How to Read a Text File and Build an Undirected Graph in Java Using Adjacency Lists

Learn how to create an undirected graph representation in Java using adjacency lists by reading from a text file. Follow our step-by-step guide for implementation and tips!
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Understanding the Challenge: Reading a Text File into a Graph

If you're a beginner in Java and looking to create an undirected graph using adjacency lists, you might run into issues when reading data from a text file. In this blog, we'll solve the problem of building a Graph class that accurately represents connected nodes (or vertices) based on the relationships defined in your text file.

The Problem

You've got a text file that defines pairs of connected nodes, like this:

[[See Video to Reveal this Text or Code Snippet]]

Each line specifies a relationship between two nodes, and your goal is to read this data into a graph structure in Java. However, your initial attempt ends with unexpected outputs—arrays filled with null values and no actual graph representation. Let's break down how you can successfully achieve this.

Step 1: Understanding the Graph Representation

An undirected graph can be represented by an adjacency list, where each key (a node) points to a list of its neighboring nodes. Here’s a quick structure:

Vertices: Represented by unique strings (e.g., "Anna")

Edges: Connectivity between two vertices (e.g., "Anna" is connected to "Noah")

In your Java implementation, you will utilize a HashMap<String, LinkedList<String>> where each key is a vertex and its value is a linked list of adjacent vertices.

Step 2: Correcting the File Reading Approach

The main issue in your initial code arises from incorrectly processing each line of the text file. The loop that reads the file does not actually retrieve the vertex names from each line. Here’s how to fix that:

Updated Code for Reading Node Connections

[[See Video to Reveal this Text or Code Snippet]]

Explanation of Changes

Splitting Lines: The line is split using split("\s+"), which breaks the string into components based on whitespace, yielding an array of node names.

Checking Array Length: Ensure that exactly two nodes are present before attempting to add them as edges. This prevents errors and helps maintain the integrity of the graph structure.

Step 3: Full Implementation of the Graph Class

Ensure your Graph class is properly structured to support adding vertices and edges effectively:

[[See Video to Reveal this Text or Code Snippet]]

Key Features:

Add Vertex: Adds a new vertex to the graph.

Add Edge: Adds a connection between two vertices, ensures both vertices exist in the graph.

Conclusion

By following these structured steps, you should be able to successfully read from a text file and construct an undirected graph in Java. The primary takeaway is the importance of correctly manipulating the input data to build your desired graph structure. Happy coding!