Processing provides a method named line() to draw a line on the screen. This code draws a white 10 pixel line on black background.

void setup() {
    size(500, 500);
    background(0);
    stroke(255);
    strokeWeight(10);
}

void draw() {
    line(0, 0, 500, 500);
}

The signature of method line() is this.

line(x1, y1, x2, y2);

x1 and y1 is a coordinate of the starting point. x2 and y2 is a coordinate of the ending point.

Method stroke() is used to specify the color of the line you will draw.

Method strokeWeight() is used to specify the thickness of the line you will draw. (in pixels)

Processing provides method rect() to draw a rectangle. This code draws a white 50 X 50 rectangle on black background.

void setup() {
    size(500, 500);
    background(0);
    fill(255);
    noStroke();
}

void draw() {
    rect(225, 225, 50, 50);
}

The signature of method rect() is this.

rect(x, y, w, h);

x and y is the coordinate of the rectangle. w and h is rectangle's width and height.

Method fill() is used to specify the filling color of the rectangle and other shapes such as ellipse, triangle, polygon.

Method noStroke() is used to specify that that there are no strokes around the rectangle. This method also affects other shapes such as ellipse, triangle, polygon.

Processing provides method ellipse in order to draw ellipse. This code draws a white circle which has radius of 25 pixels.

void setup() {
    size(500, 500);
    background(0);
    fill(255);
    noStroke();
}

void draw() {
    ellipse(225, 225, 50, 50);
}

The signature of method ellipse() is this.

ellipse(x, y, w, h);

x and y is the coordinate of the ellipse. w and h is ellipse's width and height.

Processing provides the method triangle in order to draw a triangle. The code below draws a nearly equilateral triangle of 25 pixels between each defining point.

void setup() {
    size(500, 500);
    background(0);
}
void draw() {
    triangle(0, 0, 25, 0, 12, 12);
}

The signature of triangle is as so:

triangle(x1, y1, x2, y2, x3, y3);

Each x point corresponds to the point's x axis, and y to the y axis. The three points will be joined to form a triangle.

Processing provides method triangle() to draw a triangle. This code draws a white triangle on black background.

void setup() {
    size(500, 500);
    background(0);
    fill(255);
    noStroke();
}

void draw() {
    triangle(250, 225, 225, 275, 275, 275);
}