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58 Introduction to Java Applications Chapter 2 Line 2, // A first program in Java. is a single-line comment that describes the purpose of the program. Good Programming Practice 2.2 Every program should begin with a comment describing the purpose of the program. Line 3 is simply a blank line. Programmers use blank lines and space characters to make programs easier to read. Together, blank lines, space characters and tab characters are known as white space. (Space characters and tabs are known specifically as white-space characters.) Such characters are ignored by the compiler. We discuss conventions for using white-space characters in this chapter and the next several chapters, as these spacing conventions are needed in may Java programs. Good Programming Practice 2.3 Use blank lines, space characters and tab characters to enhance program readability. Line 4, public class Welcome1 { begins a class definition for class Welcome1. Every program in Java consists of at least one class definition that is defined by you the programmer. These classes are known as programmer-defined classes, or user-defined classes. The class keyword introduces a class definition in Java and is immediately followed by the class name (Welcome1in this program). Keywords (or reserved words) are reserved for use by Java (we discuss the various keywords throughout the text) and are always spelled with all lowercase letters. The complete list of Java keywords is shown in Fig. 4.2. By convention, all class names in Java begin with a capital letter and have a capital letter for every word in the class name (e.g., SampleClassName). The name of the class is called an identifier. An identifier is a series of characters consisting of letters, digits, underscores ( _) and dollar signs ($) that does not begin with a digit and does not contain spaces. Some valid identifiers are Welcome1, $value, _value, m_inputField1 and button7. The name 7button is not a valid identifier, because it begins with a digit, and the name input field is not a valid identifier, because it contains a space. Java is case sensitive i.e., uppercase and lowercase letters are different, so a1 and A1 are different identifiers. Common Programming Error 2.2 Java is case sensitive. Not using the proper uppercase and lowercase letters for an identifier is normally a syntax error. Good Programming Practice 2.4 By convention, you should always begin a class name with a capital letter. Good Programming Practice 2.5 When reading a Java program, look for identifiers that start with capital letters. These identifiers normally represent Java classes. Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/2/01
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Chapter 2 Introduction to Java Applications 57 1 // Fig. 2.1: Welcome1.java 2 // A first program in Java. 3 4 public class Welcome1 { 5 6 // main method begins execution of Java application 7 public static void main( String args[] ) 8 { 9 System.out.println( “Welcome to Java Programming!” ); 10 11 } // end method main 12 13 } // end class Welcome1 Welcome to Java Programming! Fig. 2.1 A first program in Java. Good Programming Practice 2.1 Use comments to clarify difficult concepts used in a program. A comment that begins with //is called a single-line comment, because the comment terminates at the end of the current line. A //comment can also begin in the middle of a line and continue until the end of that line. Multiple-line comments can be written in two other forms. For example, /* This is a multiple line comment. It can be split over many lines */ is a comment that can spread over several lines. This type of comment begins with delimiter /* and ends with delimiter */; this type of comment may be called a multiple-line comment. All text between the delimiters of the comment is ignored by the compiler. A similar form of comment called a documentation comment is delimited by /**and */. Common Programming Error 2.1 Forgetting one of the delimiters of a multiple-line comment is a syntax error. Java absorbed comments delimited with /*and */from the C programming language and single-line comments delimited with // from the C++ programming language. Java programmers generally use C++-style single-line comments in preference to C-style comments. Throughout this book, we use C++-style single-line comments. The documentation comment syntax (/** and */) is special to Java. It enables programmers to embed documentation for their programs directly in the programs. The javadocutility program (provided by Sun Microsystems with the Java 2 Software Development Kit) reads those comments from the program and uses them to prepare your program s documentation. There are subtle issues to using javadoc-style comments properly. We do not use javadoc-style comments in the programs presented in this book. However, javadoc-style comments are explained thoroughly in Appendix F. Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/2/01
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56 Introduction to Java Applications Chapter 2 Outline 2.1 Introduction 2.2 A First Program in Java: Printing a Line of Text 2.2.1 Compiling and Executing your First Java Application 2.3 Modifying Our First Java Program 2.3.1 Displaying a Single Line of Text with Multiple Statements 2.3.2 Displaying Multiple Lines of Text with a Single Statement 2.4 Displaying Text in a Dialog Box 2.5 Another Java Application: Adding Integers 2.6 Memory Concepts 2.7 Arithmetic 2.8 Decision Making: Equality and Relational Operators 2.9 (Optional Case Study) Thinking About Objects: Examining the Problem Statement Summary Terminology Self-Review Exercises Answers to Self-Review Exercises Exercises 2.1 Introduction The Java language facilitates a disciplined approach to computer program design. We now introduce Java programming and present examples that illustrate several important features of Java. Each example is analyzed one line at a time. In this chapter and Chapter 3, we present two program types in Java applications and