Chapter 1 Introduction to Programs and Java
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Programs
Computer programs, known as software, are instructions to the computer.
You tell a computer what to do through programs. Without programs, a computer is an empty machine. Computers do not understand human languages, so you need to use computer languages to communicate with them.
Programs are written using programming languages.
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Programming Languages
Machine Language Assembly Language High-Level Language
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Machine language is a set of primitive instructions built into every computer. The instructions are in the form of binary code, so you have to enter binary codes for various instructions. Programs are highly difficult to read and modify. For example, to add two numbers, you might write an instruction in binary like this:
1101101010011010
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Programming Languages
Machine Language Assembly Language High-Level Language
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Assembly languages were developed to make programming easy. Since the computer cannot understand assembly language, however, a program called assembler is used to convert assembly language programs into machine code. For example, to add two numbers, you might write an instruction in assembly code like this:
ADDF3 R1, R2, R3
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Programming Languages
Machine Language Assembly Language High-Level Language
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The high-level languages are English-like and easy to learn and program. For example, the following is a high-level language statement that computes the area of a circle with radius 5:
area = 5 * 5 * 3.1415;
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Popular High-Level Languages
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Compiling Source Code
A program written in a high-level language is called a source program. Since a computer cannot understand a source program. Program called a compiler is used to translate the source program into a machine language program called an object program. The object program is often then linked with other supporting library code before the object can be executed on the machine.
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Why Java?
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The answer is that Java enables users to develop and deploy applications on the Internet for servers and desktop computers. The future of computing is being profoundly influenced by the Internet, and Java promises to remain a big part of that future. Java is the Internet programming language.
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Java, Web, and Beyond
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Examples of Java’s Versatility (Applets)
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Characteristics of Java
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Characteristics of Java
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Java is partially modeled on C++, but greatly simplified and improved. Some people refer to Java as "C++--" because it is like C++ but with more functionality and fewer negative aspects.
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Characteristics of Java
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Java is inherently object-oriented. Although many object-oriented languages began strictly as procedural languages, Java was designed from the start to be object-oriented. Object-oriented programming (OOP) is a popular programming approach that is replacing traditional procedural programming techniques.
One of the central issues in software development is how to reuse code. Object-oriented programming provides great flexibility, modularity, clarity, and reusability through encapsulation, inheritance, and polymorphism.
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Characteristics of Java
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Distributed computing involves several computers working together on a network. Java is designed to make distributed computing easy. Since networking capability is inherently integrated into Java, writing network programs is like sending and receiving data to and from a file.
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Characteristics of Java
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You need an interpreter to run Java programs. The programs are compiled into the Java Virtual Machine code called bytecode. The bytecode is machine-independent and can run on any machine that has a Java interpreter, which is part of the Java Virtual Machine (JVM).
Interpreter: A program that translates an instruction into a machine language and executes it before proceeding to the next instruction.��
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Characteristics of Java
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Java compilers can detect many problems that would first show up at execution time in other languages.
Java has eliminated certain types of error-prone programming constructs found in other languages.
Java has a runtime exception-handling feature to provide programming support for robustness.
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Characteristics of Java
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Java implements several security mechanisms to protect your system against harm caused by stray programs.
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Characteristics of Java
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Write once, run anywhere
With a Java Virtual Machine (JVM), you can write one program that will run on any platform.
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Characteristics of Java
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Because Java is architecture neutral, Java programs are portable. They can be run on any platform without being recompiled.
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Characteristics of Java
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Multithread programming is smoothly integrated in Java, whereas in other languages you have to call procedures specific to the operating system to enable multithreading.
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Characteristics of Java
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Java was designed to adapt to an evolving environment. New code can be loaded on the fly without recompilation. There is no need for developers to create, and for users to install, major new software versions. New features can be incorporated transparently as needed.
