About steering force,I'm studying The Nature of Code by Daniel Shiffman.Why does the code run with nothing?

Processing Coding Questions
1 
class Vehicle{
  PVector location;
  PVector velocity;
  PVector acceleration;
  float r;
  
  float maxforce;
  float maxspeed;
  
  Vehicle(float x,float y){
    acceleration = new PVector(0,0);
    velocity = new PVector(0,0);
    location = new PVector(x,y);
    r = 3.0;
    maxspeed = 4;
    maxforce = 0.1;
  }
  
  void update(){
    velocity.add(acceleration);
    velocity.limit(maxspeed);
    location.add(velocity);
    acceleration.mult(0);
  }
  
  void applyForce(PVector force){
    acceleration.add(force);
  }
  
  void seek(PVector target){
    PVector desired = PVector.sub(target,location);
    desired.normalize();
    desired.mult(maxspeed);
    PVector steer = PVector.sub(desired,velocity);
    steer.limit(maxforce);
    applyForce(steer);
  }

  
  void display(){
    float theta = velocity.heading2D()+ PI/2;
    fill(175);
    stroke(0);
    pushMatrix();
    translate(location.x,location.y);
    rotate(theta);
    beginShape();
    vertex(0,-r*2);
    vertex(-r,r*2);
    vertex(r,r*2);
    endShape(CLOSE);
    popMatrix();
  }
}

code
code1915×1018 89.3 KB

2
3

Hell @jiajia,

When you compile a blank sketch this is what you get and it will run once:

/* autogenerated by Processing revision 1292 on 2023-04-25 */
import processing.core.*;
import processing.data.*;
import processing.event.*;
import processing.opengl.*;

import java.util.HashMap;
import java.util.ArrayList;
import java.io.File;
import java.io.BufferedReader;
import java.io.PrintWriter;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;

public class sketch_230425a extends PApplet {

 public void setup() {


   noLoop();
 }

 static public void main(String[] passedArgs) {
   String[] appletArgs = new String[] { "sketch_230425a" };
   if (passedArgs != null) {
     PApplet.main(concat(appletArgs, passedArgs));
   } else {
     PApplet.main(appletArgs);
   }
 }
}

You can find the above in the %temp%/processing folder on W10.

When you compile a sketch with only a class it will put your class inside of the setup() and compile that.

This may provide some insight:

You should be using a class in Processing with setup() and draw() in most cases:

:)