1

Introduction to Java

Brief history of Java

Sample Java Program

Compiling & Executing

Reading:

=> Section 1.1

2

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).

3

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

>

4

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!

5

… 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.

6

… 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.

7

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.

8

… 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();

9

… 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!

10

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>

11

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!

12

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.