Java Program to Implement Binary Tree Data Structure

Introduction

Binary Trees are fundamental data structures in computer science which facilitate data storage and processing in a hierarchical order. Frequent applications of binary trees include database indexing, sorting algorithms, and much more, making them crucial for numerous programming and software development tasks.

In this article, you will learn how to implement a basic binary tree in Java. Explore how to initialize the tree, insert elements, and perform in-order traversal through practical examples and code snippets.

Setting up the Binary Tree Class

Define the Node Class

1. Start by creating the Node class, which will represent each element or node in the binary tree. Each node typically contains data and references to the left and right child nodes.

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   class Node {
       int value;
       Node left, right;
   
       public Node(int item) {
           value = item;
           left = right = null;
       }
   }
   

   This class initializes the node with a value and sets both child references to null.

Create the BinaryTree Class

1. Define the BinaryTree class with an initial root node.

2. Include methods for inserting nodes and for in-order traversal.

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   class BinaryTree {
       Node root;
   
       BinaryTree() {
           root = null;
       }
   
       void insert(int value) {
           root = insertRec(root, value);
       }
   
       Node insertRec(Node root, int value) {
           if (root == null) {
               root = new Node(value);
               return root;
           }
   
           if (value < root.value)
               root.left = insertRec(root.left, value);
           else if (value > root.value)
               root.right = insertRec(root.right, value);
   
           return root;
       }
   }
   

   The insert() method adds values by creating nodes in the correct position to maintain order. The recursive method insertRec() helps in the correct placement of a new node.

Implementing Tree Traversal

In-Order Traversal

1. Implement a method to perform in-order traversal, which visits the left branch, then the current node, followed by the right branch. This type of traversal displays the tree's elements in their natural order.

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   void inorder() {
       inorderRec(root);
   }
   
   void inorderRec(Node root) {
       if (root != null) {
           inorderRec(root.left);
           System.out.print(root.value + " ");
           inorderRec(root.right);
       }
   }
   

   This recursive function traverses the tree in-order and outputs the values of the nodes in sorted order.

Example Usage

1. Instantiate the BinaryTree and populate it with values.

2. Call the inorder() method to display the tree.

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   public class Main {
       public static void main(String[] args) {
           BinaryTree tree = new BinaryTree();
           tree.insert(50);
           tree.insert(30);
           tree.insert(20);
           tree.insert(40);
           tree.insert(70);
           tree.insert(60);
           tree.insert(80);
   
           System.out.println("Inorder traversal of binary tree is ");
           tree.inorder();
       }
   }
   

   Running this example would print the nodes in ascending order: 20, 30, 40, 50, 60, 70, 80.

Conclusion

The binary tree is an incredibly versatile and essential data structure in Java programming, useful in various real-world applications. By constructing and traversing binary trees as demonstrated, enhance applications that require structured data storage and retrieval. Adjust and expand upon this basic structure to suit more complex needs, such as balancing the tree or implementing a binary search tree (BST) for improved performance in search operations. Implementing binary trees deeply familiarizes with concepts of recursion and object-oriented programming, paving the way for tackling more advanced data structures and algorithms.