1. Chapter 3 Introduction to Classes, Objects, Methods, and Strings
Java™ How to Program, 10/e
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3. 3.1 Introduction Topics covered in this chapter Classes Objects
Methods
Parameters
double primitive data type for representing floating points numbers
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4. 3.2 Instance Variables, set Methods and get Methods
Each class you create becomes a new data type that can be used to declare variables and create objects.
You can code new classes as needed; this is one reason Java is known as an extensible language.
Create a new class (Account).
Use it to create an object.
ch03\fig03_01_02\Account.java
ch03\fig03_01_02\AccountTest.java
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5. 3.2.1 Account Class with an Instance Variable, a set Method and a get Method - I
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6. 3.2.1 Account Class with an Instance Variable, a set Method and a get Method - II
Class Declaration
Each class declaration that begins with the access modifier public must be stored in a file that has the same name as the class and ends with the .java filename extension.
Every class declaration contains keyword class followed immediately by the class’s name.
Keyword public is an access modifier.
Indicates that the class is “available to the public” – visible to any Java code.
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7. 3.2.1 Account Class with an Instance Variable, a set Method and a get Method - III
Identifiers and Camel Case Naming
Class, method and variable names are identifiers.
By Java naming conventions all use camel case names.
Class names begin with an uppercase letter, and method and variable names begin with a lowercase letter.
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8. 3.2.1 Account Class with an Instance Variable, a set Method and a get Method - IV
Instance Variable name
An object has attributes that are implemented as instance variables and carried with it throughout its lifetime.
Attributes are also called fields, data fields, data members of the class
Instance variables exist before methods are called on an object, while the methods are executing and after the methods complete execution.
A class normally contains one or more methods that manipulate the instance variables that belong to particular objects of the class.
Instance variables are declared inside a class declaration but outside the bodies of the class’s method declarations.
Each object (instance) of the class has its own copy of each of the class’s instance variables.
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10. 3.2.1 Account Class with an Instance Variable, a set Method and a get Method - V
Access Modifiers public and private
Most instance variable declarations are preceded with the keyword private, which is an access modifier.
Variables or methods declared with access modifier private are accessible only to methods of the class in which they’re declared.
private modifier prevents instance variables from being modified accidentally by a class in another part of the program.
setXXX (mutator) and getXXX (accessor) methods may be coded to access instance variables.
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11. 3.1.1 Account Class with an Instance Variable, a set Method and a get Method - VI
setName Method of Class Account
Parentheses after the method name are required – any identifier without parentheses is either a variable, a constant, or a class name.
Parameters are declared in a comma separated parameter list, which is located inside the parentheses that follow the method name in the method declaration.
Multiple parameters are separated by commas.
Each parameter must specify a type followed by a variable name.
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12. 3.1.1 Account Class with an Instance Variable, a set Method and a get Method - VII
Empty parentheses after the method name indicate that the method does not require additional information to perform its task.
Together, everything in the first line of the method is typically called the method header, signature, or prototype.
Every method’s body is delimited by left and right braces.
The method body contains one or more statements that perform the method’s task.
OOP rule: one method – one task!!!
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13. 3.1.1 Account Class with an Instance Variable, a set Method and a get Method - VIII
Parameters Are Local Variables
Variables declared in the body of a particular method are local variables and can be used only in that method.
Memory space for local variables is allocated only during the execution of the method.
When a method terminates, the values of its local variables are lost as memory is released (de-allocated).
Method’s parameters alsp are local variables of the method.
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14. 3.1.1 Account Class with an Instance Variable, a set Method and a get Method - IX
setName Method Body
Every method’s body is delimited by left and right braces ({ and }).
Each method’s body contains one or more statements that perform the method’s task(s).
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16. 3.2.1 Account Class with an Instance Variable, a set Method and a get Method - X
getName Method of Class Account
Method name follows the return type in the method declaration.
The method’s return type specifies the type of data returned to a method’s caller.
Keyword void indicates that a method will perform a task but will not return (i.e., give back) any information to its calling method when it completes its task.
