Passing objects between classes in Java

Question

I'm new to Java and I am doing a homework on constructing Decision Trees. After 2 days of continuous coding, I finally built the tree and verified it manually. But I'm stuck on validating the tree, because, everytime I try to pass the "node" object to the Validator class, it is null. I tried all sorts of previous suggestions and nothing seems to work. I need someone to point out my mistake and why its a mistake. Here is a short portion of the code that will explain what I'm trying to achieve. Please advice on how I should go about this.

//Node Class to represent a node in the tree
public class DecisionTreeNode 
{
    String attribute;
    boolean isLeaf;
    DecisionTreeBranch[] branches; //Another class to represent branch from a node

    //Default constructor for a Node: With attributes, label and isLeaf condition
    public DecisionTreeNode(String attribute) 
    {
        this.attribute = attribute;
        this.isLeaf = true;
    }
        ............
}

//Tree class with logic to build the tree
public class BuildDecisionTree 
{
    public PrepareFile config; //Need this object to get a arraylist of values to construct the tree
    DecisionTreeNode root;
        BuildDecisionTree(PrepareFile config)
    {
        this.config = config;
    }
    //Construct Decision Tree
    public void buildDecisionTree() 
    {
        root = myDecisionTreeAlgorithm(config.getExamples(), config.getAttributes());
        System.out.println("\n !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!Decision tree was constructed!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!\n");
        root.printDecisionTree("");
    }

//This is the validator where is want both the "config" and the "root" objects

import java.util.List;

public class DecisionTreeValidator 
{
    PrepareFile config;
    DecisionTreeNode node;
    BuildDecisionTree bTree;    
    public DecisionTreeValidator(BuildDecisionTree bTree, PrepareFile config)
    {
        this.bTree = bTree;
        this.node = bTree.root; //I tried adding a getter function in BuildDecisionTree class and returned the root, even then this was null. Like below
            //this.node = bTree.buildDecisionTree(); //made the return type of the buildDecisionTree function as "DecisionTreeNode"
        this.config = config;
        this.examples = config.getExamples();
    }
    public boolean validateSingleExample(Example example)
    {

        boolean result = true;
        while(node.isLeaf == false) //THIS IS WHERE I GET THE NULL POINTER EXCEPTION
                ...........................
    }
}

//Main class
       public class PredictRestaurant 
        {
            public static void main(String[] args) 
            {
                PrepareFile config = new PrepareFile();
                BuildDecisionTree bTree = new BuildDecisionTree(config);
                DecisionTreeValidator validator = new DecisionTreeValidator(bTree, config);
                boolean isTrain = true;
                        config.setTreeParameters();
                        bTree.buildDecisionTree();
                }
       }

Answer 1

Your node is null because the method signature for buildDecisionTree() is either Object or some other unspecified object; the method signature with void won't even compile.

You should change your method there to return an object of type DecisionTreeNode .

Answer 2

Your problem is that you are not initializing bTree.node properly. In your DecisionTreeValidator constructor, directly after the line

this.bTree = bTree;

add the line

bTree.buildDecisionTree();

You did not have this line in your code, and thus bTree.node was defaulted to null as it wasn't initialized. This made this.node null after the line

this.node = bTree.node;

resulting in a null pointer exception when you tried to reference this.node later on. With this change, your code should work. Tell me if you have any problems.

Answer 3

I figured out what the issue was. It wasnt related to the node being null. the node wasnt null. when i traverse the tree and hit the leaf node, I was still checking for branches. corrected it now. Its working perfect. Thank you all for your recommendations. It helped me learn.

I will now remove the "logic" for tree building.