java程序计jhtp5_02.ppt

Chapter 2 - Introduction to Java Applications 2.1 Introduction In this chapter Introduce examples to illustrate features of Java Two program styles - applications and applets 2.2 A First Program in Java: Printing a Line of Text Application Program that executes using the java interpreter Sample program Show program, then analyze each line Welcome1.javaProgram Output 1 // Fig. 2.1: Welcome1.java 2 // Text-printing program. 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 2.2 A First Program in Java: Printing a Line of Text Comments start with: // Comments ignored during program execution Document and describe code Provides code readability Traditional comments: /* ... */ /* This is a traditional comment. It can be  split over many lines */ Another line of comments Note: line numbers not part of program, added for reference 2.2 A Simple Program: Printing a Line of Text Blank line Makes program more readable Blank lines, spaces, and tabs are white-space characters Ignored by compiler Begins class declaration for class Welcome1 Every Java program has at least one user-defined class Keyword: words reserved for use by Java class keyword followed by class name Naming classes: capitalize every word SampleClassName 2.2 A Simple Program: Printing a Line of Text Name of class called identifier Series of characters consisting of letters, digits, underscores ( _ ) and dollar signs ( $ ) Does not begin with a digit, has no spaces Examples: Welcome1, $value, _value, button7 7button is invalid Java is case sensitive (capitalization matters) a1 and A1 are different For chapters 2 to 7, use public keyword Certain details not important now Mimic certain features, discussions later 2.2