Write a tree class in Java where each level has a unique object type

I need to write a tree class in Java where each level has a unique object type. The way it is written below does not take advantage of generics and causes alot of duplicate code. Is there a way to write this with Generics ?

 public class NodeB {
   private String nodeValue;
   //private List<NodeB> childNodes;
   // constructors
   // getters/setters
}

public class NodeA {
   private String value;
   private List<NodeB> childNodes;
   // constructors
   // getters/setters
}

public class Tree {
   private String value;
   priv开发者_如何学运维ate List<NodeA> childNodes;
   // constructors
   // tree methods
}

This is simplistic implementation, but enough to give general idea:

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class GenericNode {

    public static abstract class AbstractNode<V, N> {
        private V value;
        private List<N> children;

        public AbstractNode(V value, N... children) {
            this.value = value;
            this.children = children != null ? Arrays.asList(children)
                    : Collections.<N> emptyList();
        }

        public V getValue() {
            return value;
        }

        public List<N> getChildren() {
            return children;
        }

        public int getNumberOfChildren() {
            return children.size();
        }

        @Override
        public String toString() {
            return value.toString() + "->" + children.toString();
        }
    }

    // leaf node type, ignore type of children
    public static class NodeB extends AbstractNode<String, Object> {

        public NodeB(String value, Object... nodes) {
            super(value, nodes);
        }
    }

    // example of typical node in the mid of tree
    public static class NodeA extends AbstractNode<String, NodeB> {
        public NodeA(String value, NodeB... nodes) {
            super(value, nodes);
        }
    }

    // top level node type
    public static class Tree extends AbstractNode<String, NodeA> {
        public Tree(String value, NodeA... nodes) {
            super(value, nodes);
        }
    }

    @SuppressWarnings({ "rawtypes", "unchecked" })
    public static <V, N extends AbstractNode> int getNodeCount(
            AbstractNode<V, N> node) {
        int nodeCount = node.getChildren().size();
        for (N child : node.getChildren()) {
            nodeCount += getNodeCount(child);
        }
        return nodeCount;
    }

    public static void main(String[] args) {
        NodeB nodeB1 = new NodeB("Leaf node 1");
        NodeB nodeB2 = new NodeB("Leaf node 2");
        NodeA nodeA = new NodeA("Node with children", nodeB1, nodeB2);
        NodeA emptyNodeA = new NodeA("Empty node");
        Tree tree = new Tree("Tree", nodeA, emptyNodeA);
        System.out.println(tree);
        System.out.println(1 + getNodeCount(tree));
    }
}

You could make N and V types implement specific interfaces so it will be possible to call some common operations on values and/or children.

EDIT: updated implementation with recursive method for node count retrieval

All you need is a Pair<A, B>. Example of trees:

Pair<A, Pair<B, C>>
Pair<Pair<A, B>, Pair<C, D>>
Pair<Pair<Pair<A, B>, Pair<C, D>>, Pair<Pair<E, F>, Pair<G, H>>

ps: don't do this. :)

This is an ideal spot for everything inheriting from "Node", but even that is unnecessary.\

What you probably want is a single generic "Node" object that contains references to your different classes (use composition before inheritance).

At that point, each of your different classes probably has something that can be done to them (otherwise why are they all in the same data structure?) Have them implement a common interface with this common functionality. The node class can delegate to this interface, or some other class can extract the class by this interface and act on it.

This would be better than trying to force something to also BE a node--do one simple thing and do it well.

--edit--

I can't really add an example that is relevant to you because you didn't post anything about your scenario.

But let's say that you have these different classes A, B * C. First of all are they related AT ALL aside from all being children of Object? Let's say they all implement interface "Iface". (If not, you can just replace Iface with "Object", but this really implies a bad design.)

Anyway, your "Node" object is just one object--

public class Node {
    private List<node> children; 
    private Iface myObject;
    ... setters, getters, tree implementation, tree navigation, related garbage...
}

Now this is enough to create your tree. One thing that you might be able to do to make things smoother, have "Node implements Iface" and delegate any calls to it's object. For instance, if Iface contains an eat(Food foodtype) method, your node could implement Iface and have a method:

public void eat(Food foodtype) {
    myObject.eat(foodtype);
}

This would make the "Node" class act as though it was the class it contained.

By the way--another relatively good idea at this point would be to make myObject "private final" and ensure it is not null in the constructor. That way you would always know it was set and none of your delegated members would have to do null checks.

I don't think generics are going to help you much in this case. Instead of having a different class for each level in the tree. What about one node class that has children and store a different class on each level. That should help eliminate a lot of the duplication.

I'm fairly new to Java, so this might have issues I'm not aware of, but it seems to work on a simple level at least.

Define your main Node class - this one will be the root of the tree.

public class NodeA {

private String _value;
private ArrayList<NodeA> _children;
private int _depth;

public NodeA (String value, int depth) {
    _value = value;
    _children = new ArrayList<NodeA>();
    _depth = depth;
}

//probably want getters for _children and _value here

//this makes a new child, with its type depending on the depth of the tree it will
//be placed at.  NodeB and NodeC will both inherit from NodeA
public void add(String value) {
    switch (_depth) {
        case 0:
            _children.add(new NodeB(value, _depth+1));
            break;
        case 1:
            _children.add(new NodeC(value, _depth+1));
            break;
    }
}

The add() method is going to create a new child for the node using the specified value. If you initialize the root of the tree as a NodeA with depth 0, you can add children to nodes and the tree should end up populated so that the next level contains all NodeB's, and the next all NodeC's. The code for NodeB and NodeC is super simple and could be replicated to create an arbitrary amount of Node levels (here is that code).

public class NodeB extends NodeA {

public NodeB(String value, int depth) {
    super(value, depth);
}
//nothing else needed!

The code for NodeC is identical, except for the obvious replacements of B's with C's.

Hope this helps / is the kind of answer you wanted!