Foo.java

public class Foo {
  
  public int bar;
  
  public Foo(int bar) {
    this.bar = bar;
  }
  
}

Main.java

import java.util.ArrayList;

public class Main {

  public static void main(String[] args) {
    ArrayList<Foo> list = new ArrayList<>();
    Foo foo = new Foo(5);
    
    list.add(foo);
    
    System.out.println(list.get(0));
  }

}