Java căn bản - Chapter2

Chapter 2 Getting Started with Java

Objectives After you have read and studied this chapter, you should be able to Identify the basic components of Java programs Write simple Java programs Describe the difference between object declaration and creation Describe the process of creating and running Java programs Use the Date, SimpleDateFormat, String, and JOptionPane standard classes Develop Java programs, using the incremental development approach

The First Java Program The fundamental OOP concept illustrated by the program: An object-oriented program uses objects. This program displays a window on the screen. The size of the window is set to 300 pixels wide and 200 pixels high. Its title is set to My First Java Program .

Program Ch2Sample1 import javax.swing.*; class Ch2Sample1 { public static void main(String[ ] args) { JFrame myWindow; myWindow = new JFrame( ); myWindow.setSize(300, 200); myWindow.setTitle( “My First Java Program” ); myWindow.setVisible( true ); } } Declare a name Create an object Use an object

Program Diagram for Ch2Sample1 setSize(300, 200) setTitle(“My First Java Program”) myWindow : JFrame Ch2Sample1 setVisible(true)

Dependency Relationship Instead of drawing all messages, we summarize it by showing only the dependency relationship. The diagram shows that Ch2Sample1 “depends” on the service provided by myWindow . myWindow : JFrame Ch2Sample1

Object Declaration JFrame myWindow; Account customer; Student jan, jim, jon; Vehicle car1, car2; More Examples Object Name One object is declared here. Class Name This class must be defined before this declaration can be stated.

Object Creation myWindow = new JFrame ( ) ; customer = new Customer( ); jon = new Student(“John Java”); car1 = new Vehicle( ); More Examples Object Name Name of the object we are creating here. Class Name An instance of this class is created. Argument No arguments are used here.

Declaration vs. Creation Customer customer; customer = new Customer( ); 1. The identifier customer is declared and space is allocated in memory. 2. A Customer object is created and the identifier customer is set to refer to it. 1 2 customer 2 : Customer customer 1

State-of-Memory vs. Program customer : Customer State-of-Memory Notation customer : Customer Program Diagram Notation

Name vs. Objects Customer customer; customer = new Customer( ); customer = new Customer( ); Created with the first new . Created with the second new . Reference to the first Customer object is lost. customer : Customer : Customer

Sending a Message myWindow . setVisible ( true ) ; account.deposit( 200.0 ); student.setName(“john”); car1.startEngine( ); More Examples Object Name Name of the object to which we are sending a message. Method Name The name of the message we are sending. Argument The argument we are passing with the message.

Program Components A Java program is composed of comments, import statements, and class declarations.

Program Component: Comment /* Chapter 2 Sample Program: Displaying a Window File: Ch2Sample2.java */ import javax.swing.*; class Ch2Sample1 { public static void main(String[ ] args) { JFrame myWindow; myWindow = new JFrame( ); myWindow.setSize(300, 200); myWindow.setTitle( “My First Java Program” ); myWindow.setVisible( true ); } } Comment

Matching Comment Markers /* This is a comment on one line */ /* Comment number 1 */ /* Comment number 2 */ /* /* /* This is a comment */ */ Error: No matching beginning marker. These are part of the comment.

Three Types of Comments /* This is a comment with three lines of text. */ Multiline Comment Single line Comments // This is a comment // This is another comment // This is a third comment /** * This class provides basic clock functions. In addition * to reading the current time and today’s date, you can * use this class for stopwatch functions. */ javadoc Comments

Import Statement /* Chapter 2 Sample Program: Displaying a Window File: Ch2Sample2.java */ import javax.swing.*; class Ch2Sample1 { public static void main(String[ ] args) { JFrame myWindow; myWindow = new JFrame( ); myWindow.setSize(300, 200); myWindow.setTitle( “My First Java Program” ); myWindow.setVisible( true ); } } Import Statement

Import Statement Syntax and Semantics <package name> . <class name> ; e.g. dorm . Resident; import javax.swing.JFrame; import java.util.*; import com.drcaffeine.simplegui.*; More Examples Class Name The name of the class we want to import. Use asterisks to import all classes. Package Name Name of the package that contains the classes we want to use.

