Introduction

1. Introduction to Computers and Programming in JAVASection 1Professor: Sana` Odehodeh@courant.nyu.edu

2. Introduction • Introductions to Java Programming basics • Introduce a basic Java program • We will use Jcreator • to create/edit/compile/ debug and execute/run our Java programs

3. 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

4. 1 // first Java program called 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 Welcome1.javaProgram Output Welcome to Java Programming!

5. 1 // first Java program called Welcome1.java 2 // Text-printing program. Lets look at the first and second line of the previous programThis is a comment • Comments start with: // • Comments ignored during program execution • Improves code readability • Document and describe code • 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

6. 3 4 public class Welcome1 { Lets look at the following lines of the previous program • Blank line • Makes program more readable • Blank lines, spaces, and tabs are white-space characters • Ignored by compiler • Begins class declaration for class Welcome1 • Public & class are reserved key words and have to be included in every program • Reserved Keyword: words reserved for use by Java. • class keyword followed by class name ( in this case it is Welcome1) is referred to as an identifier. • An identifier is user defined • Welcome1 is the name of the program and should be same as the file name Welcome1.java • Naming classes: capitalize every word • SampleClassName

7. 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 Example 1 are public, static, and void. Their use will be introduced later in the book.

8. 4 public class Welcome1 { Lets look at the following lines of the previous program • Name of class called identifier ( in this case its Welcome1 ) • 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

9. 4 public class Welcome1 { 7 public static void main( String args[] ) 2.2 A Simple Program: Printing a Line of Text • Saving files • File name must be class name with .java extension • Welcome1.java • Left brace { • Begins body of every class • Right brace ends declarations (line 13) • Part of every Java application • Applications begin executing at main • Parenthesis indicate main is a method (ch. 6) • Java applications contain one or more methods

10. 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; }

11. Blocks A pair of braces in a program forms a block that groups components of a program.

12. Statements • A statementrepresents an action or a sequence of actions. • The statement System.out.println("Welcome to Java!"); • in the program in Example 1.1 is a statement to display the greeting "Welcome to Java!" Every statement in Java ends with a semicolon (;).

13. 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.

14. 7 public static void main( String args[] ) 8 { 2.2 A Simple Program: Printing a Line of Text • Exactly one method must be called main • Methods can perform tasks and return information • void means main returns no information • For now, mimic main's first line • Left brace begins body of method declaration • Ended by right brace } (line 11)

15. 9 System.out.println( "Welcome to Java Programming!" ); 2.2 A Simple Program: Printing a Line of Text • Instructs computer to perform an action • Prints string of characters • String - series characters inside double quotes • White-spaces in strings are not ignored by compiler • System.out • Standard output object • Print to command window (i.e., MS-DOS prompt) • Method System.out.println • Displays line of text • Argument inside parenthesis • This line known as a statement • Statements must end with semicolon ;

16. 11 } // end method main 13 } // end class Welcome1 2.2 A Simple Program: Printing a Line of Text • Ends method declaration • Ends class declaration • Can add comments to keep track of ending braces • Lines 8 and 9 could be rewritten as: • Remember, compiler ignores comments • Comments can start on same line after code

17. Compiling a Java program • Compiling a program • OpenJCreator • See website for instructions • http://www.cs.nyu.edu/courses/spring09/V22.0002-001/software.htm • Create a new project, then a new java file/class and will save it. • Type the program and compile it • If no errors, Welcome1.class created • Has bytecodes that represent application • Bytecodes passed to Java interpreter, the Java Virtual Machine (JVM) • If no errors, we will run or execute our program to get the output.

18. Executing program.. Lets take a look how to do this using JCreator • Executing a program • TypejavaWelcome1 • Interpreter loads .class file for class Welcome1 • .class extension omitted from command • Interpreter calls method main

19. Lets Modify Our First Java Program • Modify previous example to print same contents using different code

20. 9 System.out.print( "Welcome to " ); 10 System.out.println( "Java Programming!" ); Modifying Our First Java Program • Modifying programs • Welcome2.java produces same output as Welcome1.java (Fig. 2.1) • Using different code • Line 9 displays “Welcome to ” with cursor remaining on printed line • Line 10 displays “Java Programming! ” on same line with cursor on next line

21. 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.

22. System.out.print keeps the cursor on the same line, so System.out.printlncontinues on the same line. 1 // Fig. 2.3: Welcome2.java 2 // Printing a line of text with multiple statements. 3 4 public class Welcome2 { 5 6 // main method begins execution of Java application 7 public static void main( String args[] ) 8 { 9 System.out.print( "Welcome to " ); 10 System.out.println( "Java Programming!" ); 11 12 } // end method main 13 14 } // end class Welcome2 Welcome2.java1. Comments2. Blank line3. Begin class Welcome23.1 Method main4. Method System.out.print4.1 Method System.out.println5. end main,Welcome2 Welcome to Java Programming!

23. 9 System.out.println( "Welcome\nto\nJava\nProgramming!" ); 2.3 Modifying Our First Java Program • Newline characters (\n) • Interpreted as “special characters” by methods System.out.print and System.out.println • Indicates cursor should be on next line • Welcome3.java (Fig. 2.4) • Line breaks at \n • Usage • Can use in System.out.println or System.out.print to create new lines • System.out.println("Welcome\nto\nJava\nProgramming!" );

24. Notice how a new line is output for each \n escape sequence. 1 // Fig. 2.4: Welcome3.java 2 // Printing multiple lines of text with a single statement. 3 4 public class Welcome3 { 5 6 // main method begins execution of Java application 7 public static void main( String args[] ) 8 { 9 System.out.println( "Welcome\nto\nJava\nProgramming!" ); 10 11 } // end method main 12 13 } // end class Welcome3 Welcome3.java1. main2. System.out.println (uses \n for new line)Program Output Welcome to Java Programming!

25. 2.3 Modifying Our First Java Program Escape characters • Backslash ( \ ) • Indicates special characters be output

26. Find the ErrorSample 1 1 Printing multiple lines of text with a single statement. 2 3 4 public class Welcome3 { 5 6 // main method begins execution of Java application 7 public static void main( String args[] ) 8 { 9 System.out.println( "Welcome\nto\nJava\nProgramming!" ); 10 11 } // end method main 12 13 } // end class Welcome3

27. Find the ErrorSample 2 1 //Printing multiple lines of text with a single statement. 2 3 4 public Welcome3 { 5 6 // main method begins execution of Java application 7 public static void main( String args[] ) 8 { 9 System.out.println( "Welcome\nto\nJava\nProgramming!" ); 10 11 } // end method main 12 13 } // end class Welcome3

28. Find the Errors (7) 1 //Printing multiple lines of text with a single statement. 2 3 4 Public class 1firstprogram { 5 6 // main method begins execution of Java application 7 public static void main( String args[] 8 { 9 System.out.print( "Welcome\nto\nJava\nProgramming! ) 10 11 } // end method main 12 13 // end class Welcome3

29. Anatomy of a Java Program • Comments • Reserved words • Statements • Blocks • Classes • Methods • The main method