もっといい方法があるような気はします。
class Stock {
    private int maxPrice;
    private int minPrice;

    Stock(int maxPrice, int minPrice) {
        this.maxPrice = maxPrice;
        this.minPrice = minPrice;
    }

    public int getMaxPrice() {
        return this.maxPrice;
    }

    public int getMinPrice() {
        return this.minPrice;
    }
}

public class Main {
    public static void main(String[] args) {
        List<Stock> stocks = new ArrayList<>();
        ...
        
        int maxPrice = stocks.stream()
                .max(Comparator.comparing(Stock::getMaxPrice))
                .get()
                .getMaxPrice();
                        
        int minPrice = stocks.stream()
                .min(Comparator.comparing(Stock::getMinPrice))
                .get()
                .getMinPrice();
    }
}