Java basics 1 variable (lập TRÌNH NÂNG CAO SLIDE)

ADVANCED PROGRAMMING JAVA BASICS A Simple Java Program public class FirstSample{ public static void main(String[] args) { System.out.println("Hello, World!"); } }  Java is case sensitive  The keyword public is called an access modifier  The keyword class is there to remind you that everything in a Java program must be inside a class  The main method in the source file is necessary in order to execute the program  The System.out.println( ) is used to invoke method println of an object System.out Khoa CNTT – ĐH Nông Lâm TP HCM 01/2013 2/56 Comments System.out.println("We will not use 'Hello world!'"); // is this too cute? /* This is the first sample program Copyright (C) by Cay Horstmann and Gary Cornell */ public class FirstSample { public static void main(String[] args) { System.out.println("We will not use 'Hello, World!'"); } } Khoa CNTT – ĐH Nông Lâm TP HCM 01/2013 3/56 Compiling and executing the program The java compiler creates a file called 'First.class' that contains the byte codes To actually run the program, a java interpreter called java is required to execute the code Khoa CNTT – ĐH Nông Lâm TP HCM 01/2013 4/56 Passing Command Line Arguments class CommLineArg { public static void main (String [] pargs) { System.out.println("These are the arguments passed to the main method."); System.out.println(pargs [0]); System.out.println(pargs [1]); System.out.println(pargs [2]); } } Khoa CNTT – ĐH Nông Lâm TP HCM 01/2013 5/56 Passing Command Line Arguments Output Khoa CNTT – ĐH Nông Lâm TP HCM 01/2013 6/56 Identifiers  Identifiers are:  Text strings that represent variables, methods, classes or labels  Case-sensitive  Characters can be digit, letter, '$' or '_'  Identifiers cannot:  Begin with a digit  Be the same as a reserved word An_Identifier a_2nd_Identifier Go2 $10   An-Identifier 2nd_Identifier goto 10$ Khoa CNTT – ĐH Nông Lâm TP HCM 01/2013 7/56 Legal Identifiers  Identifiers must start with a letter, a currency character ($), or a connecting character such as the underscore ( _ ) Identifiers cannot start with a number!  After the first character, identifiers can contain any combination of letters, currency characters, connecting characters, or numbers  In practice, there is no limit to the number of characters an identifier can contain  You can't use a Java keyword as an identifier  Identifiers in Java are case-sensitive; foo and FOO are two different identifiers Khoa CNTT – ĐH Nông Lâm TP HCM 01/2013 8/56 Complete List of Java Keywords Khoa CNTT – ĐH Nông Lâm TP HCM 01/2013 9/56 Java Code Conventions  Classes: the names should typically be nouns  Dog  Account  PrintWriter  Interfaces: the names should be adjectives  Runnable  Serializable  Methods: the names should be verb-noun pairs  getBalance  doCalculation  setCustomerName Khoa CNTT – ĐH Nông Lâm TP HCM 01/2013 10/56 Creating Arrays  An example to create and initialize a primitive (char) array: public char[] createArray() { char[] s; s = new char[26]; for ( int i = 0; i < 26; i++ ) { s[i] = (char) ('A' + i); } return s; } Khoa CNTT – ĐH Nông Lâm TP HCM 01/2013 42/56 Creating object reference Arrays  Creating an Array of Point Objects class Point { private int x, y; public Point(int x, int y) { this.x = x; this.y = y; } } public Point[] createArray() { Point[] p; p = new Point[10]; for ( int i=0; i