How to display list data in tree structure in Java?

Question

I have an arrayList and I want to display in a hierarchal structure.

I want the results to look like this. If it doesn't have any child nodes I want the item indented with a hyphen:

User Design
    Lectures
        Week 1
            -Apr 5
            -Apr 8
        Week 2
            -Apr 12
    Activities
        Personas
            Male
                -George
            Female
                -Allison
                -Jessica

I am trying to display the list using a recursive method but not getting the desired results. This is what I've tried so far:

import java.util.*;

class Materials {
    public int id;
    public String content;
    public int pid; 

    public Materials(int id, String content, int pid) {
        this.id = id;
        this.content = content;
        this.pid = pid; 
    }
}
public class nestedList {

public static ArrayList<Materials> material;

public static void main(String[] args) {
    material = new ArrayList<Materials>();
    material.add(new Materials(1,"User Design", 0));
    material.add(new Materials(2,"Lectures", 1));
    material.add(new Materials(3,"Activities", 1));
    material.add(new Materials(4,"Week 1", 2));
    material.add(new Materials(5,"Apr 5", 4));
    material.add(new Materials(6,"Apr 8", 4));
    material.add(new Materials(7,"Week 2", 2));
    material.add(new Materials(8,"Apr 12", 7));
    material.add(new Materials(9,"Personas", 3));
    material.add(new Materials(10,"Male", 9));
    material.add(new Materials(11,"Female", 9));
    material.add(new Materials(12,"George", 10));
    material.add(new Materials(13,"Allison", 11));
    material.add(new Materials(14,"Jessica", 11));

    displayContent(material);

}

static void displayContent(ArrayList<Materials> materials) {
    ArrayList<Materials> childs = new ArrayList<Materials>();
    for (Materials material : materials) {
        childs = selectChild(material.id);
        System.out.println(material.content);
        displayContent(childs);
    }

}

static ArrayList<Materials> selectChild(int id) {
    ArrayList<Materials> list = new ArrayList<Materials>();

    for (int i = 0; i < material.size(); i++) {
        if(material.get(i).pid == id) {
            list.add(material.get(i));
        }
    }

    return list;
}
}

When I run this code the items in the arrayList repeats too many times. It initially displays the correct hierarchal structure and then starts to repeat with random variations.

Can anyone please point me in the right direction?

I am given a list of N materials. What data structure would you recommend that I use? — user2100620
– user2100620, Commented Apr 30, 2018 at 7:42
@Dukeling, thanks for that. The tab count mentioned in my answer can be managed by that answer. — Jay
– Jay, Commented Apr 30, 2018 at 7:53
So what's your question? What is the specific problem you're having? As written, your question is too broad. — Jim Mischel
– Jim Mischel, Commented Apr 30, 2018 at 13:49

Jim Mischel · Accepted Answer · 2018-04-30 17:16:19Z

1

The problem is that in your first call to displayContent, you're passing the entire list of materials. So it's going to do this:

Display the entire hierarchy for "User Design"
Display the entire hierarchy for "Lectures"
Display the entire hierarchy for "Activities"
etc.

What you really want to do is pass only the root element to displayContent in your initial call. So in your main:

ArrayList<Materials> topLevel = selectChild(0);
displayContent(topLevel);

answered Apr 30, 2018 at 17:16

Jim Mischel

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2 Comments

user2100620 Over a year ago

Oh that makes sense! I decided to try a different test case where "User Design" is no longer the root element, but both "Lectures" & "Activities" have a parentID of '0'. How can I display both at the same time without making 2 separate calls to displayContent?

Jim Mischel Over a year ago

The code I show will do that. It gets all elements that have a parentID of 0, using your selectChild function. Just pass that list to the displayContent method.

Jay · Accepted Answer · 2018-04-30 07:56:52Z

1

static void displayContent(ArrayList<Materials> materials) {
    ArrayList<Materials> childs = new ArrayList<Materials>();
    for (Materials material : materials) {
        childs = selectChild(material.id);
        if(childs.isEmpty()) {
            System.out.print(" - ");
        }
        System.out.println(material.content);
        displayContent(childs);
    }

}

Checking if the list is empty before you print the material content will add the - for the last element.

For the systematic tabs, you will have to count the traversals (child of child of child is 3, for example) and add that many tabs. This answer can help with that. Thanks Dukeling for pointing it out.

edited Apr 30, 2018 at 7:56

answered Apr 30, 2018 at 7:51

Jay

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1 Comment

user2100620 Over a year ago

Thank you for that! But I'm still having an issue with the fact that the items in the arrayList repeats too many times. It initially displays the correct hierarchal structure and then starts to repeat with random variations. Any ideas?

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