import java.util.ArrayList;

// show objects in collections do not need to be immutable
public class Test
{
  public int x; // Test instances are mutable
  public static void main(String[] args)
  { // make a list
    ArrayList<Test> list = new ArrayList<>();
    // make some Test instances
    Test obj1 = new Test();
    Test obj2 = new Test();
    Test obj3=new Test();
    // change them
    obj1.x=3;
    obj2.x=4;
    obj3.x=5;
    // put them in the list
    list.add(obj1);
    list.add(obj2);
    list.add(obj3);
    // fetch one
    System.out.println(list.get(2).x); // 5
    // change it
    list.get(2).x=99;
    // fetch it again
    System.out.println(list.get(2).x); // 99

  }
}