Main.java

import java.util.Arrays;
import java.util.List;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
import java.util.function.Supplier;


public class Main {

  volatile Object consumer;
  volatile List<String> list;

  private void run() throws Exception {

    list = Arrays.asList("true", "true", "true");
    Supplier<Void> f1 = () -> {
      exceptionalParse();
      return null;
    };

    Supplier<Void> f2 = () -> {
      iterate();
      return null;
    };

    Supplier<Void> f3 = () -> {
      iterate();
      return null;
    };

    Supplier<Void> f4 = () -> {
      iterate();
      return null;
    };

    Supplier<Void>[] arr = new Supplier[]{f1, f2, f3, f4};

    for (Supplier<Void> supplier : arr) {
      new Thread(() -> {
        while (true) {
          depth(20, supplier);
        }
      }).start();
    }

    Thread.sleep(TimeUnit.MINUTES.toMillis(5));
  }


  private void exceptionalParse() {
    double sum = 0.0;
    for (String s : list) {
      try {
        sum += Double.parseDouble(s);
      }
      catch (NumberFormatException e) {

      }
    }

    consumer = sum;
    if (consumer == null) {
      throw new NullPointerException();
    }
  }

  private void iterate() {
    int size = 0;
    for (String s : list) {
      ++size;
    }

    consumer = size;
    if (consumer == null) {
      throw new NullPointerException();
    }
  }

  private void depth(int n, Supplier<Void> supplier) {
    if (--n != 0) {
      depth(n, supplier);
    }
    else {
      supplier.get();
    }
  }

  public static void main(String[] args) throws Exception {
    new Main().run();
  }

}