When a method that specifies a return type other than void is called and completes its task, the method must return a result to its calling method.
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17. 3.2.1 Account Class with an Instance Variable, a set Method and a get Method - XI
The return statement passes a value from a called method back to its caller.
Classes often provide public methods to allow the class’s clients to set or get private instance variables.
The names of these methods need not begin with set or get, but this naming convention is recommended.
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18. 3.2.1 Account Class with an Instance Variable, a set Method and a get Method - XII
Class methods can (and should) use class attributes even though these attributes are not declared in any of the methods.
Can use an instance variable of the class in each of the class methods.
Exception to this are static methods.
The order in which methods are declared in a class does not determine when they are called at execution time.
One method of a class can call another method of the same class by using just the method name.
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19. 3.2.1 Account Class with an Instance Variable, a set Method and a get Method - XIII
setXXX and getXXX methods
A class’s private fields can be manipulated only by the class’s methods.
The code that uses an object must call the class’s public methods to manipulate the private fields of an object of the class.
Classes may provide public methods to allow clients to set (i.e., assign values to) or get (i.e., obtain the values of) private instance variables.
Convention - the names of these methods should begin with set or get followed by the variable name with the first letter capitalized,.
setXXX methods are usually called modifier methods.
getXXX methods are usually called accessor methods.
Provide only if required for the code usage.
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20. 3.2.2 AccountTest Class That Creates and Uses an Object of Class Account - I
Driver Class AccountTest
A class that creates an object of another class, then calls the object’s methods, is a driver class.
Class declarations in Java include:
Access modifier.
Keyword class.
Class name.
A matching pair of left and right braces.
Code inside the class body.
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23. 3.2.2 AccountTest Class That Creates and Uses an Object of Class Account - II
Scanner Object for Receiving Input from the User
Scanner method nextLine reads characters until a newline character is encountered, then returns the characters as a String.
Scanner method next reads characters until any white space character is encountered, then returns the characters as a String.
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24. 3.2.2 AccountTest Class That Creates and Uses an Object of Class Account - III
Instantiating an Object: Keyword new and Constructors
A class instance creation expression begins with keyword new and creates a new object.
A constructor is codrd similar to a method but is called by the new operator to initialize an object’s instance variables at the time the object is created.
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25. 3.2.2 AccountTest Class That Creates and Uses an Object of Class Account - IV
Calling Class Account’s getName Method
To call a method of an object, follow the object name by a dot separator, the method name and a set of parentheses containing the method’s arguments.
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27. 3.2.2 AccountTest Class That Creates and Uses an Object of Class Account - V
null is the Default Initial Value for String Variables
Method’s local variables are not automatically initialized.
Every instance variable has a default initial value provided by Java compiler when you do not specify the instance variable’s initial value.
The default value for an instance variable of type String (and all reference types) is null.
Class fields are not required to be explicitly initialized before they are used in a program, unless they must be initialized to values other than their default values.
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28. 3.2.2 AccountTest Class That Creates and Uses an Object of Class Account - VI
Calling Class Account’s setName Method
A method call supplies values known as arguments for each of the method’s parameters.
Each argument’s value is assigned to the corresponding parameter in the method header.
The number of arguments in a method call must match the number of parameters in the method declaration’s parameter list.
Arguments and parameters are paired left to right.
The argument types in the method call must be consistent (assignment compatible) with the types of the corresponding parameters in the method’s declaration.
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29. 3.2.2 AccountTest Class That Creates and Uses an Object of Class Account - VII
An argument is a value we pass to a method.
A parameter is a placeholder in the called method to hold the value of the passed argument.
A parameter is also called a formal parameter and an argument - an actual parameter.
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30. Intro to OOP with Java, C. Thomas Wu
Method Declaration
<modifier> <return type> <method name> ( <parameters> ){
<statements> }
Modifier
Return Type
Method Name
Parameters
public void displayMessage ( String courseName ) {
System.out.printf( "Welcome to the grade book for\n%s!\n", courseName ); }
Statements
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31. 3.2.3 Compiling and Executing an Application with Multiple Classes
The javac command can compile multiple classes at once.