Class Declaration /* Chapter 2 Sample Program: Displaying a Window File: Ch2Sample2.java */ import javax.swing.*; class Ch2Sample1 { public static void main(String[ ] args) { JFrame myWindow; myWindow = new JFrame( ); myWindow.setSize(300, 200); myWindow.setTitle( “My First Java Program” ); myWindow.setVisible( true ); } } Class Declaration

Method Declaration /* Chapter 2 Sample Program: Displaying a Window File: Ch2Sample2.java */ import javax.swing.*; class Ch2Sample1 { public static void main(String[ ] args) { JFrame myWindow; myWindow = new JFrame( ); myWindow.setSize(300, 200); myWindow.setTitle( “My First Java Program” ); myWindow.setVisible( true ); } } Method Declaration

Method Declaration Elements public static void main( String[ ] args ){ JFrame myWindow; myWindow = new JFrame( ); myWindow.setSize(300, 200); myWindow.setTitle( “My First Java Program” ); myWindow.setVisible( true ); } Method Body Modifier Modifier Return Type Method Name Parameter

Template for Simple Java Programs /* Chapter 2 Sample Program: Displaying a Window File: Ch2Sample2.java */ import javax.swing.*; class Ch2Sample1 { public static void main(String[ ] args) { JFrame myWindow; myWindow = new JFrame( ); myWindow.setSize(300, 200); myWindow.setTitle(“My First Java Program”); myWindow.setVisible(true); } } Import Statements Class Name Comment Method Body

Why Use Standard Classes Don’t reinvent the wheel. When there are existing objects that satisfy our needs, use them. Learning how to use standard Java classes is the first step toward mastering OOP. Before we can learn how to define our own classes, we need to learn how to use existing classes We will introduce four standard classes here: JOptionPane String Date SimpleDateFormat.

JOptionPane Using showMessageDialog of the JOptionPane class is a simple way to display a result of a computation to the user. JOptionPane.showMessageDialog ( null , “I Love Java” ) ; This dialog will appear at the center of the screen.

Displaying Multiple Lines of Text We can display multiple lines of text by separating lines with a new line marker \n . JOptionPane.showMessageDialog ( null , “ one\ntwo\nthree” ) ;

String The textual values passed to the showMessageDialog method are instances of the String class. A sequence of characters separated by double quotes is a String constant. There are close to 50 methods defined in the String class. We will introduce three of them here: substring , length , and indexOf . We will also introduce a string operation called concatenation .

String is an Object 1. The identifier name is declared and space is allocated in memory. 2. A String object is created and the identifier name is set to refer to it. 1 2 1 2 name String name; name = new String(“Jon Java”); : String Jon Java name

String Indexing The position, or index, of the first character is 0.

Definition: substring Assume str is a String object and properly initialized to a string. str.substring( i, j ) will return a new string by extracting characters of str from position i to j-1 where 0  i  length of str, 0  j  length of str, and i  j. If str is “programming” , then str.substring(3, 7) will create a new string whose value is “gram” because g is at position 3 and m is at position 6. The original string str remains unchanged.

Examples: substring text.substring(6,8) text.substring(0,8) text.substring(1,5) text.substring(3,3) text.substring(4,2) String text = “Espresso” ; “ so” “ Espresso” “ spre” error “”

Definition: length Assume str is a String object and properly initialized to a string. str.length( ) will return the number of characters in str. If str is “programming” , then str.length( ) will return 11 because there are 11 characters in it. The original string str remains unchanged.

Examples: length String str1, str2, str3, str4; str1 = “Hello” ; str2 = “Java” ; str3 = “” ; //empty string str4 = “ “ ; //one space str1.length( ) str2.length( ) str3.length( ) str4.length( ) 5 4 1 0

Definition: indexOf Assume str and substr are String objects and properly initialized. str.indexOf( substr ) will return the first position substr occurs in str. If str is “programming” and substr is “gram” , then str.indexOf(substr ) will return 3 because the position of the first character of substr in str is 3. If substr does not occur in str, then –1 is returned. The search is case-sensitive.

Examples: indexOf String str; str = “I Love Java and Java loves me.” ; str.indexOf( “J” ) str2.indexOf( “love” ) str3. indexOf( “ove” ) str4. indexOf( “Me” ) 7 21 -1 3 3 7 21

Definition: concatenation Assume str1 and str2 are String objects and properly initialized. str1 + str2 will return a new string that is a concatenation of two strings. If str1 is “pro” and str2 is “gram” , then str1 + str2 will return “program” . Notice that this is an operator and not a method of the String class. The strings str1 and str2 remains the same.