applets. In Chapter 4 and Chapter 5, we present a detailed treatment of program development and program control in Java. 2.2 A First Program in Java: Printing a Line of Text Java uses notations that may appear strange to nonprogrammers. We begin by considering a simple application that displays a line of text. An application is a program that executes using the javainterpreter (discussed later in this section). The program and its output are shown in Fig. 2.1. This program illustrates several important features of the Java language. We consider each line of the program in detail. Each program we present in this book has line numbers included for the reader s convenience; line numbers are not part of actual Java programs. Line 9 does the real work of the program, namely displaying the phrase Welcome to JavaProgramming! on the screen. But let us consider each line in order. Line 1, // Fig. 2.1: Welcome1.java begins with //, indicating that the remainder of the line is a comment. Programmers insert comments to document programs and improve program readability. Comments also help other people read and understand a program. Comments do not cause the computer to perform any action when the program is run. The Java compiler ignores comments. We begin every program with a comment indicating the figure number and file name (line 1). Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/2/01
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2 Introduction to Java Applications Objectives To be able to write simple Java applications. To be able to use input and output statements. To become familiar with primitive data types. To understand basic memory concepts. To be able to use arithmetic operators. To understand the precedence of arithmetic operators. To be able to write decision-making statements. To be able to use relational and equality operators. Comment is free, but facts are sacred. C. P. Scott The creditor hath a better memory than the debtor. James Howell When faced with a decision, I always ask, What would be the most fun? Peggy Walker He has left his body to science and science is contesting the will. David Frost Classes struggle, some classes triumph, others are eliminated. Mao Zedong Equality, in a social sense, may be divided into that of condition and that of rights. James Fenimore Cooper Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/2/01
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54 Introduction to Computers, the Internet and the Web Chapter 1 i) The only language that a computer can directly understand is called that computer s . j) Which logical unit of the computer coordinates the activities of all the other logical units? . 1.8 Distinguish between the terms fatal error and nonfatal error. Why might you prefer to experience a fatal error rather than a nonfatal error? 1.9 Fill in the blanks in each of the following statements: a) Java are designed to be transported over the Internet and executed in World Wide Web browsers. b) programming causes a program to perform a task in response to user inter actions with graphical user interface (GUI) components. c) Java s graphics capabilities are and, hence portable. d) The standard can be used to provide identical user interfaces across all com puter platforms. e) Languages that cannot perform multiple activities in parallel are called languages or languages. f) Aggregations of data such as linked lists, stacks, queues and trees are called . 1.10 Fill in the blanks in each of the following statements (based on Sections 1.15 and 1.16): a) design patterns describe techniques to instantiate objects (or groups of objects). b) The is now the most widely used graphical representation scheme for modeling object-oriented systems. c) Java classes contain (which implement class behaviors) and (which implement class data). d) design patterns allow designers to organize classes and objects into larger structures. e) design patterns assign responsibilities to objects. f) In Java, the unit of programming is the , from which are eventu ally instantiated. Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/8/01
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Chapter 1 Introduction to Computers, the Internet and the Web 53 1.4 Fill in the blanks in each of the following statements (based on Sections 1.15 and 1.16): a) Over the past decade, the software-engineering industry has made significant progress in the field of proven architectures for constructing flexible and maintainable object-oriented software. b) Objects have the property of . c) Java programmers concentrate on creating their own user-defined types, called . d) Classes can also have relationships with other classes. These relationships are called . e) The process of analyzing and designing a system from an object-oriented point of view is called . ANSWERS TO SELF-REVIEW EXERCISES 1.1 a) Apple. b) IBM Personal Computer. c) programs. d) input unit, output unit, memory unit, arithmetic and logic unit, central processing unit, secondary storage unit. e) machine languages, assembly languages, high-level languages. f) compilers. 1.2 a) appletviewer. b) java. c) javac. d) HTML. e) .java. f) .class. g) byte- codes. 1.3 a) World Wide Web. b) applications. c) arrays. d) Swing. e) Multithreading. f) Collections. 1.4 a) design patterns. b) information hiding. c) classes. d) associations. e) object-oriented analysis and design (OOAD). EXERCISES 1.5 Categorize each of the following items as either hardware or software: a) CPU b) Java compiler c) ALU d) Java interpreter e) input unit f) editor 1.6 Why might you want to write a program in a machine-independent language instead of a machine-dependent language? Why might a machine-dependent language be more appropriate for writing certain types of programs? 1.7 Fill in the blanks in each of the following statements: a) Which logical unit of the computer receives information from outside the computer for use by the computer? . b) The process of instructing the computer to solve specific problems is called . c) What type of computer language uses English-like abbreviations for machine language instructions? . d) Which logical unit of the computer sends information that has already been processed by the computer to various devices so that the information may be used outside the computer? . e) Which logical unit of the computer retains information? . f) Which logical unit of the computer performs calculations? . g) Which logical unit of the computer makes logical decisions? . h) The level of computer language most convenient to the programmer for writing programs quickly and easily is . Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/8/01
