check their contents. If string1 and string2 have the same content but are different objects, (I.e. different locations in memory), the above comparison will not evaluate to true.

44. Comparing Strings
To compare two strings to determine if they contain the same data, we must use the .equals method(). This method is inherited from java.lang.Object.
Here's an example of how to correctly check a String's contents :
if ( string1.equals("abcdef") ) {
System.out.println (“String contains “abcdef"); }

45. Strings - Initialization
You can initialize a string with:
String Quote = “ ‘‘; or String Quote ;
In the first case, the string is initialized to the “empty string” , and in the second it is uninitialized and contains a null value.

46. 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
Other class libraries can be obtained through third party vendors, or you can create them yourself

47. Packages
The classes of the Java standard class library are organized into packages
Some of the packages in the standard class library are:
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

48. 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
import java.util.Scanner;
To import all classes in a particular package, you can use the * wildcard character
import java.util.*;

49. 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

50. 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 (page 126)

51. Random Numbers
The Random class provides methods to simulate a random number generator.
You can give the generator a seed value as in:
Random randomizer1 = new Random(123)
Which will produce the same sequence of numbers each time
Or you can give it no seed and the values are based on the system time. Because the start value is unpredictable, this produces a different sequence each time it is run:
Random randomizer1 = new Random()

52. Random class
The nextInt method of the random class returns a number from the whole spectrum of Integer values.
The nextFloat returns a numer greater than 0 and less than or equal to 1. Usually, we scale and shift the numbers to get a particular range.

53. 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

54. value = Math.cos(90) + Math.sqrt(delta);
The Math Class
The methods of the Math class are static methods (also called class methods)
Static methods can be invoked through the class name – no object of the Math class is needed.
value = Math.cos(90) + Math.sqrt(delta);
See Quadratic.java (page 129)
We discuss static methods further in Chapter 6

55. Wrapper Classes
The java.lang package contains wrapper classes that correspond to each primitive type:
Primitive Type
Wrapper Class
byte
Byte
short
Short
int
Integer
long
Long
float
Float
double
Double
char
Character
boolean
Boolean
void
Void

56. Integer age = new Integer(40);
Wrapper Classes
The following declaration creates an Integer object which represents the integer 40 as an object
Integer age = new Integer(40);
An object of a wrapper class can be used in any situation where a primitive value will not suffice
For example, some objects serve as containers of other objects
Primitive values could not be stored in such containers, but wrapper objects could be

57. num = Integer.parseInt(str);
Wrapper Classes
Wrapper classes also contain static methods that help manage the associated type
For example, the Integer class contains a method to convert an integer stored in a String to an int value:
num = Integer.parseInt(str);
The wrapper classes often contain useful constants as well
For example, the Integer class contains MIN_VALUE and MAX_VALUE which hold the smallest and largest int values

58. Autoboxing
Autoboxing is the automatic conversion of a primitive value to a corresponding wrapper object:
int num = 42;
Integer obj = num;
The assignment creates the appropriate Integer object
The reverse conversion (called unboxing) also occurs automatically as needed:
int num = obj