public class Demo {
  public static void main(String[] args){

    Tree sampleTree = new Tree("dog",
                               new Tree("bat",
                                        null,
                                        new Tree("camel", null, null)),
                               new Tree("mouse", null, null));

    // The outermost procedure has a static link pointing at a frame
    //  containing any global variables.
    GlobalFrame globals = new GlobalFrame();

    String result = new PrettyprintFrame(globals).prettyprint(sampleTree);

    System.out.println(result);
  }
}

class Tree {
  String key;
  Tree left, right;
  Tree(String key, Tree left, Tree right){
    this.key = key;
    this.left = left;
    this.right = right;
  }
}

class GlobalFrame {
  // any globals would go in here; Program 6.3 has none
}

class PrettyprintFrame {
  GlobalFrame staticLink;

  String output;  // this is the one escaping variable

  PrettyprintFrame(GlobalFrame staticLink){
    this.staticLink = staticLink;
  }

  String prettyprint(Tree tree){
    output = "";
    new ShowFrame(this).show(0, tree);
    return output;
  }
}

class WriteFrame {
  PrettyprintFrame staticLink;

  // no escaping variables

  WriteFrame(PrettyprintFrame staticLink){
    this.staticLink = staticLink;
  }

  void write(String s){
    staticLink.output = staticLink.output + s;
  }
}

class ShowFrame {
  PrettyprintFrame staticLink;

  int n; // escaping variable

  ShowFrame(PrettyprintFrame staticLink){
    this.staticLink = staticLink;
  }

  void show(int nArg, Tree t){

    n = nArg;  // store in frame since it escapes

    if(t == null){
      new IndentFrame(this).indent(".");
    } else {
      new IndentFrame(this).indent(t.key);
      new ShowFrame(staticLink).show(n+1, t.left);
      new ShowFrame(staticLink).show(n+1, t.right);
    }
  }
}

class IndentFrame {
  ShowFrame staticLink;

  // no escaping variables

  IndentFrame(ShowFrame staticLink){
    this.staticLink = staticLink;
  }

  void indent(String s){
    for(int i = 1; i <= staticLink.n; i++){
      new WriteFrame(staticLink.staticLink).write(" ");
    }
    staticLink.staticLink.output = staticLink.staticLink.output + s;
    new WriteFrame(staticLink.staticLink).write("\n");
  }
}