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  • Re: Common Java Mistakes

    01:58 PM
    1
    Problem description: Forgetting to initialize a variable
    Problem category: Runtime problem

    Diagnosis Difficulty: Easy-medium
    Difficulty to Fix: Easy-medium


    This problem occurs when you forget to initialize an object variable. It will usually manifest itself as a Null Pointer Exception when you try to run the code.

    public class Node
{
    public Node[] neighbors;
    public int value;
    public Node(int value)
    {
        this.value = value;
    }
}

    public class NodeTester
{
    public static void main(String[] args)
    {
        Node myNode = new Node(5);
        myNode.neighbors[0] = new Node(3);
    }
}

    Error Message

    Probably 90% of the time this problem will manifest itself with a Null Pointer Exception when you try to run it. However, this exception will pop up at where the null value will cause a problem, not where you need to fix the problem.

    Exception in thread "main" java.lang.NullPointerException
    at NodeTester.main(NodeTester.java:6)
    Suggested fixes

    Look for where the value should have been initialized (almost always in the constructor if it's an object field), then initialize it to an appropriate value.

    public class Node
{
    public Node[] neighbors;
    public int value;
    public Node(int value)
    {
        this.value = value;
        this.neighbors = new Node[5]; // can have 2 neighbors
    }
}
    Common Java Mistakes helloworld922
    2. Common Java Mistakes
    1
    1. patricajohnson51 - April 16th, 2024, 01:32 AM
      The issue in the provided code is that the neighbors array is not initialized when a Node object is created. Therefore, when trying to access myNode.neighbors[0], a NullPointerException occurs because neighbors is still null.

      To fix this, you need to initialize the neighbors array either when declaring it or in the constructor. Here's the modified code:
      public class Node {
      public Node[] neighbors;
      public int value;

      public Node(int value) {
      this.value = value;
      // Initialize neighbors array to an appropriate size
      this.neighbors = new Node[5]; // Assuming a maximum of 5 neighbors
      }
      }

      public class NodeTester {
      public static void main(String[] args) {
      Node myNode = new Node(5);
      myNode.neighbors[0] = new Node(3);
      }
      }
      Now, when you create a Node object, its neighbors array will be initialized, and you won't encounter a NullPointerException when accessing its elements. You can find more assistance with Java assignments on https://www.programminghomeworkhelp....va-assignment/