Simply list the source code filenames after the command with each filename separated by a space from the next.
If the directory containing the app includes only one application’s files, you can compile all of its classes with the command javac *.java.
The asterisk (*) in *.java indicates that all files in the current directory ending with the filename extension “.java” should be compiled.
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32. 3.2.4 Account Class UML Diagram with an Instance Variable and set and get Methods - I
Top Compartment
In the UML, each class is modeled in a class diagram as a rectangle with three compartments. The top one contains the class’s name centered horizontally in boldface.
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34. 3.2.4 Account Class UML Diagram with an Instance Variable and set and get Methods - II
Middle Compartment
The middle compartment contains the class’s attributes, which correspond to instance variables in Java.
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35. 3.2.4 Account Class UML Diagram with an Instance Variable and set and get Methods - III
Bottom Compartment
The bottom compartment contains the class’s operations, which correspond to methods and constructors in Java.
The UML represents instance variables as an attribute name, followed by a colon and the type.
Private attributes are preceded by a minus sign (–) in the UML.
The UML models operations by listing the operation name followed by a set of parentheses.
A plus sign (+) in front of the operation name indicates that the operation is a public one in the UML (i.e., a public method in Java).
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36. 3.2.4 Account Class UML Diagram with an Instance Variable and set and get Methods - IV
Return Types
The UML indicates an operation’s return type by placing a colon and the return type after the parentheses following the operation name.
UML class diagrams do not specify return types for operations that do not return values.
Declaring instance variables private is known as data hiding or information hiding.
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37. 3.2.4 Account Class UML Diagram with an Instance Variable and set and get Methods - V
Parameters
The UML models a parameter of an operation by listing the parameter name, followed by a colon and the parameter type between the parentheses after the operation name
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38. 3.2.5 Additional Notes on Class AccountTest - I
static Method main
The main method is called automatically by the Java Virtual Machine (JVM) when you execute an application.
You must call methods other than main explicitly to tell them to perform their tasks.
A key part of enabling the JVM to locate and call method main to begin the app’s execution is the static keyword, which indicates that main is a static method that can be called without first creating an object of the class in which the method is declared.
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39. 3.2.5 Additional Notes on Class AccountTest - II
A static method (such as main) is special
It can be called without first creating an object of the class in which the method is declared.
Typically, you cannot call a method that belongs to another class until you create an object of that class.
Declare a variable of the class type
Each new class you create becomes a new Java data type that can be used to declare variables and create objects.
You declare new class types as needed; this is one reason why Java is known as an extensible language.
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40. 3.2.5 Additional Notes on Class AccountTest - III
Any Java class can contain a main method.
The JVM invokes the main method only in the class used to execute the application.
If multiple classes contain main, then one that is invoked is the one in the class named in the java command call to the JVM.
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41. 3.2.5 Additional Notes on Class AccountTest - IV
Notes on import Declarations
Most classes you’ll use in Java programs must be imported explicitly.
There’s a special relationship between classes that are compiled in the same directory.
By default, such classes are considered to be in the same package.
Classes in the same package are implicitly imported into the source code files of other classes in that package.
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42. 3.2.5 Additional Notes on Class AccountTest - V
An import declaration is not required when one class in a package uses another in the same package.
An import declaration is not required if you always refer to a class with its fully qualified class name, which includes its package name and class name.
Classes System and String are in package java.lang
Implicitly imported into every Java program.
Can use the java.lang classes without explicitly importing them.
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44. 3.2.6 Software Engineering with private Instance Variables and public set and get Methods - I
Declaring instance variables private is known as encapsulation, data hiding or information hiding.
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46. 3.2.7 Software Engineering with private Instance Variables and public set and get Methods - II
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47. 3.3 Primitive Types vs. Reference Types - I
Java data types are divided into two categories: primitive types and reference types.
The 8 Java primitive types are boolean, byte, char, short, int, long, float and double.
