How to display flat data structure into hierarchical data structure (Java)?

Question

I have recently faced this question in a practical test for a job .

Suppose you are given a flat data structure like this :

**Category**         **Name**         **Parent**
1                   electronics          0
2                   Television           1
3                    21inch              2
4                    23inch              2
5                   LCD display          2
6                   player               1
7                   mp3player            6
8                   vcd player           6
9                   dvd player           6
10                  hd quality           8

Now from the above flat data structure we want to show something like the below hierarchical tree structure .

 -Electronics
|   -Television
|   |   -21 inch
|   |   -23 inch
|   |   -lcd display
|   -Player
|   |   -mp3player
|   |   -vcdplayer
|   |   | -HD display
|   |   -DVD player

Then If I am adding another entry to my array like :

11                 Test               3

then it should show Test entry just below 21inch .

So for this sort of thing I am currently using ArrayList and have been able to traverse till second level but can't do so for third level . So what is the perfect way for doing this ?

Thanks

EDIT :

I was asked to build this concept using DOS based Java application only.

Answer 1

Here is some sample code that lists them in a hierarchy using recursion. The Item class has a List of children. The trick is adding any new children to the right parent. Here is the method I created to do this:

public Item getItemWithParent(int parentID){
    Item result = null;
    if(this.categoryID == parentID){
        result = this;
    } else {
        for(Item nextChild : children){
            result = nextChild.getItemWithParent(parentID);
            if(result != null){
                break;
            }
        }
    }
    return result;
}

There is probably a more efficient way, but this works.

Then, when you want to add new items to your hierarchy, do something like this:

public void addItem(int categoryID, String name, int parentID) {
    Item parentItem = findParent(parentID);
    parentItem.addChild(new Item(categoryID, name, parentID));
}
private Item findParent(int parentID) {
    return rootNode.getItemWithParent(parentID);
}

For the actual display, I just pass in a "tab level" that says how far to tab in, then increment it for each child like this:

public String toStringHierarchy(int tabLevel){
    StringBuilder builder = new StringBuilder();
    for(int i = 0; i < tabLevel; i++){
        builder.append("\t");
    }
    builder.append("-" + name);
    builder.append("\n");
    for(Item nextChild : children){
        builder.append(nextChild.toStringHierarchy(tabLevel + 1));
    }
    return builder.toString();
}

Which gives me this:

-electronics
    -Television
        -21inch
            -Test
        -23inch
        -LCD display
    -player
        -mp3player
        -vcd player
            -hd quality
        -dvd player

Answer 2

You could have a design inspired by Swing TreeModel.

EDIT When I say that, I mean you could use a class implementing a likewise interface; Noticeyou can even go as far as using directly this interface, as Swing is a part of standard JRE and is available everywhere standard Java is avaiable.

Furthermore, as it is an interface (and not a class), it's only a way for you to structure your calls. As a consequence, you can easily use it in a console based application.