从 arrayList 格式的树中删除子树

Question

I'm studying trees in Java using an ArrayList, and I'm trying to implement a method to remove an entire subtree from a child node on my tree, but I keep getting a NullPointerException error. Here's my code:

public class GeneralTree<Tree>
{
    private Tree data = null; //create a tree
    private List<GeneralTree> children = new ArrayList<>(); //create an arraylist
    private GeneralTree parent = null; //create a parent

    public GeneralTree(Tree data) //constructor
    {
        this.data = data;
    }
    public void addChild(GeneralTree child) //create a child method to create a child
    {
        child.setParent(this);
        this.children.add(child);
    }
    public void addChild(Tree data) //create a method to put data into the children
    {
        GeneralTree<Tree> newChild = new GeneralTree<>(data);
        this.addChild(newChild);
    }
    public void addChildren(List<GeneralTree> children) //create a method to add children to a parent
    {
        for(GeneralTree treeAdd: children)
        {
            treeAdd.setParent(this);
        }
        this.children.addAll(children);
    }
    public List<GeneralTree> getChildren() //get the children
    {
        return this.children;
    }
    public Tree getData() //get the data in the children
    {
        return data;
    }
    public void setData(Tree data) //set the data of the children together
    {
        this.data = data;
    }
    public void setParent(GeneralTree parent) //set the parent together
    {
        this.parent = parent;
    }
    public GeneralTree getParent() //get the parent
    {
        return this.parent;
    }
    public boolean isEmpty()
    {
        return this.children.isEmpty();
    }
    public List removeSubtree(GeneralTree childs)
    {
        GeneralTree removes = removeTree(this.parent);

        return removes.children;
    }

removeSubTree() and removeTree() methods to implement this

    public List removeSubtree(GeneralTree childs)
    {
        GeneralTree removes = removeTree(this.parent);

        return removes.children;
    }
    public GeneralTree removeTree(GeneralTree remover)
    {
        return null;
    }

main method

    public static void main(String[] args)
    {
        GeneralTree<String> root = new GeneralTree<>("Root"); //create a root node

        GeneralTree<String> child1 = new GeneralTree<>("Child 1"); //first child node
        child1.addChild("Grandchild 1"); //first grandchild node
        child1.addChild("Grandchild 2"); //second grandchild node
        GeneralTree<String> child2 = new GeneralTree<>("Child 2"); //second child node
        child2.addChild("Grandchild 3"); //third grandchild node
        root.addChild(child1);
        root.addChild(child2);
        root.addChild("Child 3"); //third child node
        root.addChildren(Arrays.asList(new GeneralTree<>("Child 4"), new GeneralTree<>("Child 5"), new GeneralTree<>("Child 6")));//add fourth, fifth, and sixth children nodes

        System.out.println(root.isEmpty()? "Empty": "Not Empty");
        root.removeSubtree(child1);

        for(GeneralTree node: root.getChildren()) //get and print the children as long as they're under the root
        {
            System.out.println(node.getData()); //get the data
        }
    }
}

I'm not fully sure where I've gone wrong, but I think the problem is that I'm using a remove Object method to try & remove a series of elements from my array list. Is that right, or am I going about this process the wrong way?

Answer 1

The removeTree method returns always null. The removeSubtree method returns removes.children but the removes object is always null.

You should implement the removeTree method