All other types are reference types, so classes, which specify the types of objects, are reference types.
A primitive type variable can store exactly one value of its declared type at a time.
Primitive type instance variables are initialized by default.
Variables of types byte, char, short, int, long, float and double are initialized to 0.
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48. 3.3 Primitive Types vs. Reference Types - II
Variables of type boolean are initialized to false.
Reference type variables (called references) store the location of an object in the computer’s memory.
Such variables refer to objects in the program.
The object that’s referenced may contain many instance variables and methods.
Reference type instance variables are initialized by default to the value null.
A reference to an object is required to invoke an object’s methods.
A primitive type variable does not refer to an object and therefore cannot be used to invoke a method.
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49. 3.4 Account Class: Initializing Objects with Constructors
You can and should specify your own initial values for all primitive or reference instance variables by assigning the variable a value in its declaration (or in the class’s constructor).
Each class you declare can optionally provide a constructor with parameters that can be used to initialize an object of a class when the object is created.
Java requires a single constructor call for every object that’s created.
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50. 3.4.1 Declaring an Account Constructor for Custom Object Initialization - I
When an object of a class is created, its instance variables are initialized by default.
Each class can and should include one or more constructors that properly initialize an object of a class when the object is created.
Operator new requests memory from the system to store an object, then calls the corresponding class’s constructor to initialize the object.
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51. 3.4.1 Declaring an Account Constructor for Custom Object Initialization - II
Constructor is called once and only once for every object created.
A constructor must have the same name as the class.
ch03\fig03_05_06\Account.java
ch03\fig03_01_06\AccountTest.java
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53. 3.4.1 Declaring an Account Constructor for Custom Object Initialization - II
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54. 3.4.2 Class AccountTest: Initializing Account Objects When They’re Created - I
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55. Instance variables are initialized to their default values.
Class AccountTest: Initializing Account Objects When They’re Created - II
The compiler inserts a default (no-argument) constructor with no parameters for any class that does not explicitly include a constructor.
Instance variables are initialized to their default values.
Always provide your own constructors to specify meaningful instance field initializations for objects of your class.
A constructor’s parameter list specifies the data it requires to perform its task – instance fields’ initialization.
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56. 3.4.2 Class AccountTest: Initializing Account Objects When They’re Created - III
Normally, constructors are declared public.
Constructors Cannot Return Values
Constructor declaration can specify parameters but not return types.
Default Constructor
If a class does not define constructors, the compiler provides a default constructor with no parameters, and the class’s instance variables are initialized to their default values.
There’s No Default Constructor in a Class That Declares a Constructor
If you declare a constructor for a class, the compiler will not create a default constructor for that class.
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58. 3.4.2 Class AccountTest: Initializing Account Objects When They’re Created - IV
Adding the Constructor to Class Account’s UML Class Diagram
The UML models constructors in the third compartment of a class diagram.
To distinguish a constructor from a class’s operations, the UML places the word “constructor” between guillemets (« and ») before the constructor’s name.
List constructors before other operations in the third compartment.
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60. Constructor Basics - I
Always executed by JVM when a new instance of the class is created.
public <class name> ( <parameters> ){ <statements> }
Modifier
Class Name
Parameters
public GradeBook ( String name ) {
courseName = name; // initializes courseName
} // end constructor
Statements
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61. Constructor Basics - II
The name of a constructor must be the same as the name of the class.
If no constructor is defined for a class, then the Java compiler will include a default (no-argument) constructor.
The default constructor will have the following form:
public <class name> ( ){ }
public GradeBook( ) {
It is recommended to provide an explicit default constructor to prevent potential inheritance related problems.
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62. Constructor Basics - III
A constructor does not have a return type.
It is possible to create multiple (overloaded) constructors for a class, as long as the constructors have different signatures which means either:
A different number of parameters, or
Different data types for the parameters if the number of parameters is the same.
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63. Template for Class Definition
Intro to OOP with Java, C. Thomas Wu
Template for Class Definition
class { }
Import Statements
Class Comments
Class Name
Data Members
Methods
(incl. Constructors)
This is the template we use when creating programmer-defined classes.
