Algorithm1.class

Algorithm1.java

package primes;

import java.util.*;

public class Algorithm1 {
    // Find the nth prime number
    public static int nthPrime(int n) {
        // We are at the kth prime number, so far
        int k = 1;

        // We start our counter at 1
        int i = 1;

        // Count up by kth primes until we've reached
        // the nth prime.
        while (k <= n) {
            i += 1;
            if (isPrime(i)) {
                k += 1;
            }
        }

        return i;
    }

    // Check if the integer i is prime by computing the
    // remainder of i against all the numbers less than i.
    //
    // Very brute force.
    public static boolean isPrime(int i) {
        for (int j = 2; j < i; j++) {
            if (i % j == 0) {
                return false;
            }
        }

        return true;
    }
}

Algorithm2.class

Algorithm2.java

package primes;

import java.util.*;

public class Algorithm2 {
    static ArrayList<Integer> primes;

    public static void main(String[] args) {
        int n = Integer.parseInt(args[0]);
        System.out.println(nthPrime(n));
    }

    // Find the nth prime number
    public static int nthPrime(int n) {
        // Initialize our list of primes
        primes = new ArrayList<Integer>();

        // We are at the kth prime number, so far
        int k = 1;

        // We start our counter at 1
        int i = 1;

        // Count up by kth primes until we've reached
        // the nth prime.
        while (k <= n) {
            i += 1;
            if (isPrime(i)) {
                k += 1;
            }
        }

        return i;
    }

    // A better way to check for primes by keeping track of
    // previously found primes.
    public static boolean isPrime(int i) {
        int j = 0;

        while (j < primes.size() && primes.get(j) < i) {
            if (i % primes.get(j) == 0) {
                return false;
            }
            j += 1;
        }

        // Once we find a prime, add it to the list
        primes.add(i);
        return true;
    }

    public void testPrimes() {
        // ...
    }
}

Algorithm3.class

Algorithm3.java

package primes;

import java.util.*;

public class Algorithm3 {
    static ArrayList<Integer> primes;

    public static void main(String[] args) {
        int n = Integer.parseInt(args[0]);
        System.out.println(nthPrime(n));
    }

    // Find the nth prime number
    public static int nthPrime(int n) {
        // Initialize our list of primes
        primes = new ArrayList<Integer>();

        // We are at the kth prime number, so far
        int k = 1;

        // We start our counter at 1
        int i = 1;

        // Count up by kth primes until we've reached
        // the nth prime.
        while (k <= n) {
            i += 1;
            if (isPrime(i)) {
                k += 1;
            }
        }

        return i;
    }

    // A better way to check for primes by keeping track of
    // previously found primes.
    public static boolean isPrime(int i) {
        int j = 0;

        // We only need to check the primes less than or equal to the sqrt(i)
        while (j < primes.size() && primes.get(j) <= Math.sqrt(i)) {
            if (i % primes.get(j) == 0) {
                return false;
            }
            j += 1;
        }

        primes.add(i);
        return true;
    }
}

build.xml

<project name="PrimeNumberAlgos" default="dist" basedir=".">
  <description>
    A few prime number algorithms written in Java.
  </description>

  <!-- set global properties for this build -->
  <property name="src"     location="src"/>
  <property name="lib"     location="lib"/>
  <property name="build"   location="build"/>
  <property name="dist"    location="dist"/>

  <path id="classpath.base" />
  <path id="classpath.test">
    <pathelement location="/usr/share/java/junit.jar" />
    <pathelement location="${build}" />
    <pathelement location="${src}" />
    <path refid="classpath.base" />
  </path>

  <target name="init" depends="clean">
    <tstamp/>
    <mkdir dir="${build}"/>
  </target>

  <target name="compile" depends="init" description="compile the source">
    <javac srcdir="${src}" destdir="${build}"/>
  </target>

  <target name="dist" depends="compile" description="generate the distribution">
    <mkdir dir="${dist}/lib"/>
    <jar jarfile="${dist}/lib/PrimeNumberAlgos-${DSTAMP}.jar" basedir="${build}"/>
  </target>

  <target name="test" depends="compile" description="run the unit tests">
    <junit>
      <classpath refid="classpath.test" />
      <formatter type="brief" usefile="false" />
      <test name="primes.TestAlgos" />
    </junit>
  </target>

  <target name="clean" description="clean up" >
    <delete dir="${build}"/>
    <delete dir="${dist}"/>
  </target>
</project>

JUnitTestExample.class

JUnitTestExample.java

import junit.framework.*;

public class JUnitTestExample extends TestCase{
  public void testCase1(){
  assertTrue( "TestExample", true );
  }
}

Main.class

Main.java

package primes;

import java.util.*;

public class Main {
    public static void main(String[] args) {
        // Do stuff here..
    }
}

PrimeNumber2.class

PrimeNumber2.java

import java.util.*;

import org.junit.Test;
import static org.junit.Assert.*;

public class PrimeNumber2 {
    static ArrayList<Integer> primes;

    // Find the nth prime number
    public static int nthPrime(int n) {
        // Initialize our list of primes
        primes = new ArrayList<Integer>();

        // We are at the kth prime number, so far
        int k = 1;

        // We start our counter at 1
        int i = 1;

        // Count up by kth primes until we've reached
        // the nth prime.
        while (k <= n) {
            i += 1;
            if (isPrime(i)) {
                k += 1;
            }
        }

        return i;
    }

    // A better way to check for primes by keeping track of
    // previously found primes.
    public static boolean isPrime(int i) {
        int j = 0;

        while (j < primes.size() && primes.get(j) < i) {
            if (i % primes.get(j) == 0) {
                return false;
            }
            j += 1;
        }

        // Once we find a prime, add it to the list
        primes.add(i);
        return true;
    }
}

TestAlgos.java

package primes;

import junit.framework.*;

public class TestAlgos extends TestCase {

    public static int primes[] = {1,2,3,5,7,11,13,17,19,23,29,31,37,
                                  41,43,47,53,59,61,67,71,73,79,83,89,
                                  97,101,103,107,109,113,127,131,137,
                                  139,149,151,157,163,167,173,179,181,
                                  191,193,197,199,211,223,227,229,233,
                                  239,241,251,257,263,269,271,277,281,
                                  283,293,307,311,313,317,331,337,347,
                                  349,353,359,367,373,379,383,389,397,
                                  401,409,419,421,431,433,439,443,449,
                                  457,461,463,467,479,487,491,499,503,
                                  509,521,523,541};


    // Test the first 100 primes, including the 0th prime being 1
    public void testAlgo1() {
        for (int i = 0; i < 100; i++) {
            assertEquals("Check prime " + i, primes[i], Algorithm1.nthPrime(i));
        }
    }

    public void testAlgo2() {
        for (int i = 0; i < 100; i++) {
            assertEquals("Check prime " + i, primes[i], Algorithm2.nthPrime(i));
        }
    }

    public void testAlgo3() {
        for (int i = 0; i < 100; i++) {
            assertEquals("Check prime " + i, primes[i], Algorithm3.nthPrime(i));
        }
    }
}