Lesson 2: First Java Programs

1. Lesson 2: First Java Programs

2. Lesson 2: First Java Programs Objectives: • Discuss why Java is an important programming language. • Explain the Java virtual machine and byte code. • Choose a user interface style. • Describe the structure of a simple Java program.

3. Lesson 2: First Java Programs Objectives: • Write a simple program. • Edit, compile, and run a program using a Java development environment. • Format a program to give a pleasing, consistent appearance. • Understand compile-time errors. • Write a graphics program.

4. Lesson 2: First Java Programs Vocabulary: • Applet • Assignment operator • Byte code • DOS development environment • Graphical user interface (GUI) • Hacking • Import statement • Integrated development environment (IDE) • Interpreter • Java virtual machine(JVM) • Just-in-time compilation (JIT) • Panel • Panes • Parameter • Source code • Statement • Terminal I/O interface • Variable

5. 2.1 Why Java? • Java is the fastest growing programming language in the world. • Java is a modern object-oriented programming language. • Java has benefited by learning from the less desirable features of early object-oriented programming languages.

6. 2.1 Why Java? Java is ideal for distributed, network-based applications. • Secure: Virus-free, tamper-free systems. • Robust: Supports development of programs that do not overwrite memory. • Portable: Yields programs that can be run on different computer types.

7. 2.1 Why Java? • Java is ideally suited to develop distributed, network-based applications because it: • Enables the construction of virus-free, tamper-free systems (security) • Supports the development of programs that do not overwrite memory (robust) • Yields programs that can be run on different types of computers without change (portable)

8. 2.1 Why Java? • Java supports advanced programming concepts such as threads. • A thread is a process that can run concurrently with other processes. • Java resembles C++, the world’s most popular industrial strength programming language. • Java however, runs more slowly than most modern programming languages because it is interpreted.

9. 2.2 The Java Virtual Machine and Byte Code • Java compilers translate Java into pseudomachine language called java byte code. • To run java byte code on a particular computer, a Java virtual machine (JVM) must be installed.

10. 2.2 The Java Virtual Machine and Byte Code • A Java virtual machine is a program that runs like a computer. It is called an interpreter. • Disadvantage: • Runs more slowly than an actual computer • To combat slower processing, some JVMs translate code when first encountered. This is known as just-in-time compilation (JIT).

11. 2.2 The Java Virtual Machine and Byte Code • Advantages: • Portability. Any computer can run an interpreter.This makes byte code portable. • Applets. Applets are small Java programs already translated into byte code. • Applets run in a JVM incorporated in a web browser • Applets can be decorative (like animated characters on a web page.) • Applets can be practical (like continuous streams of stock market quotes.) • Security. It is possible to limit the capabilities of a Java program since it runs inside a virtual machine.

12. 2.2 The Java Virtual Machine and Byte Code • Advantages: • JVMs are getting faster. • Using JIT (just-in-time) compilations, which translate byte code into machine language.

13. 2.3 Choosing a User Interface Style • There are two types of user interfaces available to use to create Java programs. • Graphical User Interface (GUI) • Terminal I/O interface • Figure 2-1 illustrates both interfaces used to create the same program.

14. 2.3 Choosing a User Interface Style Graphical user interface (GUI)

15. 2.3 Choosing a User Interface Style Terminal I/O user interface

16. 2.3 Choosing a User Interface Style • There are 3 reasons for beginning with terminal I/O: • It is easier to implement than a GUI • There are programming situations that require terminal I/O • Terminal-oriented programs are similar in structure to programs that process files of sequentially organized data. (What is learned here is easily transferred to that setting.)

17. 2.4 HelloWorld • Figure 2-2 displays the results of a small Java program, entitled “HelloWorld”

18. 2.4 HelloWorld • A program is a sequence of instructions for a computer. • The following is the bulk of instructions, or source code, for the HelloWorld” program.

19. 2.4 HelloWorld The Explanation: • System.out is an object that displays characters in a terminal window. • println is the message being sent to the object. • The quotations indicate what is to be displayed. • Semicolons mark the end of each statement. • The characters between the parentheses are the parameters. • The period (.) is the method selector operator. Sending messages to objects always takes the following form: <name of object>.<name of message>(<parameters>)

20. 2.4 HelloWorld The Larger Framework: The program must be embedded in several lines of code, such as: Program comments are in green, reserved words in blue, and code in black.

21. 2.5 Edit, Compile, and Execute • Figure 2-3 illustrates the edit, compile and execute steps.

