Task: Hilbert curve
C++: #include <algorithm>
#include <iostream>
#include <vector>

struct Point {
    int x, y;

    
    void rot(int n, bool rx, bool ry) {
        if (!ry) {
            if (rx) {
                x = (n - 1) - x;
                y = (n - 1) - y;
            }
            std::swap(x, y);
        }
    }
};

Point fromD(int n, int d) {
    Point p = { 0, 0 };
    bool rx, ry;
    int t = d;
    for (int s = 1; s < n; s <<= 1) {
        rx = ((t & 2) != 0);
        ry = (((t ^ (rx ? 1 : 0)) & 1) != 0);
        p.rot(s, rx, ry);
        p.x += (rx ? s : 0);
        p.y += (ry ? s : 0);
        t >>= 2;
    }
    return p;
}

std::vector<Point> getPointsForCurve(int n) {
    std::vector<Point> points;
    for (int d = 0; d < n * n; ++d) {
        points.push_back(fromD(n, d));
    }
    return points;
}

std::vector<std::string> drawCurve(const std::vector<Point> &points, int n) {
    auto canvas = new char *[n];
    for (size_t i = 0; i < n; i++) {
        canvas[i] = new char[n * 3 - 2];
        std::memset(canvas[i], ' ', n * 3 - 2);
    }

    for (int i = 1; i < points.size(); i++) {
        auto lastPoint = points[i - 1];
        auto curPoint = points[i];
        int deltaX = curPoint.x - lastPoint.x;
        int deltaY = curPoint.y - lastPoint.y;
        if (deltaX == 0) {
            
            int row = std::max(curPoint.y, lastPoint.y);
            int col = curPoint.x * 3;
            canvas[row][col] = '|';
        } else {
            
            int row = curPoint.y;
            int col = std::min(curPoint.x, lastPoint.x) * 3 + 1;
            canvas[row][col] = '_';
            canvas[row][col + 1] = '_';
        }
    }

    std::vector<std::string> lines;
    for (size_t i = 0; i < n; i++) {
        std::string temp;
        temp.assign(canvas[i], n * 3 - 2);
        lines.push_back(temp);
    }
    return lines;
}

int main() {
    for (int order = 1; order < 6; order++) {
        int n = 1 << order;
        auto points = getPointsForCurve(n);
        std::cout << "Hilbert curve, order=" << order << '\n';
        auto lines = drawCurve(points, n);
        for (auto &line : lines) {
            std::cout << line << '\n';
        }
        std::cout << '\n';
    }
    return 0;
}

Java: 

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class HilbertCurve {
    public static class Point {
        public int x;
        public int y;
        
        public Point(int x, int y) {
            this.x = x;
            this.y = y;
        }
        
        public String toString() {
            return "(" + x + ", " + y + ")";
        }
        
        
        public void rot(int n, boolean rx, boolean ry) {
            if (!ry) {
                if (rx) {
                    x = (n - 1) - x;
                    y = (n - 1) - y;
                }
        
                
                int t  = x;
                x = y;
                y = t;
            }
            
            return;
        }
        
        public int calcD(int n) {
            boolean rx, ry;
            int d = 0;
            for (int s = n >>> 1; s > 0; s >>>= 1) {
                rx = ((x & s) != 0);
                ry = ((y & s) != 0);
                d += s * s * ((rx ? 3 : 0) ^ (ry ? 1 : 0));
                rot(s, rx, ry);
            }
            
            return d;
        }
        
    }

    public static Point fromD(int n, int d) {
        Point p = new Point(0, 0);
        boolean rx, ry;
        int t = d;
        for (int s = 1; s < n; s <<= 1) {
            rx = ((t & 2) != 0);
            ry = (((t ^ (rx ? 1 : 0)) & 1) != 0);
            p.rot(s, rx, ry);
            p.x += (rx ? s : 0);
            p.y += (ry ? s : 0);
            t >>>= 2;
        }
        return p;
    }
    
    public static List<Point> getPointsForCurve(int n) {
        List<Point> points = new ArrayList<Point>();
        for (int d = 0; d < (n * n); d++) {
            Point p = fromD(n, d);
            points.add(p);
        }
        
        return points;
    }
    
    public static List<String> drawCurve(List<Point> points, int n) {
        char[][] canvas = new char[n][n * 3 - 2];
        for (char[] line : canvas) {
            Arrays.fill(line, ' ');
        }
        for (int i = 1; i < points.size(); i++) {
             Point lastPoint = points.get(i - 1);
            Point curPoint = points.get(i);
            int deltaX = curPoint.x - lastPoint.x;
            int deltaY = curPoint.y - lastPoint.y;
            if (deltaX == 0) {
                if (deltaY == 0) {
                    
                    throw new IllegalStateException("Duplicate point, deltaX=" + deltaX + ", deltaY=" + deltaY);
                }
                
                int row = Math.max(curPoint.y, lastPoint.y);
                int col = curPoint.x * 3;
                canvas[row][col] = '|';
            }
            else {
                if (deltaY != 0) {
                    
                    throw new IllegalStateException("Diagonal line, deltaX=" + deltaX + ", deltaY=" + deltaY);
                }
                
                int row = curPoint.y;
                int col = Math.min(curPoint.x, lastPoint.x) * 3 + 1;
                canvas[row][col] = '_';
                canvas[row][col + 1] = '_';
            }
            
        }
        List<String> lines = new ArrayList<String>();
        for (char[] row : canvas) {
            String line = new String(row);
            lines.add(line);
        }
        
        return lines;
    }
    
    public static void main(String... args) {
        for (int order = 1; order <= 5; order++) {
            int n = (1 << order);
            List<Point> points = getPointsForCurve(n);
            System.out.println("Hilbert curve, order=" + order);
            List<String> lines = drawCurve(points, n);
            for (String line : lines) {
                System.out.println(line);
            }
            System.out.println();
        }
        return;
    }
}