My solution for https://dzone.com/articles/java-code-challenge-play-the-stock-market

StockMarket.java

package software.schwering.javacodechallenge;

import java.math.BigDecimal;
import java.util.List;
import java.util.SortedSet;
import java.util.TreeSet;
import java.util.regex.Pattern;
import java.util.stream.Collectors;

public class StockMarket {
    private final BigDecimal[] prices;

    public StockMarket(List<BigDecimal> prices) {
        this.prices = prices.toArray(new BigDecimal[prices.size()]);
    }

    public StockMarket(String s) {
        this(Pattern.compile(" ").splitAsStream(s).map(BigDecimal::new).collect(Collectors.toList()));
    }

    public static void main(String... args) {
        System.out.println(new StockMarket(args[0]).bestDeal());
    }

    public PriceRange bestDeal() {
        return findPossibleDeals().last();
    }

    SortedSet<PriceRange> findPossibleDeals() {
        TreeSet<PriceRange> priceRanges = new TreeSet<>();
        for (int i = 0; i < prices.length - 2; i++) {
            for (int j = i + 2; j < prices.length; j++) {
                if (prices[j].compareTo(prices[i]) > 0) {
                    priceRanges.add(new PriceRange(prices[i], prices[j]));
                }
            }
        }
        return priceRanges;
    }

    public static class PriceRange implements Comparable<PriceRange> {
        private final BigDecimal buyPrice;
        private final BigDecimal sellPrice;

        public PriceRange(BigDecimal buyPrice, BigDecimal sellPrice) {
            this.buyPrice = buyPrice;
            this.sellPrice = sellPrice;
        }

        public BigDecimal getGain() {
            return sellPrice.subtract(buyPrice);
        }

        @Override
        public int compareTo(PriceRange o) {
            return getGain().compareTo(o.getGain());
        }

        @Override
        public String toString() {
            return buyPrice.toString() + ' ' + sellPrice.toString();
        }

        @Override
        public int hashCode() {
            return buyPrice.hashCode() * 31 + sellPrice.hashCode();
        }

        @Override
        public boolean equals(Object obj) {
            if (!(obj instanceof PriceRange)) {
                return false;
            }
            PriceRange other = ((PriceRange) obj);
            return buyPrice.equals(other.buyPrice) && sellPrice.equals(other.sellPrice);
        }
    }
}

StockMarketTest.java

package software.schwering.javacodechallenge;

import org.junit.Test;

import java.math.BigDecimal;

import static org.hamcrest.CoreMatchers.equalTo;
import static org.hamcrest.CoreMatchers.hasItems;
import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;

public class StockMarketTest {

    @Test
    public void findPossibleDeals() {
        assertThat(new StockMarket("1.0 2.0 0.5 3.0").findPossibleDeals(), hasItems(r("2.0", "3.0"), r("1.0", "3.0")));
    }

    private static StockMarket.PriceRange r(String buy, String sell) {
        return new StockMarket.PriceRange(new BigDecimal(buy), new BigDecimal(sell));
    }

    @Test
    public void findPossibleDeals_emptyIfNotEnoughPricesWithAtLeastOneTickDistance() {
        assertThat(new StockMarket("1.0 2.0").findPossibleDeals(), hasItems());
    }

    @Test
    public void findPossibleDeals_emptyIfOnlyBadDeals() {
        assertThat(new StockMarket("5.0 4.0 3.0 2.0 1.0").findPossibleDeals(), hasItems());
    }

    @Test
    public void bestDeal() {
        StockMarket stockMarket = new StockMarket("9.20 8.03 10.02 8.08 8.14 8.10 8.31 8.28 8.35 8.34 8.39 8.45 8.38 8.38 8.32 8.36 8.28 8.28 8.38 8.48 8.49 8.54 8.73 8.72 8.76 8.74 8.87 8.82 8.81 8.82 8.85 8.85 8.86 8.63 8.70 8.68 8.72 8.77 8.69 8.65 8.70 8.98 8.98 8.87 8.71 9.17 9.34 9.28 8.98 9.02 9.16 9.15 9.07 9.14 9.13 9.10 9.16 9.06 9.10 9.15 9.11 8.72 8.86 8.83 8.70 8.69 8.73 8.73 8.67 8.70 8.69 8.81 8.82 8.83 8.91 8.80 8.97 8.86 8.81 8.87 8.82 8.78 8.82 8.77 8.54 8.32 8.33 8.32 8.51 8.53 8.52 8.41 8.55 8.31 8.38 8.34 8.34 8.19 8.17 8.16");
        StockMarket.PriceRange bestDeal = stockMarket.bestDeal();
        assertThat(bestDeal.toString(), is(equalTo("8.03 9.34")));
    }

}