22. 2.5 Edit, Compile, and Execute • Edit • The programmer uses a word processor or editor to enter the source code. • Save it as a text file with the extension .java. • Compile • The programmer invokes the Java language compiler. • Translates the source code into Java byte code. • Execute • The programmer instructs the JVM to load the byte code into memory and execute. • The user and program can now interact.

23. 2.5 Edit, Compile, and Execute • Development environments: • Unix • standard text editor • command line activation of compiler and JVM • DOS, using Microsoft Windows and NT OS • notepad text editor • command line activation of compiler and JVM from a DOS window • Integrated development environment, using Windows, NT, or MAC OS • Examples: Symantec’s Visual Café, Microsoft’s Visual J++, NetBeans, or Borland’s J Builder

24. 2.5 Edit, Compile, and Execute

25. 2.5 Edit, Compile, and Execute • Preparing your development environment: • Create a directory, open a terminal window, use the cd command to move to your new directory • Open notepad, create the file HelloWorld.java, type in the lines of code • Save the file, go back to the terminal window, compile the program • Run the program

26. 2.5 Edit, Compile, and Execute The program as typed into Notepad

27. 2.5 Edit, Compile, and Execute • The following figures illustrate the steps necessary for preparing your development environment.

28. 2.5 Edit, Compile, and Execute • Compile-Time Errors: • Mistakes detected by the compiler are called syntax errors or compile-time errors. • Typos made when editing. • Compiler prints a list of errors in the terminal window.

29. 2.5 Edit, Compile, and Execute • Readability: • Programs may be maintained by other people. • Layout affects readability. • Use indentation, blank lines, and spaces.

30. 2.5 Edit, Compile, and Execute

31. 2.6 Temperature Conversion • View the program’s source code: import java.util.Scanner; public class Convert { public static void main (String [ ] args) { Scanner reader = new Scanner(System.in);// This allows the user // to enter data from // the keyboard double fahrenheit; // The temperature in // Fahrenheit double celsius; // The temperature in // Celsius System.out.print(“Enter degrees Fahrenheit: ”); fahrenheit = reader.readDouble(); celsius = (Fahrenheit – 32.0) * 5.0 / 9.0; System.out.print(“The equivalent in Celsius is ”); System.out.println(celsius); } }

32. 2.6 Temperature Conversion • The following is an explanation of the program code: • Import statement • Instantiate or create an object • Declare the variables • Position the cursor after “Enter degrees Fahrenheit” • Assignment operators • Assignment statements are evaluated • Print text (and position the cursor) • Print the value of the variable • Statement to prevent the terminal window from disappearing from the display (optional, only needed with certain development environments)

33. 2.6 Temperature Conversion • Temperature conversion program reads user input and performs computations. • The first line of code is an import statement. • Variables for Fahrenheit and Celsius. • Assignment statements use an operator such as *, /, +, and -.

34. 2.6 Temperature Conversion • Variables and objects used in the conversion program.

35. 2.7 Graphics and GUIs: Windows and Panels • A Simple Application Window: • Graphics and GUI programs in Java can be stand-alone applications or applets. • Consistent features: • Title bar with controls (maximize, zoom, etc.) • Width and height can be resized • Code for application windows is in the class Jframe. • JFrame responds to messages to set the title bar and window size.

36. 2.7 Graphics and GUIs: Windows and Panels Some commonly used JFrame methods

37. 2.7 Graphics and GUIs: Windows and Panels Panels and Colors: • A Jframe has a container or pane to fill with objects. • A panel is a rectangle used to display objects such a shapes and images. • Panes are panels that contain related objects such as images and widgets. • Colors in most computer system use RGB. • Red, green, blue • Values 0-255

38. 2.7 Graphics and GUIs: Windows and Panels Layout Managers and Multiple Panels: • Each container object uses a layout manager to control panel placement. • BorderLayout class allows arrangement of up to five objects. • North, south, east, west, center • GridLayout uses rows and columns to arrange objects.

39. Summary In this chapter, you learned: • Java is the fastest growing programming language in the world. It is secure, robust, and portable. It is also similar to C++, the world’s most popular programming language.

40. Summary • The Java compiler translates Java into a pseudomachine language called Java byte code. Byte code can be run on any computer that has a Java virtual machine installed. The Java virtual machine (JVM) is a program that behaves like a computer—an interpreter. • Java programs include variables, arithmetic expressions, statements, objects, messages, and methods.

41. Summary • Three basic steps in the coding process are editing, compiling, and running a program using a Java development environment. Programmers should pay attention to a program’s format to ensure readability.

42. Summary • Java programs accomplish many tasks by sending messages to objects. Examples are sending text to the terminal window for output and receiving input data from the keyboard. • There are several user interface styles, among them terminal based and graphical based.

43. THE END