Sending arguments into Java native application

THE PROBLEM: Write a program that will read an input file mentioned in your command line, and display several lines of text from the input file.

THE SAMPLE OUTPUT:

In this tutorial we need simple code editor (Notepad++) and JDK (Java Development Kits). If you’re using Mac, JDK is already available inside (read more for Mac user).

SETTING THE JAVA PATH

In Windows environment, if you’d like to use the command line to compile (JAVAC) and execute a java class (JAVA) you need to set the Java home PATH.

Find the Java installation folder, for example: C:\Program Files\Java\jdk1.6.0_29\bin

This is the folder where the JAVAC and JAVA program is available.

Figure 1

Right-click MyComputer icon on your desktop, and select Properties. Click the Environment Variables as in Figure 2.

Figure 2

In the system variables, select PATH and click EDIT button (Figure 3).

Figure 3

Add the Java bin path to the Variable Value: C:\Program Files\Java\jdk1.6.0_29\bin as in Figure 4. Use the semicolon “;” to separate different variable value.

Figure 4

STEP 1: THE CODE SAMPLE

Use Notepad++ or your favourite code editor to key-in this code. This program will actually read an input file mentioned in your command line, and display several lines of text from the input file.

Java source code: Head.java

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
public class Head {
    
    //declare the BufferdReader to read the input file
    private static BufferedReader  failin;
    public static void main (String args[]) throws IOException{
        
        //baris - the content of each lines in the input file
        String baris;
        //lines - the number of lines of the input text from the file 
        //that will be displayed
        int lines=Integer.parseInt(args[0]);
        //filen - name of the file send by the user thru command prompt
        String filen=args[1];
        int i=0;
        
        //try catch any file input error
        try{
            failin=new BufferedReader (new FileReader(filen));
            //extract one line at a time
            while((baris=failin.readLine())!=null){
                
                System.out.println(baris);
                i++;
                if (i==lines){
                    System.exit(0);
                }
            }
        }
        //if any error on fetching the file, print here
        catch(IOException e){
            System.out.println("Ralat "+e.toString());
        }
    }

}

STEP 2: CREATE THE INPUT TEXT

input.txt

Before you can start having fun programming in Java, 
you need to download
and install 
the Java SE Development Kit. 
The JDK is the most important Java download 
you can make because once it's installed,
 your computer will be able 
to understand and execute 
the programs you create.

Save these files in a folder in your storage, for example C:\kamarul

Figure 5

STEP 3: COMPILE

Launch the command prompt by typing cmd in the Start Button search.

Figure 6

And you will see this black screen.

Figure 7

Type cd\ <Enter> to get back to C:\ prompt (refer Figure 7).

One you get to C:\ , go inside your folder (which is kamarul) by typing cd kamarul <Enter> (as in Figure 8).

Figure 8

Run dir to check the content of your folder (as in Figure 9).

Figure 9

To compile Head.java, type

javac Head.java

and <Enter> (Figure 10).

Figure 10

Run dir, to list all files. Now you can see Head.class . This Java bytecode file is produced from the compilation proses using JAVAC (Figure 11).

Figure 11

STEP 4: EXECUTE

Now use JAVA to execute the Head.class (this file contain the Java bytecode).

java Head 3 info.txt

The sample output are displayed in Figure 12 and 13.

Figure 12

Figure 13