Chapter 2 Using Objects

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Chapter 2Using Objects
2
Chapter Goals

• Learn about variables
• Type of variables, assignment operators, etc.
• Classes, objects and methods
• Methods
• parameters and return values
• Accessor and mutator methods
• Number types

3
Chapter Goals

• Constructing objects
• Write test programs
• Browse the API documentation
• Objects versus object references

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Types and Variables

• Every value has a type and a name
• Variable declaration examples
• Variables
• Store values
• Can be used in place of the objects they store

String greeting "Hello, World!"PrintStream
printer System.outint luckyNumber 13
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Syntax 2.1 Variable Definition
 typeName variableName value or typeName
variableName Example   String greeting
"Hello, Dave!" Purpose To define a new
variable of a particular type, (optionally)
supply an initial value
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Identifiers

• Identifier
• Name of a variable, method, or class
• Rules for identifiers in Java
• Can be made up of letters, digits, and the
  underscore (_) character
• Cannot start with a digit
• Cannot use other symbols such as ? or
• Spaces are not permitted inside identifiers
• You cannot use reserved words
• Identifiers are case sensitive

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Identifiers

• By convention, variable names start with a
  lowercase letter
• But things like luckyNumber are OK
• This is sometimes called camel case
• By convention, class names start with an
  uppercase letter

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Self Check

• What is the type of the values 0 and "0"?
• Which of the following are legal identifiers?
  
• Define a variable to hold your name. Use camel
  case in the variable name.

Greeting1gvoid101dalmatiansHello,
Worldltgreetinggt
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Answers

• int and String
• Only the first two are legal identifiers

String myName "John Q. Public"
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The Assignment Operator

• Assignment operator is
• Not used as a statement about equality
• a b assigns value of b to a
• Not the same meaning as in math
• Used to change the value of a variableint
  luckyNumber 13 luckyNumber 12

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Uninitialized Variables

• Compile time error

int luckyNumberSystem.out.println(luckyNumber)
   // ERROR - uninitialized variable
Figure 2An Uninitialized Object Variable
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Initialized Variable

• To initialize a variable
• Or, better yet

int luckyNumberluckyNumber 13 System.out.prin
tln(luckyNumber)
int luckyNumber 13 System.out.println(luckyNumb
er)
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Syntax 2.2 Assignment

• Assign a value to a variable

 variableName value Example  luckyNumber
12 Purpose To assign a new value to a
previously defined variable.
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Self Check

• Is 12 12 a valid expression in the Java
  language?
• How do you change the value of the greeting
  variable to "Hello, Nina!"?

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Answers

• No, the left-hand side of the operator must be
  a variable
• Note thatis not the right answerthat
  statement defines a new variable

greeting "Hello, Nina!"
String greeting "Hello, Nina!"
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Objects and Classes

• Object entity that you can manipulate in your
  programs (by calling a method or methods)
• Each object belongs to a class. For example,
  System.out belongs to the class PrintStream

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Methods

• Method Sequence of instructions that accesses
  the data of an object
• You manipulate objects by calling methods
• Class Set of objects with the same behavior
• Class defines methods that can be applied to an
  object

String greeting "Hello"greeting.println() //
Errorgreeting.length() // OK
println not in String class length is in String
class
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Methods

• Public Interface Specifies what you can do with
  the objects of a class
• Private (hidden) implementation describes how
  these actions are carried out

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Representation of 2 String Objects
Figure 4A Representation of Two String Objects
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Example of a String Method

• length counts the number of characters in a
  string

String greeting "Hello, World!" int n
greeting.length() // sets n to 13
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String Methods

• toUpperCase creates another String object
  that contains the characters of the original
  string, with lowercase letters converted to
  uppercase

String river "Mississippi" String bigRiver
river.toUpperCase() // sets bigRiver to
"MISSISSIPPI"
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String Methods

• When applying a method to an object, method must
  be defined in the appropriate class
• length is not defined in PrintStream class to
  which System.out belongs

System.out.length() // This method call is an
error
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Self Check

• How can you compute the length of the string
  "Mississippi"?
• How can you print out the uppercase version of
  "Hello, World!"?
• Is it legal to call river.println()? Why or why
  not?

