Approximate PI in almost one line with Lambdas in Java 8 :) - Theory here: http://www.stealthcopter.com/blog/2009/09/python-calculating-pi-using-random-numbers/

LambdaPi.java

package de.tutorials.training.java8.lambda;

import java.util.stream.IntStream;

import static java.lang.Math.*;
import static java.util.concurrent.ThreadLocalRandom.*;

/**
 * Author: Thomas Darimont
 */
public class LambdaPi {

    public static void main(String[] args) {
        int n = 9999999;
        double pi = IntStream.range(0, n)
                             .map((a) -> sqrt(pow(current().nextDouble(),2)+pow(current().nextDouble(),2)) <= 1 ? 1 : 0)
                             .parallel()
                             .sum() * 4.0 / n;
        System.out.printf("%s delta-abs: %s",pi, PI-pi);
    }

}

LambdaPi2.java

package de.tutorials.training.java8.lambda;

import java.util.stream.IntStream;

import static java.lang.Math.*;
import static java.util.concurrent.ThreadLocalRandom.*;

/**
 * Author: Thomas Darimont
 */
public class LambdaPi {

    public static void main(String[] args) {
        int n = 9999999;
        double pi = IntStream.range(0, n)
                .mapToDouble((a) -> 4.0 * 1.0 / n * (sqrt(pow(current().nextDouble(),2)+pow(current().nextDouble(),2)) <= 1 ? 1 : 0))
                .parallel()
                .sum();
        System.out.printf("%s delta-abs: %s",pi, PI-pi);
    }
}

LambdaPi3SingleThreaded.java

package de.tutorials.training.java8.lambda;

import java.util.stream.IntStream;

import static java.lang.Math.*;

/**
 * Author: Thomas Darimont
 */
public class LambdaPi {

    public static void main(String[] args) {
        int n = 999999;
        double pi = IntStream.range(0,n)
                             .mapToDouble((a) -> 4.0 * 1.0 / n * (sqrt(pow(random(),2)+pow(random(),2)) <= 1 ? 1 : 0))
                             .sum();
        System.out.printf("%s delta-abs: %s",pi, PI-pi);
    }
}