This class should be generalized to allow trees with any type of value to be used. Add a type parameter E to the PreOrderList and TreeNode classes. Your getPreOrderList and preOrder method should be erased entirely; that should be implemented inside the TreeNode class. In fact, there is no need for the PreOrderList class at all; all that behavior could be implemented inside TreeNode:

public class TreeNode<E>{

    public static TreeNode<E> fromIterator(Iterator<E> it){
        if(!it.hasNext()) return null;
        TreeNode<E> root = new TreeNode<E>();
        for(E e: list){
            
    }
    public static TreeNode<E> fromIterable(Iterable<E> iterable){
        return fromIterator(iterable.iterator());
    }
    public static TreeNode<E> fromArray(E ... array){
        return fromIterable(Arrays.asList(array));
    }


    TreeNode<E> [] children;
    E item;

    private TreeNode(E item, TreeNode ... children) {
        this.children = children;
        this.item = item;
    }

    int childIdx;
    public void add(E item){
        if(children[childIdx] == null)
            children[childIdx] = new TreeNode(item, new TreeNode[children.length]);
        else children[childIdx].add(item);

        childIdx++;
    }

    public boolean contains(Object item){
        if (item==null?this.item==null:item.equals(this.item)) return true;
        for(TreeNode<?> child: children)
            if(child.contains(item)) return true;
        return false;
    }

    @Override
    public String toString(){
        return String.format("%s(%s,%s)",item,left,right);
    }
}

This class should be generalized to allow trees with any type of value to be used. Add a type parameter E to the PreOrderList and TreeNode classes. Your getPreOrderList and preOrder method should be erased entirely; that should be implemented inside the TreeNode class. In fact, there is no need for the PreOrderList class at all; all that behavior could be implemented inside TreeNode:

public class TreeNode<E>{

    public static TreeNode<E> fromIterator(Iterator<E> it){
        if(!it.hasNext()) return null;
        TreeNode<E> root = new TreeNode<E>();
        while(it.hasNext())
            root.add(it.next());
    }
    public static TreeNode<E> fromIterable(Iterable<E> iterable){
        return fromIterator(iterable.iterator());
    }
    public static TreeNode<E> fromArray(E ... array){
        return fromIterable(Arrays.asList(array));
    }


    TreeNode<E> [] children;
    E item;

    private TreeNode(E item, TreeNode ... children) {
        this.children = children;
        this.item = item;
    }

    int childIdx;
    public void add(E item){
        if(children[childIdx] == null)
            children[childIdx] = new TreeNode(item, new TreeNode[children.length]);
        else children[childIdx].add(item);

        childIdx++;
    }

    public boolean contains(Object item){
        if (item==null?this.item==null:item.equals(this.item)) return true;
        for(TreeNode<?> child: children)
            if(child.contains(item)) return true;
        return false;
    }

    @Override
    public String toString(){
        return String.format("%s(%s,%s)",item,left,right);
    }
}

This class should be generalized to allow trees with any type of value to be used. Add a type parameter E to the PreOrderList and TreeNode classes. Your getPreOrderList and preOrder method should be erased entirely; that should be implemented inside the TreeNode class. In fact, there is no need for the PreOrderList class at all; all that behavior should be implemented inside TreeNode.

private static class TreeNode<E> {
    TreeNode left;
    E item;
    TreeNode right;

    TreeNode(TreeNode left, E item, TreeNode right) {
        this.left = left;
        this.item = item;
        this.right = right;
    }

    public String preOrderString(){
        return String.format("%s(%s,%s)",
                String.valueOf(item),
                left.preOrderString(),
                right.preOrderString());
    }
}

This class should be generalized to allow trees with any type of value to be used. Add a type parameter E to the PreOrderList and TreeNode classes. Your getPreOrderList and preOrder method should be erased entirely; that should be implemented inside the TreeNode class. In fact, there is no need for the PreOrderList class at all; all that behavior could be implemented inside TreeNode:

public class TreeNode<E>{

    public static TreeNode<E> fromIterator(Iterator<E> it){
        if(!it.hasNext()) return null;
        TreeNode<E> root = new TreeNode<E>();
        for(E e: list){
            
    }
    public static TreeNode<E> fromIterable(Iterable<E> iterable){
        return fromIterator(iterable.iterator());
    }
    public static TreeNode<E> fromArray(E ... array){
        return fromIterable(Arrays.asList(array));
    }


    TreeNode<E> [] children;
    E item;

    private TreeNode(E item, TreeNode ... children) {
        this.children = children;
        this.item = item;
    }

    int childIdx;
    public void add(E item){
        if(children[childIdx] == null)
            children[childIdx] = new TreeNode(item, new TreeNode[children.length]);
        else children[childIdx].add(item);

        childIdx++;
    }

    public boolean contains(Object item){
        if (item==null?this.item==null:item.equals(this.item)) return true;
        for(TreeNode<?> child: children)
            if(child.contains(item)) return true;
        return false;
    }

    @Override
    public String toString(){
        return String.format("%s(%s,%s)",item,left,right);
    }
}