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Answers

• It is not legal. The variable river has type
  String and the println method is not a method of
  the String class.

river.length() or "Mississippi".length()
System.out.println(greeting.toUpperCase())
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Implicit and Explicit Parameters

• Parameter (explicit parameter) is an input to a
  method. Not all methods have explicit
  parameters.
• The object on which a method is invoked is an
  implicit parameter

System.out.println(greeting) greeting.length()
// has no explicit parameter
System.out.println(greeting)
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Implicit and Explicit Parameters
Figure 5Passing a parameter to the println
method
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Return Values

• A return value is a result that the method has
  computed for use by the code that called it

int n greeting.length() // return value stored
in n
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Return Values
Figure 6Invoking the length Method on a String
Object
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Passing Return Values

• You can also use the return value as a parameter
  of another method
• Not all methods return values. For example,

System.out.println(greeting.length())
println
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Passing Return Values
Figure 7Passing the Result of a Method Call to
Another Method
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A More Complex Call

• replace method carries out a search-and-
  replace operation
• This method call has
• one implicit parameter string "Mississippi"
• two explicit parameters "issipp" and "our"
• a return value the string "Missouri"

river.replace("issipp", "our") // constructs a
new string ("Missouri")
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A More Complex Call
Figure 8Calling the replace Method
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Method Definitions

• Method definition specifies types of explicit
  parameters and return value
• Type of implicit parameter is current class, so
  not mentioned in method definition

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Method Definitions

• Example Class String defines
• Search for target string and replace with
  replacement at each occurrence of string

public int length() // return type int //
no explicit parameterpublic String
replace(String target, String replacement) //
return type String // two explicit
parameters of type String
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Method Definitions

• If method returns no value, the return type is
  declared as void
• A method name is overloaded if a class has more
  than one method with the same name (but different
  parameter types)

public void println(String output) // in class
PrintStream
public void println(String output)public void
println(int output)
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Self Check

• What are the implicit parameters, explicit
  parameters, and return values in the method call
  river.length()?
• What is the result of the call
  river.replace("p", "s")?
• What is the result of the call greeting.replace("W
  orld","Dave").length()?
• How is the toUpperCase method defined in the
  String class?

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Answers

• The implicit parameter is river. There is no
  explicit parameter and return value is 11
• "Missississi"
• 12
• As public String toUpperCase() with no
  explicit parameter and return type String

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Number Types

• Integers short, int, long
• For example
• 13
• Floating point numbers float, double
• For example1.30.00013

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Number Types

• When a floating-point number is multiplied or
  divided by 10, only the position of the decimal
  point changes it "floats". This representation
  is related to the scientific notation 1.3 10-4.
  
• Numbers are not objects numbers types are
  primitive types

1.3E-4       // 1.3 ? 10-4 written in Java
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Arithmetic Operations

• Math operators - ?
• As in math, the ? operator binds more strongly
  than the or - operator

10 nn - 110 n       // 10 x n
x y 2    // means the sum of x and y 2(x
y) 2  // multiplies the sum of x and y with 2
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Self Check

• Which number type would you use for storing the
  area of a circle?
• Why is the expression 13.println() an error?
• Write an expression to compute the average of the
  values x and y.

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Answers

• double
• An int is not an object, and you cannot call a
  method on it

(x y) 0.5
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Rectangular Shapes and Rectangle Objects

• Objects of type Rectangle describe rectangular
  shapes

Figure 9Rectangular Shapes
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Rectangular Shapes and Rectangle Objects

• A Rectangle object is not a shape
• It is an object that contains a set of numbers
  that describe a rectangle

Figure 10Rectangular Objects
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Constructing Objects

• Details
• The new operator makes a Rectangle object
• It uses the parameters (in this example, 5, 10,
  20, and 30) to initialize the data of the object
• It returns the object
• Usually the output of the new operator is stored
  in a variable

new Rectangle(5, 10, 20, 30)
Rectangle box new Rectangle(5, 10, 20, 30)
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Constructing Objects

• The process of creating a new object is called
  construction (since it uses a constructor method)
• The four values 5, 10, 20, and 30 are called the
  construction parameters
• Some classes let you construct objects in
  multiple ways

new Rectangle() // constructs a rectangle
with its top-left corner // at the origin (0,
0), width 0, and height 0
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Syntax 2.3 Object Construction
new ClassName(parameters) Example new
Rectangle(5, 10, 20, 30)new Rectangle() Purpose
To construct a new object, initialize it with
the construction parameters, and return a
reference to the constructed object
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Self Check

• How do you construct a square with center (100,
  100) and side length 20?
• What does the following statement print?