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52 Introduction to Computers, the Internet and the Web Chapter 1 reference Swing GUI components requirements document syntax error reusable componentry throughput runtime error throw an exception secondary storage unit timesharing shareware translator programs software Unified Modeling Language (UML) software reuse verify phase structural design pattern video structured programming World Wide Web Sun Microsystems SELF-REVIEW EXERCISES 1.1 Fill in the blanks in each of the following statements: a) The company that popularized personal computing was . b) The computer that made personal computing legitimate in business and industry was the . c) Computers process data under the control of sets of instructions called . d) The six key logical units of the computer are the , , , , and . e) The three classes of languages discussed in the chapter are , and . f) The programs that translate high-level language programs into machine language are called . 1.2 Fill in the blanks in each of the following sentences about the Java environment: a) The command from the Java 2 Software Development Kit executes a Java applet. b) The command from the Java 2 Software Development Kit executes a Java application c) The command from the Java 2 Software Development Kit compiles a Java program. d) A(n) file is required to invoke a Java applet. e) A Java program file must end with the file extension. f) When a Java program is compiled, the file produced by the compiler ends with the file extension. g) The file produced by the Java compiler contains that are interpreted to execute a Java applet or application. 1.3 Fill in the blanks in each of the following statements: a) The allows computer users to locate and view multimedia-based documents on almost any subject over the Internet. b) Java typically are stored on your computer and are designed to execute in dependent of a World Wide Web browsers. c) Lists and tables of values are called . d) The GUI components are written completely in Java. e) allows an applet or application to perform multiple activities in parallel. f) provide Java programmers with a standard set of data structures for storing and retrieving data and a standard set of algorithms that allow programmers to manipulate the data. Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/8/01
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Chapter 1 Introduction to Computers, the Internet and the Web 51 applet IDE (Integrated Development Environment) appletviewercommand information hiding application inheritance arithmetic and logic unit (ALU) input device array input unit assembly language input/output (I/O) attribute instance variable Basic Internet behavior interpreter behavioral design pattern Java bytecode .java extension bytecode verifier Java 2 Software Development Kit (J2SDK) C java interpreter C standard library Java Virtual Machine C++ javac compiler central processing unit (CPU) JIT (just-in-time) compiler class KIS (keep it simple) .class file legacy systems class libraries live-code approach class loader load phase client/server computing logic error COBOL machine dependent collections machine independent compile phase machine language compiler memory unit compile-time error method computer Microsoft computer program Microsoft Internet Explorer Web browser computer programmer modeling condition multiprocessor CPU (central processing unit) multitasking creational design pattern multithreading design pattern Netscape Navigator Web browser disk nonfatal run-time error distributed computing object dynamic content object edit phase object-oriented analysis and design (OOAD) editor object-oriented design (OOD) encapsulation object-oriented programming (OOP) event-driven programming open source execute phase output device execution-time error output unit fatal runtime error Pascal file server personal computing Fortran platforms freeware portability hardware primary memory high-level language problem statement HotSpot compiler procedural programming HTML (Hypertext Markup Language) programming language Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/8/01
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50 Introduction to Computers, the Internet and the Web Chapter 1 Humans learn about objects by studying their attributes and observing their behaviors. Different objects can have similar attributes and can exhibit similar behaviors. Object-oriented design (OOD) models real-world objects. It takes advantage of class relationships, where objects of a certain class such as a class of vehicles have the same characteristics. It takes advantage of inheritance relationships, and even multiple-inheritance relationships, where newly created classes of objects are derived by absorbing characteristics of existing classes and adding unique characteristics of their own. OOD encapsulates data (attributes) and functions (behavior) into objects; the data and functions of an object are intimately tied together. Objects have the property of information hiding. This means that, although objects may know how to communicate with one another across well-defined interfaces, objects normally are not allowed to know how other objects are implemented. Languages such as Java are object-oriented programming in such a language is called object-oriented programming (OOP) and allows designers to implement the object-oriented design as a working system. In Java, the unit