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JDK Versions
�
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Popular Java IDEs
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A Simple Java Program
//This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
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Listing 1.1
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Creating and Editing Using NotePad
To use NotePad, type
notepad Welcome.java
from the DOS prompt.
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Creating, Compiling, and Running Programs
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Compiling Java Source Code
You can port a source program to any machine with appropriate compilers. The source program must be recompiled, however, because the object program can only run on a specific machine. Nowadays computers are networked to work together. Java was designed to run object programs on any platform. With Java, you write the program once, and compile the source program into a special type of object code, known as bytecode. The bytecode can then run on any computer with a Java Virtual Machine, as shown below. Java Virtual Machine is a software that interprets Java bytecode.
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Trace a Program Execution
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//This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
Enter main method
animation
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Trace a Program Execution
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//This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
Execute statement
animation
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Trace a Program Execution
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//This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
animation
print a message to the console
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Two More Simple Examples
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Run
Welcome1
Run
ComputeExpression
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Anatomy of a Java Program
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Comments
Line comment: A line comment is preceded by two slashes (//) in a line.
Paragraph comment: A paragraph comment is enclosed between /* and */ in one or multiple lines.
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javadoc comment: javadoc comments begin with /** and end with */. They are used for documenting classes, data, and methods. They can be extracted into an HTML file using JDK's javadoc command.
Three types of comments in Java.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807
Reserved Words
Reserved words or keywords are words that have a specific meaning to the compiler and cannot be used for other purposes in the program. For example, when the compiler sees the word class, it understands that the word after class is the name for the class. Other reserved words in Listing 1.1 are public, static, and void. Their use will be introduced later in the book.
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Modifiers
Java uses certain reserved words called modifiers that specify the properties of the data, methods, and classes and how they can be used. Examples of modifiers are public and static. Other modifiers are private, final, abstract, and protected. A public datum, method, or class can be accessed by other programs. A private datum or method cannot be accessed by other programs. Modifiers are discussed in Chapter 6, “Objects and Classes.”
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Statements
A statement represents an action or a sequence of actions. The statement System.out.println("Welcome to Java!") in the program in Listing 1.1 is a statement to display the greeting "Welcome to Java!" Every statement in Java ends with a semicolon (;).
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Blocks
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A pair of braces in a program forms a block that groups components of a program.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807
Classes
The class is the essential Java construct. A class is a template or blueprint for objects. To program in Java, you must understand classes and be able to write and use them. The mystery of the class will continue to be unveiled throughout this book. For now, though, understand that a program is defined by using one or more classes.
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Methods
What is System.out.println? It is a method: a collection of statements that performs a sequence of operations to display a message on the console. It can be used even without fully understanding the details of how it works. It is used by invoking a statement with a string argument. The string argument is enclosed within parentheses. In this case, the argument is "Welcome to Java!" You can call the same println method with a different argument to print a different message.
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main Method
The main method provides the control of program flow. The Java interpreter executes the application by invoking the main method.
The main method looks like this:
public static void main(String[] args) {
// Statements;
}
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Displaying Text in a Message Dialog Box
you can use the showMessageDialog method in the JOptionPane class. JOptionPane is one of the many predefined classes in the Java system, which can be reused rather than “reinventing the wheel.”
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WelcomeInMessageDialogBox
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The showMessageDialog Method
JOptionPane.showMessageDialog(null,
"Welcome to Java!",
"Display Message",
JOptionPane.INFORMATION_MESSAGE);
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Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807
Two Ways to Invoke the Method
There are several ways to use the showMessageDialog method. For the time being, all you need to know are two ways to invoke it.
One is to use a statement as shown in the example:
JOptionPane.showMessageDialog(null, x,
y, JOptionPane.INFORMATION_MESSAGE);
where x is a string for the text to be displayed, and y is a string for the title of the message dialog box.
The other is to use a statement like this:
JOptionPane.showMessageDialog(null, x);
where x is a string for the text to be displayed.
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Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807