System.out.println(new Rectangle().getWidth())
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Answers

• 0

new Rectangle(90, 90, 20, 20)
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Accessor and Mutator Methods

• An accessor method does not change the state of
  its implicit parameter
• A mutator method changes the state of its
  implicit parameter

double width box.getWidth()
box.translate(15, 25)
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Accessor and Mutator Methods
Figure 11Using the translate Method to Move a
Rectangle
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Self Check

• Is the toUpperCase method of the String class an
  accessor or a mutator?
• Which call to translate is needed to move the box
  rectangle so that its top-left corner is the
  origin (0, 0)? Recall that

Rectangle box new Rectangle(5, 10, 20, 30)
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Answers

• An accessor since it does not modify the original
  string (it returns a new string with uppercase
  letters)
• box.translate(-5, -10) (provided the box
  rectangle has not been changed)

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Implementing a Test Program

• Create a new class
• Supply a main method
• Inside the main method, construct one or more
  objects
• Apply methods to the objects
• Display the results of the method calls

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Importing Packages

• Must include appropriate packages
• Java classes are grouped into packages
• Import library classes by specifying the package
  and class name, for example,
• You do not need to import classes in the
  java.lang package such as String and System

import java.awt.Rectangle
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Importing a Class from a Package
 import packageName.ClassName Example  import
java.awt.Rectangle Purpose To import a class
from a package for use in a program.
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File MoveTester.java
01 import java.awt.Rectangle 02 03 public
class MoveTester 04 05 public static void
main(String args) 06 07 Rectangle
box new Rectangle(5, 10, 20, 30) 08 09
// Move the rectangle 10
box.translate(15, 25) 11 12 // Print
information about the moved rectangle 13
System.out.println("After moving, the top-left
corner is") 14
System.out.println(box.getX()) 15
System.out.println(box.getY()) 16 17
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Self Check

• The Random class is defined in the java.util
  package. What do you need to do in order to use
  that class in your program?
• Why doesn't the MoveTester program print the
  width and height of the rectangle?

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Answers

• Add the statement import java.util.Random
  at the top of your program
• It is not necessary since the translate method
  does not modify the shape (width, height) of the
  rectangle

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Testing Classes in an IDE
Figure 12Testing a Method Call in Bluej
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The API Documentation

• API Application Programming Interface
• Lists classes and methods in the Java library
• http//java.sun.com/javase/6/docs/api/index.html

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The API Documentation of the Standard Java
Library
Figure 13The API Documentation of the Standard
Java Library
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The API Documentation for the Rectangle Class
Figure 14The API Documentation of the Rectangle
Class
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Javadoc Method Summary
Figure 15The Method Summary for the Rectangle
Class
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translate Method Documentation
Figure 16The API Documentation of the translate
Method
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Self Check

• Look at the API documentation of the String
  class. Which method would you use to obtain the
  string "hello, world!" from the string "Hello,
  World!" ?
• In the API documentation of the String class,
  look at the description of the trim method. What
  is the result of applying trim to the string "
  Hello, Space ! " (note the spaces in the string)?

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Answers

• toLowerCase
• "Hello, Space !" (only the leading and
  trailing spaces are trimmed)

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Object References

• References describe the location of objects
• The new operator returns a reference to a new
  object
• Multiple object variables can refer to the same
  object

Rectangle box new Rectangle()
Rectangle box new Rectangle(5, 10, 20,
30)Rectangle box2 boxbox2.translate(15, 25)
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Object References

• Primitive type variables are not object variables
  
• So, for example, cannot call a method on a
  primitive variable

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Object Variables
Figure 17 An Object Variable containing an
Object Reference
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Object Variables
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Object Variables and Number Variables
Figure 19 A Number Variable Stores a Number
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Copying Numbers
int luckyNumber 13int luckyNumber2
luckyNumberluckyNumber2 12
Figure 20 Copying Numbers
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Copying Object References

Rectangle box new Rectangle(5, 10, 20,
30)Rectangle box2 boxbox2.translate(15, 25)
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Copying Object References
Figure 21 Copying Object References
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Copying Object References

• Why are objects and primitive variables (such as
  numbers) treated differently in Java?

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Self Check

• Assuming that greeting and greeting2 are String
  objects, what is the effect of the assignment
  greeting2 greeting?
• After calling greeting2.toUpperCase(), what are
  the contents of greeting and greeting2?

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Answers

• Now greeting and greeting2 both refer to the same
  String object.
• Both variables still refer to the same string,
  and the string has not been modified. Recall that
  the toUpperCase method constructs a new string
  that contains uppercase characters, leaving the
  original string unchanged (tricky)

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Mainframes When Dinosaurs Ruled the Earth
Figure 22 A Mainframe Computer