Core Java Coding Questions: Creating a custom hierarchical List in Java -- Part 3

Q. Can you write a custom list class that supports following features?

1. Allows you to maintain a parent/child hierachy of sub lists.
2. Allows you to create sublists from a given list and a predicate (i.e a condition)
3. If you add an item or element to a list, the addition must be propagated to its parent and child lists.
4. If you remove an element from a list, the removal must be propagated to its parent and child lists.

In the previous parts

• Core Java Coding Questions: Creating a custom hierarchical List in Java -- Part 1: subList with a predicate functionality.
• Core Java Coding Questions: Creating a custom hierarchical List in Java -- Part 2: toString( ) method to print the hierarchy.

In this part, you will be adding an add(...) method to add an item to the hierarchical list.


Step 1: Remove the UnsupportedOperationException with the following recursive method call. Recursion is used to add to the parent and children.

//....removed methods

public class CustomList<E> implements List<E> {

    //...removed constructors & methos  

 @Override
 public boolean add(E e) {
  return add(e, null);
 }
 
 protected boolean add(E e, CustomList<E> givenList) {

  // check if the predicate (i.e. condition) is met
  if (!evaluate(e)) {
   return false;
  }

  boolean hasAdded = true;

  // if parent exists, add to the parent
  if (parent != null && parent != givenList) {
   // Recursive method call. The parent takes care of siblings...
   hasAdded = parent.add(e, this);
  }

  // if addition fails, return false
  if (!hasAdded) {
   return false;
  }

  //add it to the actual list
  hasAdded = this.list.add(e);

  if (!hasAdded) {
   if (parent != null) {
    throw new IllegalStateException("Failed to add !!");
   } else {
    return false;
   }
  }

  // Add to the sublists
  for (CustomList<E> sublist : sublists) {
   if (sublist != givenList) {
    // recursive method call
    sublist.add(e, this);
   }
  }

  return true;
 }

 private boolean evaluate(E e) {
  return (this.predicate != null) ? this.predicate.evaluate(e) : true;
 }
 
 
 //...removed some methods 
}

Step 2: The CustomListTest that adds items to the CustomList. Note that the initial list creation was slightly modified to make "initialList" mutable, so that new items can be added.

package test;

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

public class CustomListTest {

 public static void main(String[] args) {
  /**
   * Arrays.asList returns a List wrapper around an array. This wrapper
   * has a fixed size and is directly backed by the array, and as such
   * calls to set will modify the array, and any other method that
   * modifies the list will throw an UnsupportedOperationException.
   * hence creating a new ArrayList(...) instance
   */
  List<Integer> initialList = new ArrayList<Integer>(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10));

  CustomList<Integer> customList = new CustomList<Integer>(initialList);
  System.out.println("customList: " + customList);

  Predicate<Integer> oddNumberPredicate = new OddNumberPredicate<Integer>();
  CustomList<Integer> oddNumbersSubList = customList.subList(oddNumberPredicate);
  System.out.println("oddNumbersSubList: " + oddNumbersSubList);

  Predicate<Integer> factorOf5Predicate = new FactorOf5Predicate<Integer>();
  CustomList<Integer> factorOf5SubList = customList.subList(factorOf5Predicate);
  System.out.println("factorOf5SubList: " + factorOf5SubList);

  Predicate<Integer> factorOf3Predicate = new FactorOf3Predicate<Integer>();
  CustomList<Integer> factorOf3SubList = oddNumbersSubList.subList(factorOf3Predicate);
  System.out.println("factorOf3SubList : " + factorOf3SubList);

  System.out.println("Demonstrate printing customList again");
  System.out.println("customList : " + customList);

  // add an item or element
  customList.add(11); // this should be added to customList and
       // oddNumbersSubList
  customList.add(15); // this should be added to all four lists.

  System.out.println("After adding 11 & 15: ");
  System.out.println("customList : " + customList);
 }
}

The output is:

customList: list: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

oddNumbersSubList: parent: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
list: [1, 3, 5, 7, 9]

factorOf5SubList: parent: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
list: [5, 10]

factorOf3SubList : parent: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
parent: [1, 3, 5, 7, 9]
list: [3, 9]

Demonstrate printing customList again
customList : list: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
child: [1, 3, 5, 7, 9]
child: [5, 10]
child: [3, 9]

After adding 11 & 15: 
customList : list: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 15]
child: [1, 3, 5, 7, 9, 11, 15]
child: [5, 10, 15]
child: [3, 9, 15]