Java Iterate Over List

Question

How do you iterate over a list that has a "parent/child/grandChild/etc."?

Sample data:

{ID, Parent ID}

Object parent = {1, 0};
Object childA = {2, 1};
Object childB = {3, 1};
Object grandChildA = {4, 3};
Object grandChildB = {5, 2};

And the list would be, {parent, childA, childB, grandChildA, grandChildB}

How do you iterate it by "roots"?

Sample output:

Parent
- ChildA
  - GrandChildB
- ChildB
  - GrandChildA

Thanks!

Sample data:

Constructor: SampleObject(int id,int parentId)

SampleObject parent = new SampleObject(1, 0);
SampleObject childA = new SampleObject(2, 1);
SampleObject childB = new SampleObject(3, 1);
SampleObject grandChildA = new SampleObject(4, 3);
SampleObject grandChildB = new SampleObject(5, 2);

I then placed the objects in an ArrayList: ArrayList testList

so, problem is, how to iterate over the list, so that the result would:

Parent
- ChildA
  - GrandChildB
- ChildB
  - GrandChildA

And the list would be, {parent, childA, childB, grandChildA, grandChildB} if that were true you could just use 1 simple loop without any recursion or nesting — Tim
– Tim, Commented Sep 11, 2015 at 7:40
I would start by building a tree structure to match your data set and format the output based on the level of nesting. — Peter Lawrey
– Peter Lawrey, Commented Sep 11, 2015 at 7:43

user1438038 · Accepted Answer · 2015-09-11 07:50:25Z

I do recommend a tree-like data structure, such as:

package com.stackoverflow.test;

import java.util.ArrayList;
import java.util.List;

public class TreeNode {
   private String         name       = "";
   private List<TreeNode> childNodes = new ArrayList<TreeNode>();

   public TreeNode(final String name) {
      this.name = name;
   }

   public String getName() {
      return this.name;
   }

   public void add(final TreeNode child) {
      childNodes.add(child);
   }

   public List<TreeNode> getChildren() {
      return this.childNodes;
   }

   public static void main(String[] args) {

      TreeNode parent = new TreeNode("Parent");

      TreeNode childA = new TreeNode("ChildA");
      childA.add(new TreeNode("GrandChildA"));

      TreeNode childB = new TreeNode("ChildB");
      childB.add(new TreeNode("GrandChildB"));

      parent.add(childA);
      parent.add(childB);

      TreeNode.printRecursively(parent, 0);
   }

   private static void printRecursively(final TreeNode root, final int level) {
      if (null != root && null != root.getChildren()) {

         for (int i = 0; i < level; ++i) {
            System.out.print("   ");
         }

         System.out.println(root.getName());

         for (TreeNode child: root.getChildren()) {
            TreeNode.printRecursively(child, level + 1);
         }
      }
   }

}

Will print:

Parent
   ChildA
      GrandChildA
   ChildB
      GrandChildB

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