Introduction to Java Brief history of Java Sample Java Program - - PowerPoint PPT Presentation

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Introduction to Java

 Brief history of Java  Sample Java Program  Compiling & Executing

Reading: => Section 1.1

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Introduction to Java

History

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Invented in 1991 - Sun Microsystems, Inc (James Gosling).

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Sun Microsystems was purchased by Oracle Corporation in 2010.

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Originally a language for programming home appliances.

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Later (1994) used for internet applications and general-purpose programming.

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Why the name “Java”?

• Supposedly came from a list of random words (wikipedia).

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A Java Application

import java.util.Scanner; public class FirstProgram { public static void main(String[] args) { int n1, n2; Scanner keyboard = new Scanner(System.in); System.out.println(“Enter an integer:”); n1 = keyboard.nextInt(); System.out.println(“Enter another integer:”); n2 = keyboard.nextInt(); System.out.println(“The sum is:”); System.out.println(n1 + n2); } }

>javac FirstProgram.java >java FirstProgram Enter an integer: 15 Enter another integer: 7 The sum is: 22 >

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Explanation of Code ...

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Code at the beginning of the program (to be explained later):

import java.util.Scanner; public class FirstProgram { public static void main(String[] args) { 

Java applications all have similar code at the beginning

• The name of the class differs from one program to another.
• The name of the class is also the name of the file.
• Notice that the blank line is gone!

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… Explanation of Code ...

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The following creates two variables named n1, n2 for storing two whole numbers (integer): int n1, n2;

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These are called “variable declarations.”

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In this program they are used to store the user’s response.

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… Explanation of Code ...

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The following creates an object called keyboard of the Scanner class:

Scanner keyboard = new Scanner(System.in); 

System.in refers to the keyboard

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The Scanner class provides the program with access to keyboard input.

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Explanation of Code ...

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Displays a “text” string to the screen:

System.out.println(“Enter an integer:”); 

Note the “dot” delimiter:

• System is a class
• out an object
• println is a method that outputs something

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Double-quoted text inside the parentheses is referred to as an argument or parameter to the method.

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… Explanation of Code ...

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The following inputs (or “reads”) an integer typed in using the keyboard and stores it in the variable n1:

n1 = keyboard.nextInt(); 

The next two lines are similar:

System.out.println(“Enter another integer:”); n2 = keyboard.nextInt();

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… Explanation of Code

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The following prints the sum to the console (or screen):

System.out.println(“The sum is:”); System.out.println(n1 + n2);

By the way, every character counts!

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Compiling and Running a Java Program

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Type the program into a file:

• FirstProgram.java

Make sure you get the file name correct! 

Compile (from the command-line, i.e., DOS):

• javac <file>.java

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Run (and link):

• java <file>

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Extra Work – Yippee!

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Type-in, compile and run the previous program.

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Experiment with “what if” scenarios:

• Add some syntax errors
• Run the program incorrectly – input improper data
• Modify the program to work with 3 ints

 Note that if you change the program, you have to recompile

before you rerun!

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Some Helpful Commands

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Some helpful commands:

• cd <dir-name>
• change directory
• cd ..
• move up a directory
• dir <pattern>
• list directory contents matching the pattern
• mkdir <dir-name>
• create a new directory
• del <pattern>
• delete a file or files matching the pattern

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Suggestion – create a new directory/subfolder on your “C” drive to store all class related programs.

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Create subfolders as appropriate.