java - How to control indivdual particles in a chain in JBox2D -

i have series of particles chained vertically using processing , box2d. want move particles in controlled way follows:

rather continuous sinusoidal wave along particles, able simple move top 5 particles in , out along x-axis , maybe 5 particles somewhere else (as shown in diagram). dont want move whole chain when this:

i don't seem able control of particles in order this. have far:

import shiffman.box2d.*; import org.jbox2d.common.*; import org.jbox2d.dynamics.joints.*; import org.jbox2d.collision.shapes.*; import org.jbox2d.collision.shapes.shape; import org.jbox2d.common.*; import org.jbox2d.dynamics.*; import org.jbox2d.dynamics.contacts.*;  // reference our box2d world box2dprocessing box2d;  // object describe bridget (a list of particles joint connections) bridge bridge; void setup() {   size(1040,1060);   smooth();   // initialize box2d physics , create world   box2d = new box2dprocessing(this);   box2d.createworld();    // make bridge   //total length,number of points,x start position   bridge = new bridge(width/2,width/10,20);  } void draw() {   background(255);   box2d.step();   bridge.display();      (int = 0; < 50; i++) {        (float x = 0; x < 500; x++){          vec2 pos = box2d.getbodypixelcoord(bridge.particles.get(i).body);          bridge.particles.get(i).body.getposition().set(pos.mul(sin(x) * 10));       }     }   fill(0); }  class bridge {    // bridge properties   float totallength;  // how long   int numpoints;      // how many points    // our chain list of particles   arraylist<particle> particles;    // chain constructor   bridge(float l, int n,int start) {      totallength = l;     numpoints = n;      particles = new arraylist();       float len = totallength / numpoints;      // go through , add particles chain     for(int i=0; < numpoints+1; i++) {       // make new particle       particle p = null;            if (i == 0) p = new particle(start,i*len,4,true);       else if (i == numpoints) p = new particle(start,i*len,4,true);       else p = new particle(start,i*len,4,false);       particles.add(p);         // connect particles distance joint       if (i > 0) {          distancejointdef djd = new distancejointdef();          particle previous = particles.get(i-1);          // connection between previous particle , 1          djd.bodya = previous.body;          djd.bodyb = p.body;          // equilibrium length          djd.length = box2d.scalarpixelstoworld(len);          // these properties affect how springy joint           djd.frequencyhz = 0;          djd.dampingratio = 0;          // make joint.          distancejoint dj = (distancejoint) box2d.world.createjoint(djd);       }     }   }  class particle {    // need keep track of body , radius   body body;   float r;    color col;    particle(float x, float y, float r_, boolean fixed) {     r = r_;      // define body     bodydef bd = new bodydef();     if (fixed) bd.type = bodytype.static;     else bd.type = bodytype.static;      // set position     bd.position = box2d.coordpixelstoworld(x,y);     body = box2d.world.createbody(bd);      // make body's shape circle     // make body's shape circle     circleshape cs = new circleshape();     cs.m_radius = box2d.scalarpixelstoworld(r);      fixturedef fd = new fixturedef();     fd.shape = cs;     // parameters affect physics     fd.density = 1;     fd.friction = 0.3;     fd.restitution = 0.5;        body.createfixture(fd);     col = color(175);   }      void display() {     // @ each body , screen position     vec2 pos = box2d.getbodypixelcoord(body);     // angle of rotation     float = body.getangle();     pushmatrix();     translate(pos.x,pos.y);     rotate(a);     fill(col);     stroke(0);     strokeweight(1);     ellipse(0,0,r*2,r*2);     // let's add line can see rotation     line(0,0,r,0);     popmatrix();   } }        // draw bridge       void display() {         (particle p: particles) {           p.display();         }       }      }