From Javascript P5 to Processing 3.x

So far my progress:

~~
final int W = 680, H = 200; // dimensions of canvas
final int time = 400; // number of x tick values
final int step = W/time; // time step

float data ; // to store number of infected people
int count = 0; // steps counter
int pos, fy, c, infected, colors, l, f;

void setup() {
size(W, H);
fill(255, 30, 70, 90);

// array containing the x positions of the line graph, scaled to fit the canvas
//posx = Float32Array.from({ length: time }, (_, i) => map(i, 0, time, 0, W));
float posx = new float[time];
for (int i = 0; i < time; posx[i] = map(i++, 0, time, 0, W));

// function to map the number of infected people to a specific height (here the height of the canvas)
fy = _ => map(_, 3, 0, H, 10);

// colors based on height stored in an array list.
colors = d3.range(H).map(i => d3.interpolateWarm(norm(i, 0, H)))

}

I don't know how to translate the last two lines.
I do not understand them:
fy, is declared as in int
but I don't understand the oprarator "_"! which follows after the sign =.
and d3 is declared internally within void? !!