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64. 3.5 Account Class with a Balance; Floating Point Numbers and Type double - I
A floating point number is a number with a decimal point.
Java provides two primitive types for storing floating point numbers in memory: float and double.
Variables of type float represent single precision floating point numbers and have up to seven significant digits.
Variables of type double represent double precision floating point numbers.
double variables require twice as much memory as float variables and provide 15 significant digits - more than double the precision of float variables.
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65. 3.5 Account Class with a Balance; Floating Point Numbers and Type double - II
Floating point literals (such as 7.33 and ) are of type double by default.
ch03\fig03_08_09\Account.java
ch03\fig03_08_09\AccountTest.java
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66. 3.5.1 Account Class with a balance Instance Variable of Type double
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69. 3.5.2 AccountTest Class to Use Class Account - I
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73. 3.5.2 AccountTest Class to Use Class Account - II
Scanner method nextDouble returns a double value entered by the user.
System.out.printf
Format specifier %.2f
%f is used to output values of type float or double.
.2 represents the number of decimal places (2) to output to the right of the decimal point (also known as the number’s display precision).
Any floating point value output with %.2f will be rounded to the hundredths position.
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74. 3.5.2 AccountTest Class to Use Class Account- III
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75. 3.5.2 AccountTest Class to Use Class Account - IV
The default value for an instance variable of type double is 0.0, and the default value for an instance variable of type int is 0.
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76. 3.5.2 AccountTest Class to Use Class Account - V
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77. 3.5.2 AccountTest Class to Use Class Account - VI
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78. Java Numeric Data Type Precisions
The six Java numeric data types differ in the precision of values they can store in memory.
This is for information only. You do not have to memorize this table. Just get some general idea on the range of values that can be represented by the six numerical data types.
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79. Floating Point Numbers in Programs
Never use float in your code – double should be used to provide necessary precision.
Don’t use floating point numbers as loop control variables.
Never compare floating point numbers for exact equality
1.0/ / /3 != 1.0
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80. 3.6 GUI and Graphics Case Study: Using Dialog Boxes - I
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81. 3.6 GUI and Graphics Case Study: Using Dialog Boxes - II
Many applications use windows or dialog boxes (also called dialogs) to display output.
Typically, dialog boxes are windows in which programs display important messages to users.
Class JOptionPane provides prebuilt dialog boxes that enable programs to display windows containing messages - such windows are called message dialogs.
ch03\fig03_12\Dialog1.java
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83. 3.6 GUI and Graphics Case Study: Using Dialog Boxes - III
Package javax.swing contains many classes that help you create graphical user interfaces (GUIs).
GUI components facilitate data entry by a program’s user and presentation of outputs to the user.
JOptionPane method showMessageDialog displays a dialog box containing a message.
Requires two arguments.
The first helps the Java application determine where to position the dialog box.
If the first argument is null, the dialog box is displayed at the center of your screen.
The second argument is the String to display in the dialog box.
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84. 3.6 GUI and Graphics Case Study: Using Dialog Boxes - IV
JOptionPane method showMessageDialog is a static method.
Such methods often define frequently used tasks.
Typically called by using method’s class name followed by a dot (.) and the method name, as in
ClassName.methodName(arguments);
Notice that you do not need to create an object of class JOptionPane to use its static method showMessageDialog.
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85. 3.6 GUI and Graphics Case Study: Using Dialog Boxes - V
An input dialog allows the user to enter data into a program.
JOptionPane method showInputDialog displays an input dialog
Contains a prompt and a field (known as a text field) in which the user can enter text.
Method showInputDialog returns a String containing the characters typed by the user.
If you press the dialog’s Cancel button or press the Esc key, the method returns null reference.
ch03\Fig03_13\NameDialog.java
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86. 3.6 GUI and Graphics Case Study: Using Dialog Boxes - VI
static String method format returns a formatted String.
Method format works like method System.out.printf, except that format returns the formatted String rather than displaying it in a command window.
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