Moving Snake Logic in Processing - processing

Question

I am in the process of making a Snake Game, but I am confused on how to implement the Snake's movement in Processing. I have created a class for the snake, which includes a function for movement that can detect key presses, but I'm stuck on how to actually code the movement of the snake. Can anyone give me a brief explanation on how to implement the Snake movement based on the code below?
int score = 0;
int unit = 20;
PFont courierNew24, courierNew40;
ArrayList unitList;
String direction = "right";
String nextDirection = "";
int directionCount = 0;
class Snake
{
Snake() {
unitList = new ArrayList();
unitList.add(new Unit(4, 3));
unitList.add(new Unit(4, 4));
unitList.add(new Unit(4, 5));
unitList.add(new Unit(4, 6));
unitList.add(new Unit(4, 7));
unitList.add(new Unit(4, 8));
}
void drawSnake()
{
for (int i = 0; i < unitList.size (); i++)
{
Unit snakePiece = (Unit) unitList.get(i);
snakePiece.drawSnakePiece();
}
}
void moveSnake()
{
if (direction != "left" && nextDirection == "right")
{
//Move Snake
}
if (direction != "right" && nextDirection == "left")
{
}
if (direction != "up" && nextDirection == "down")
{
}
if (direction != "down" && nextDirection == "up")
{
}
}
}
class Unit
{
int row, column;
Unit (int unitRow, int unitColumn)
{
row = unitRow;
column = unitColumn;
}
void drawSnakePiece()
{
fill(0, 255, 0);
rect(column*unit, row*unit, unit, unit);
}
void drawApple()
{
fill(255, 0, 0);
ellipse(column*unit+(unit/2), row*unit+(unit/2), unit, unit);
}
void collision(int unitRow, int unitColumn)
{
if (row == unitRow && column == unitColumn)
{
}
}
}
//Functions
void scoreBoard()
{
fill(255);
textFont(courierNew24, 24);
text("Score: " + score, 20, 670);
}
void gameOver()
{
fill(255);
textFont(courierNew40, 40);
textAlign(CENTER, CENTER);
text("Game Over, Score of " + score, 500, 350);
}
void setup()
{
size(1000, 700);
background(0);
courierNew24 = loadFont("CourierNewPSMT-24.vlw");
courierNew40 = loadFont("CourierNewPSMT-40.vlw");
scoreBoard();
}
void draw()
{
smooth();
Snake snake = new Snake();
snake.drawSnake();
snake.moveSnake();
Unit apple = new Unit(10, 10);
apple.drawApple();
}
void keyPressed()
{
switch(key)
{
case 'a':
case 'A':
directionCount += 1;
if (directionCount > 1)
{
direction = nextDirection;
}
nextDirection = "left";
break;
case 'd':
case 'D':
directionCount += 1;
if (directionCount > 1)
{
direction = nextDirection;
}
nextDirection = "right";
break;
case 'w':
case 'W':
directionCount += 1;
if (directionCount > 1)
{
direction = nextDirection;
}
nextDirection = "up";
break;
case 's':
case 'S':
directionCount += 1;
if (directionCount > 1)
{
direction = nextDirection;
}
nextDirection = "down";
break;
}
}

Answer 1

A brief explanation:
You hold all snake units in a list - that's already done. There are head and tail which are the first and the last elements of the list. So it is actually a queue.
On each tick, determine the direction in which you should move. For example, if the direction is left, then next head coordinates will be at (-1,0) relative to current head.
Insert new unit in the list at the head position with the coordinates determined in step 2.
Remove the tail unit from the list (and from the screen).
That will arrange the movement. If you find an apple at the head position, initialize a growth counter. On each tick, if growth_counter > 0, decrease it and skip the tail unit removal. Thus, only head will move until it has grown.