How To: SPOJ TEST Using JAR File

September 12, 2010 at 07:39 (java, tutorial)

Background: Sphere Online Judge (SPOJ) is an online judge system accepting a wide variety of programming languages. It’s a great way to test your skills as a programmer, and can be quite addictive if you like a good challenge.

While browsing the SPOJ forums, I did not find a thread detailing how to successfully submit a JAR for the first problem, TEST. So, here’s a quick guide. I’m using Windows, but there should be little difference with other operating systems since everything is done through the command line. I will assume you already have the JDK installed and paths set up such that if you type java, javac, or jar from the command line the appropriate programs will be executed.

First, a simple Java implementation:

import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main (String[] args) throws Exception {
		BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
		String s;
		while (!(s=r.readLine()).startsWith("42")) System.out.println(s);
	}
}

Note that since all input numbers are one or two digits, it’s all right to use startsWith(). Also, it seems the import statements cause a small drop in performance, but this is easier to read than specifying the package names in the main program.

Now, the simplest way to get AC with JAR is to save this code to a file Main.java. Then using the command line, go to the directory where your put your file and type

javac Main.java

as usual. Then in the same directory create a file manifest.txt (the name is arbitrary) containing this text:

Main-Class: Main

and make sure the file is terminated with a newline. Then from the command line type

jar cfm Main.jar manifest.txt Main.class

The .jar file is now complete, and if you’d like to execute it you can type

java -jar Main.jar

From Windows/DOS you can pipe input/output using files in the usual manner as follows

java -jar Main.jar < in.txt > out.txt

More information about JAR files can be found on Oracle’s Java Tutorials.