Скачать или смотреть Solving the Java Dijkstra Algorithm Problem Without Reading a Text File

This guide explains how to troubleshoot a Java Dijkstra algorithm implementation issue, detailing how to correctly manage vertex creation when reading from a text file.
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This video is based on the question https://stackoverflow.com/q/69938252/ asked by the user 'Sarah Cross' ( https://stackoverflow.com/u/17390143/ ) and on the answer https://stackoverflow.com/a/69945244/ provided by the user 'b.GHILAS' ( https://stackoverflow.com/u/12436427/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Dijkstra's Algorithm Implementation in Java

Dijkstra's algorithm is a fundamental method in computer science for finding the shortest path between nodes in a weighted graph. While implementing this algorithm in Java, many learners may encounter issues, especially when reading graph data from a text file. This guide will explore a common error faced when implementing Dijkstra’s algorithm in Java and provide a solution to overcome this challenge.

The Problem

As presented in a recent coding scenario, a student implemented Dijkstra’s algorithm based on classroom pseudocode, but ran into difficulties when reading graph data from a text file. The program successfully outputs the correct adjacency list but fails to compute the shortest paths correctly when data is fetched from the file. The student noticed that during the first iteration of the relaxation process, the adjacent vertex distances and parent were updated but later reverted to the previous state.

Here’s the example text file format that the program was using:

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

Identifying the Root Cause

The crux of the issue stems from how the code is creating new instances of Vertex whenever it encounters adjacent vertices from the text file. In particular, the following code snippet creates a new vertex each time, even if a vertex with the same name already exists in the graph:

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

This means that even if a vertex with that name has already been added to the graph, the program does not re-use it for connecting edges but, instead, generates a new instance, which leads to inconsistencies in distance calculations and ultimately incorrect path outputs.

Solutions

To resolve this issue, we need to implement a mechanism in the graph class that either retrieves an existing vertex by name or creates a new one if it does not exist. This can be accomplished by adding a method to the Graph class.

Step 1: Implementing the getOrCreateVertex Method

In the Graph class, add the following method:

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

Step 2: Modifying File Reading Logic

Next, adjust the file reading section in your code to utilize this new method:

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

Conclusion

By implementing the getOrCreateVertex method, you can ensure that your graph maintains the integrity of vertices throughout the execution of Dijkstra’s algorithm. This small change will allow your program to read data correctly from a text file, enabling accurate shortest path calculations to be produced.

For anyone who is still grappling with Dijkstra's implementations, I hope this breakdown helps clarify how to effectively manage your graph’s vertices and improve your understanding of graph algorithms in Java. Happy coding!