I agree with this analysis.

Refactoring for clarity, rather than compactness, as well as formatting,
can help with understanding messy code. I haven't fixed the choices of
identifiers, because I wanted to keep the relationship to the existing code.

import java.util.ArrayList;

public class cc {
  ArrayList arl = new ArrayList();

  boolean flag = false;

  static Thread create(final int i, final Thread t1) {
    Thread t2 = new Thread(new T2(i,t1));
    System.out.println(i + 3);
    t2.start();
    System.out.println(i + 4);
    return t2;
  }

  static class T2 implements Runnable{
    Thread t1;
    int i;
    T2(int i, Thread t1){
      this.t1 = t1;
      this.i = i;
    }
    public void run() {
      System.out.println(i + 1);
      try {
        t1.join();
      } catch (InterruptedException e) {
      }
      System.out.println(i + 2);
    }
  }

  public static void main(String[] args) {
    Thread thread0 = Thread.currentThread();
    Thread thread1 = create(20,thread0);
    Thread thread2 = create(10,thread1);
  }
}