BinaryTree2.java

class TreeNode {
    int data;
    TreeNode left, right;

    public TreeNode(int data) {
        this.data = data;
        left = right = null;
    }
}

class BinaryTree {
    TreeNode root;

    BinaryTree() {
        root = null;
    }

    int countNodes(TreeNode node) {
        if (node == null)
            return 0;
        return 1 + countNodes(node.left) + countNodes(node.right);
    }

    int getNumberOfNodes() {
        return countNodes(root);
    }
}

public class Main {
    public static void main(String[] args) {
        BinaryTree tree = new BinaryTree();
        tree.root = new TreeNode(1);
        tree.root.left = new TreeNode(2);
        tree.root.right = new TreeNode(3);
        tree.root.left.left = new TreeNode(4);
        tree.root.left.right = new TreeNode(5);

        int nodeCount = tree.getNumberOfNodes();
        System.out.println("Number of nodes in the binary tree: " + nodeCount);
    }
}