Transcript Chapter 1: Introduction

Introduction to Java
 Brief history of Java
 Sample Java Program
 Compiling & Executing
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Introduction to Java
History
 Invented in 1991 - Sun Microsystems, Inc (James Gosling).
 Sun Microsystems was purchased by Oracle Corporation in 2010.
 Originally a language for programming home appliances.
 Later (1994) used for internet applications and general-purpose
programming.
 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);
}
}
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>javac FirstProgram.java
>java FirstProgram
Enter an integer:
15
Enter another integer:
7
The sum is:
22
>
Explanation of Code ...
 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 ...
 The following creates two variables named n1, n2 for storing two
whole numbers (integer):
int n1, n2;
 These are called “variable declarations.”
 In this program they are used to store the user’s response.
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… Explanation of Code ...
 The following creates an object called keyboard of the Scanner class:
Scanner keyboard = new Scanner(System.in);
 System.in refers to the keyboard
 The Scanner class provides the program with access to keyboard input.
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Explanation of Code ...
 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
 Double-quoted text inside the parentheses is referred to as an argument or
parameter to the method.
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… Explanation of Code ...
 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
 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
 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
 Run (and link):
 java <file>
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Extra Work – Yippee!
 Type-in, compile and run the previous program.
 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
 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
 Suggestion – create a new directory/subfolder on your “C” drive to store
all class related programs.
 Create subfolders as appropriate.
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