HelloWorld ofSketch Gist.

Ball.sketch

class Ball{
    
public:

    ofVec2f position;
    ofVec2f velocity;
    
    int radius;
    
    ofColor color;
    
    Ball() {
        
        // Choose a random radius.
        radius = ofRandom(5, 10);
        
        // Choose a random position.
        position = ofVec2f(ofRandom(radius, ofGetWidth() - radius), 
                           ofRandom(radius, ofGetHeight() - radius));
        
        // Choose a random velocity.
        velocity = ofVec2f(ofRandom(-5, 5), ofRandom(-5, 5));
      
        // Choose a random color between red and yellow.
        color = ofColor(255, ofRandom(255), 0, 255);
    
    }
    
    void update() {
        
        // Update the x velocity.
        if (position.x + radius >= ofGetWidth() ||
            position.x - radius <= 0) {
            velocity.x *= -1;
        }
        
        // Update the y velocity.
        if (position.y + radius >= ofGetHeight() ||
            position.y - radius <= 0) {
            velocity.y *= -1;
        }
        
        // Update the position.
        position += velocity;
    
    }

    void draw() {
        ofFill();
    
        // Set the fill color for this ball.
        ofSetColor(color.r, color.g, color.b, 127);
        
        // Draw the ball.
        ofCircle(position, radius);
        
        ofNoFill();
        
        // Set the outline for this ball.
        ofSetColor(color);
        
        // Draw the ball.
        ofCircle(position, radius); 
    
    }

};

HelloWorld.sketch

#include "Ball.h"

int numBalls; // The total number of balls we'll use.

vector<Ball> balls; // A collection of all balls.

void setup() {
    ofSetFrameRate(60);
    
    numBalls = 750; // The total number of balls we'll use.
    
    // Enable vertical sync.
    ofSetVerticalSync(true);
    
    // Set the window size.
    ofSetWindowShape(500, 500);
    
    // Set the background color.
    ofBackground(255);

    // Create the balls
    for (int i = 0; i < numBalls; i++) {
        balls.push_back(Ball());
    }
}

void update() {
    // Update all balls.
    for (int i = 0; i < balls.size(); i++) {
        balls[i].update();
    }
}

void draw() {
    // Draw all balls.
    for (int i = 0; i < balls.size(); i++) {
        balls[i].draw();
    }
    
    // Log a message.
    ofLogNotice("draw()") << ofGetTimestampString();
}