of programming is the class from which objects are eventually instantiated (a fancy term for created ). Java classes contain methods (which implement class behaviors) and attributes (which implement class data). Java programmers concentrate on creating their own user-defined types, called classes. Each class contains data and the set of functions that manipulate that data. The data components of a Java class are called attributes. The function components of a Java class are called methods. An instance of a user-defined type (i.e., a class) is called an object. Classes can also have relationships with other classes. These relationships are called associations. With object technology, we can build much of the software we will need by combining standardized, interchangeable parts called classes. The process of analyzing and designing a system from an object-oriented point of view is called object-oriented analysis and design (OOAD). The Unified Modeling Language (the UML) is now the most widely used graphical representation scheme for modeling object-oriented systems. Those who design systems use the language (in the form of graphical diagrams) to model their systems. Over the past decade, the software-engineering industry has made significant progress in the field of design patterns proven architectures for constructing flexible and maintainable object-oriented software. Using design patterns can substantially reduce the complexity of the design process. Design patterns benefit system developers by helping to construct reliable software using proven architectures and accumulated industry expertise, promoting design reuse in future systems, identifying common mistakes and pitfalls that occur when building systems, helping to design systems independently of the language in which they will be implemented, establishing a common design vocabulary among developers and shortening the design phase in a software-development process. Designers use design patterns to construct sets of classes and objects. Creational design patterns describe techniques to instantiate objects (or groups of objects). Structural design patterns allow designers to organize classes and objects into larger structures. Behavioral design patterns assign responsibilities to objects. TERMINOLOGY abstraction ALU (arithmetic and logic unit) Ada ANSI C Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/8/01
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Chapter 1 Introduction to Computers, the Internet and the Web 49 ties and research organizations, the Internet today is accessible by hundreds of millions of computers worldwide. The Web allows computer users to locate and view multimedia-intensive documents over the Internet. Java systems generally consist of several parts: an environment, the language, the Java Applications Programming Interface (API) and various class libraries. Java programs normally go through five phases to be executed edit, compile, load, verify and execute. Java program file names end with the .java extension. The Java compiler (javac) translates a Java program into bytecodes the language understood by the Java interpreter. If a program compiles correctly, the compiler produces a file with the .class extension. This is the file containing the bytecodes that are interpreted during the execution phase. A Java program must first be placed in memory before it can execute. This is done by the class loader, which takes the .class file (or files) containing the bytecodes and transfers it to memory. The .classfile can be loaded from a disk on your system or over a network. An application is a program that is normally stored and executed on the user s local computer. An applet is a small program that is normally stored on a remote computer that users connect to via a Web browser. Applets are loaded from a remote computer into the browser, executed in the browser and discarded when execution completes. Applications are loaded into memory, then executed by the java interpreter. Browsers are used to view HTML (Hypertext Markup Language) documents on the World Wide Web. When the browser sees an applet in an HTML document, the browser launches the Java class loader to load the applet. The browsers that support Java each have a built-in Java interpreter. Once the applet is loaded, the Java interpreter in the browser begins executing the applet. Applets can also be executed from the command line using the appletviewercommand provided with the Java 2 Software Development Kit (J2SDK). The appletviewer is commonly referred to as the minimum browser it knows only how to interpret applets. Before the bytecodes in an applet are executed by the Java interpreter built into a browser or the appletviewer, they are verified by the bytecode verifier to ensure that the bytecodes for downloaded classes are valid and that they do not violate Java s security restrictions. An intermediate step between interpreters and compilers is a just-in-time (JIT) compiler that, as the interpreter runs, produces compiled code for the programs and executes the programs in machine language rather than reinterpreting them. JIT compilers do not produce machine language that is as efficient as a full compiler. For organizations wanting to do heavy-duty information-systems development, Integrated Development Environments (IDEs) are available from the major software suppliers. The IDEs provide many tools for supporting the software-development process. Object orientation is a natural way of thinking about the world and of writing computer programs. The Unified Modeling Language (UML) is a graphical language that allows people who build systems to represent their object-oriented designs in a common notation. Humans think in terms of objects. We possess the marvelous ability of abstraction, which enables us to view screen images as people, planes, trees and mountains rather than as individual dots of color (called pixels for picture elements ). Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/8/01
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