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Introduction to Computers and Programming in JAVA Section 1 Professor: Sana` Odeh [email protected] 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.

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introduction
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
2 2 a first program in java printing a line of text
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 java program output

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!

lets look at the first and second line of the previous program t his is a comment

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
lets look at the following lines of the previous program

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
reserved words
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.
lets look at the following lines of the previous program1

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
2 2 a simple program printing a line of text

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
main method
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;

}

blocks
Blocks

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

statements
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 (;).
classes
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.
2 2 a simple program printing a line of text1

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)
2 2 a simple program printing a line of text2

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 ;
2 2 a simple program printing a line of text3

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
compiling a java program
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.
executing program lets take a look how to do this using jcreator
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
lets modify our first java program
Lets Modify Our First Java Program
  • Modify previous example to print same contents using different code
modifying our first java program

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
methods
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.
slide22

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!

2 3 modifying our first java program

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!" );
welcome3 java 1 main 2 system out println uses n for new line program output

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!

2 3 modifying our first java program1
2.3 Modifying Our First Java Program

Escape characters

  • Backslash ( \ )
  • Indicates special characters be output
find the error sample 1
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

find the error sample 2
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

find the errors 7
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

anatomy of a java program
Anatomy of a Java Program
  • Comments
  • Reserved words
  • Statements
  • Blocks
  • Classes
  • Methods
  • The main method
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