Examples: concatenation String str1, str2; str1 = “Jon” ; str2 = “Java” ; str1 + str2 str1 + “ “ + str2 str2 + “, “ + str1 “ Are you “ + str1 + “?” “ JonJava” “ Jon Java” “ Java, Jon” “ Are you Jon?”

Date The Date class from the java.util package is used to represent a date. When a Date object is created, it is set to today (the current date set in the computer) The class has toString method that converts the internal format to a string. Date today; today = new Date( ); today.toString( ); “ Fri Oct 31 10:05:18 PST 2003”

SimpleDateFormat The SimpleDateFormat class allows the Date information to be displayed with various format. Table 2.1 page 64 shows the formatting options. Date today = new Date( ); SimpleDateFormat sdf1, sdf2; sdf1 = new SimpleDateFormat( “MM/dd/yy” ); sdf2 = new SimpleDateFormat( “MMMM dd, yyyy” ); sdf1.format(today); sdf2.format(today); “ 10/31/03” “ October 31, 2003”

JOptionPane for Input Using showInputDialog of the JOptionPane class is a simple way to input a string. String name; name = JOptionPane.showInputDialog ( null , “What is your name?” ) ; This dialog will appear at the center of the screen ready to accept an input.

Problem Statement Problem statement: Write a program that asks for the user’s first, middle, and last names and replies with their initials. Example: input: Andrew Lloyd Weber output: ALW

Overall Plan Identify the major tasks the program has to perform. We need to know what to develop before we develop! Tasks: Get the user’s first, middle, and last names Extract the initials and create the monogram Output the monogram

Development Steps We will develop this program in two steps: Start with the program template and add code to get input Add code to compute and display the monogram

Step 1 Design The program specification states “get the user’s name” but doesn’t say how. We will consider “how” in the Step 1 design We will use JOptionPane for input Input Style Choice #1 Input first, middle, and last names separately Input Style Choice #2 Input the full name at once We choose Style #2 because it is easier and quicker for the user to enter the information

Step 1 Code /* Chapter 2 Sample Program: Displays the Monogram File: Step1/Ch2Monogram.java */ import javax.swing.*; class Ch2Monogram { public static void main ( String [ ] args ) { String name; name = JOptionPane.showInputDialog( null , "Enter your full name (first, middle, last):“ ) ; JOptionPane.showMessageDialog ( null , name ) ; } }

Step 1 Test In the testing phase, we run the program and verify that we can enter the name the name we enter is displayed correctly

Step 2 Design Our programming skills are limited, so we will make the following assumptions: input string contains first, middle, and last names first, middle, and last names are separated by single blank spaces Example John Quincy Adams (okay) John Kennedy (not okay) Harrison, William Henry (not okay)

Step 2 Design (cont’d) Given the valid input, we can compute the monogram by breaking the input name into first, middle, and last extracting the first character from them concatenating three first characters “ Aaron Ben Cosner” “ Aaron” “ Ben Cosner” “ Ben” “ Cosner” “ ABC”

Step 2 Code /* Chapter 2 Sample Program: Displays the Monogram File: Step 2/Ch2MonogramStep2.java */ import javax.swing.*; class Ch2Monogram { public static void main ( String [ ] args ) { String name, first, middle, last, space, monogram; space = " “ ; //Input the full name name = JOptionPane.showInputDialog ( null , "Enter your full name (first, middle, last):“ ) ;

Step 2 Code (cont’d) //Extract first, middle, and last names first = name.substring ( 0, name.indexOf ( space )) ; name = name.substring ( name.indexOf ( space ) +1, name.length ()) ; middle = name.substring ( 0, name.indexOf ( space )) ; last = name.substring ( name.indexOf ( space ) +1, name.length ()) ; //Compute the monogram monogram = first.substring ( 0, 1 ) + middle.substring ( 0, 1) + last.substring ( 0,1 ) ; //Output the result JOptionPane.showMessageDialog ( null , "Your monogram is " + monogram ) ; } }

Step 2 Test In the testing phase, we run the program and verify that, for all valid input values, correct monograms are displayed. We run the program numerous times. Seeing one correct answer is not enough. We have to try out many different types of (valid) input values.

Program Review The work of a programmer is not done yet. Once the working program is developed, we perform a critical review and see if there are any missing features or possible improvements One suggestion Improve the initial prompt so the user knows the valid input format requires single spaces between the first, middle, and last names

Java căn bản - Chapter2

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Java căn bản - Chapter2