How can I modify my Java ray casting algorithm to create walls with variable heights in my 'Windows' maze' project?

Sure, here is the Ray Casting with different height size article:

Ray Casting with different height size

Problem

I have a java project who makes the "windows' maze" and use the ray casting algorithm. Here's a screenshot :

Like you can see all the walls have the same height size. I would like to do the same but with different height size

private void castRay(int xOnScreen,double angle,double direction) {
    R rx = castRayInX(angle,direction);
    R ry = castRayInY(angle,direction);
    // In case of out-of-space rays
    if (rx.getDistance()==Double.MAX_VALUE && ry.getDistance()==Double.MAX_VALUE) {
        graphics.setColor(BACKGROUND);
        graphics.drawLine(xOnScreen,0,xOnScreen,this.image.getHeight());
        return;
    }
    double distance = rx.getDistance();
    double normal = rx.getNormal();
    Color c = rx.getColor();
    double coef = Math.cos((angle+direction+Math.PI)-normal);
    Plot collision = rx.getPlot();

    if (ry.getDistance()<rx.getDistance()) {
        distance = ry.getDistance();
        normal = ry.getNormal();
        c = ry.getColor();
        coef = Math.cos((angle+direction+Math.PI)-normal);
        collision = ry.getPlot();
    }

    coef = Math.abs(coef);
    int factor = map.length*SQUARE_SIZE;
    double d = (double)(distance+factor)/factor;
    coef *= 1/(d*d);
    Color c2 = new Color((int)(c.getRed()*coef),(int)(c.getGreen()*coef),(int)(c.getBlue()*coef));
    graphics.setColor(c2);
//  graphics.setColor(c);  // no illumination
    distance *= Math.cos(angle); // lens correction
    int h = (int)(this.screenDistance/distance*WALL_HEIGHT); // perspective height
    int vh = this.image.getHeight();
    graphics.drawLine(xOnScreen,(vh-h)/2,xOnScreen,(vh+h)/2);
    drawEye(direction,collision);
}

private R castRayInX(double angleRay,double direction) {
    double angle = angleRay+direction;
    double x1 = eye.getX()+SQUARE_SIZE*Math.cos(angle);
    double y1 = eye.getY()+SQUARE_SIZE*Math.sin(angle);
    double slope = (y1-eye.getY())/(x1-eye.getX());
    if (Math.cos(angle)==0) {
        if (Math.sin(angle)>0)
            return new R(Double.MAX_VALUE,3*Math.PI/2,BACKGROUND,null);
        else
            return new R(Double.MAX_VALUE,Math.PI/2,BACKGROUND,null);
    }
    if (Math.cos(angle)>0) {
        int firstX = ((eye.getX()/SQUARE_SIZE)+1)*SQUARE_SIZE;
        R r = new R(Double.MAX_VALUE,angle+Math.PI,BACKGROUND,null);
        for (int x = firstX; x<map[0].length*SQUARE_SIZE; x += SQUARE_SIZE) {
            int y = (int)(slope*(x-eye.getX())+eye.getY());
            if (isOutside(x,y,Color.MAGENTA,this.showRayCastingX)) break;
            Color c = colorAt(x,y);
            if (c==null) c = colorAt(x,y-1);
            if (c==null) c = colorAt(x-1,y);
            if (c==null) c = colorAt(x-1,y-1);
            if (c!=null) {
                int DX = x-eye.getX();
                double DY = y-eye.getY();
                return new R(Math.sqrt(DX*DX+DY*DY),Math.PI,c,new Plot((int)x,(int)y, WALL_HEIGHT));
            }
        }
        return r;
    } else {
        int firstX = ((eye.getX()/SQUARE_SIZE))*SQUARE_SIZE;
        R r = new R(Double.MAX_VALUE,angle+Math.PI,BACKGROUND,null);
        for (int x = firstX; x>=0; x -= SQUARE_SIZE) {
            int y = (int)(slope*(x-eye.getX())+eye.getY());
            if (isOutside(x,y,Color.MAGENTA,this.showRayCastingX)) break;
            Color c = colorAt(x,y);
            if (c==null) c = colorAt(x,y-1);
            if (c==null) c = colorAt(x-1,y);
            if (c==null) c = colorAt(x-1,y-1);
            if (c!=null) {
                int DX = x-eye.getX();
                double DY = y-eye.getY();
                return new R(Math.sqrt(DX*DX+DY*DY),0,c,new Plot((int)x,(int)y, WALL_HEIGHT));
            }
        }
        return r;           
    }
}
private R castRayInY(double angleRay,double direction) {
//  System.out.println("cast ray 2 Y "+angleRay+" "+direction);
    double angle = angleRay+direction;
    double x1 = eye.getX()+SQUARE_SIZE*Math.cos(angle);
    double y1 = eye.getY()+SQUARE_SIZE*Math.sin(angle);
//  System.out.println(eye+" "+x1+" "+y1);
    double slope = (y1-eye.getY())/(x1-eye.getX());
    if (Math.sin(angle)==0) {
        if (Math.cos(angle)>0)
            return new R(Double.MAX_VALUE,Math.PI,BACKGROUND,null);
        else
            return new R(Double.MAX_VALUE,0,BACKGROUND,null);
    }
    if (Math.sin(angle)>0) {
        int firstY = ((eye.getY()/SQUARE_SIZE)+1)*SQUARE_SIZE;
        R r = new R(Double.MAX_VALUE,angle+Math.PI,BACKGROUND,null);
        for (int y = firstY; y<map.length*SQUARE_SIZE; y += SQUARE_SIZE) {
            int x = (int)((y-eye.getY())/slope)+eye.getX();
            if (isOutside(x,y,Color.CYAN,this.showRayCastingY)) break;
            Color c = colorAt(x,y);
            if (c==null) c = colorAt(x,y-1);
            if (c==null) c = colorAt(x-1,y);
            if (c==null) c = colorAt(x-1,y-1);
            if (c!=null) {
                double DX = x-eye.getX();
                int DY = y-eye.getY();
                return new R(Math.sqrt(DX*DX+DY*DY),3*Math.PI/2,c,new Plot((int)x,(int)y, WALL_HEIGHT));
                }
            }
            return r;
        } else {
            int firstY = ((eye.getY()/SQUARE_SIZE))*SQUARE_SIZE;
            R r = new R(Double.MAX_VALUE,angle+Math.PI,BACKGROUND,null);
            for (int y = firstY; y>=0; y -= SQUARE_SIZE) {
                int x = (int)((y-eye.getY())/slope)+eye.getX();
                if (isOutside(x,y,Color.CYAN,this.showRayCastingY)) break;
                Color c = colorAt(x,y);
                if (c==null) c = colorAt(x,y-1);
                if (c==null) c = colorAt(x-1,y);
                if (c==null) c = colorAt(x-1,y-1);
                if (c!=null) {
                    double DX = x-eye.getX();
                    int DY = y-eye.getY();
                    return new R(Math.sqrt(DX*DX+DY*DY),Math.PI/2,c,new Plot((int)x,(int)y, WALL_HEIGHT));
                }
            }
            return r;           
        }
    }

**Answer** In order to add variable height, do these:

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