Comparing two different objects using contains(Object o) returns false, when equals(Object o) returns true

[FIXED PROBLEM BELOW]

I have this class:

Class Doc() {
  private String D;

  Doc(String d) {
    D = d;
  }

  @Override
  public boolean equals(Object o) {
    if (o instanceOf String) {
      if (o.equals(D)) return true;
      else return false;
    } else return false;
  }

  @Override
  public int hashCode() {
    ...
  }
}

If I run a test, initiating a Doc() using e.g. "abc" as parameter and then use Doc().equals("abc") it returns true - as I would expect. If I use:

ArrayList docs = new ArrayList<Doc>();
Doc d = new Doc("123");
Docs.add(d);
System.out.println(docs.contains("123"));

it returns false, and I am quite unsure of why. I have tried adding output to the .equals() method to check if it is used, and it is. The code for the hashCode() is just some bogus (I know I should do something proper, I'm just not sure how) - but I am only ever going to use equals().

I read somewhere that equals() also checks hashCode(), however; if I add a System.out.println(".") as first line in hashCode() it doesn't output anything, so it doesn't seem like it is called. My best guess is that I can't compare two Objects of different class - is this correct? Or am I overlooking something else?

For future reference, this problem was solved using the suggestion of @JonSkeet - code is now:

Class Doc() {
  private String D;
  Doc(String d) {
    D = d;
  }

  public String getD() {
    return D;
  }

  @Override
  public boolean equals(Object o) {
    if (o instanceOf Doc) {
      Doc tmp = (Doc) o;
      if (tmp.getD().equals(D)) return true;
      else return false;
    } else return false;
  }

  @Override
  public int hashCode() {
    ...
  }
}