我在我的代码中得到nullpointerexception ..不知道该代码中有什么错误

问题描述

我得到Null指针异常...不知道该代码中有什么错误...如何解决它。



我是什么尝试过：

I am getting Null pointer exception...don't know whats wrong in that code...how to resolve it.

What I have tried:

import java.util.Scanner;

class Node
{
	private int data;
	private Node next;



	public Node(int d , Node n)
	{
		data = d;
		next = n;
	}

	public void setData(int d)
	{
		data = d;
	}

	public void setNext(Node n)
	{
		next = n;
	}

	public int getData()
	{
		return(data);
	}

	public Node getNext()
	{
		return(next);
	}
}

class Linkedlist
{
	private int size;
	private Node start;

	public Linkedlist()
	{
		size = 0;
		start = null;
	}

	public void insetAtfirst(int val)
	{
		Node n = new Node(val,start);

		n.setData(val);
		n.setNext(start);

		start = n;
		size++;
	}

	public void viewlist()
	{
		Node t;

		t = start;

		for(int i=1;i<=size;i++)
		{
			System.out.println(" " + t.getData());
			t = t.getNext();
		}
	}

	public void deleteAltNode()
	{

		Node t1;

		t1 = start;

		for(int i=1;i<=size;i++)
		{
			System.out.println(" " + t1.getData());
			t1 = t1.getNext().getNext();

		}


		size--;




	}
}



public class Delete_alternate_node
{


	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);

		Linkedlist l = new Linkedlist();

		System.out.println("Enter the no of node ");
		int k = sc.nextInt();

		for(int i=1;i<=k;i++)
		{
			System.out.println("Enter the elements for linkedlist");
			int val = sc.nextInt();
			l.insetAtfirst(val);
		}

		System.out.println();
		l.viewlist();

		System.out.println();
		l.deleteAltNode();



	}

}

推荐答案

import java.util.Scanner;

class Node
{
  private int data;
  private Node next;

  public Node(int d , Node n)
  {
    data = d;
    next = n;
  }

  public void setData(int d)
  {
    data = d;
  }

  public void setNext(Node n)
  {
    next = n;
  }

  public int getData()
  {
    return(data);
  }

  public Node getNext()
  {
    return(next);
  }
}

class Linkedlist
{
  private int size;
  private Node start;

  public Linkedlist()
  {
    size = 0;
    start = null;
  }

  public void insertAtfirst(int val)
  {
    Node n = new Node(val,start);

    n.setData(val);
    n.setNext(start);

    start = n;
    size++;
  }

  public void viewlist()
  {
    Node t;

    t = start;

    while ( t != null )
    {
      System.out.print(" " + t.getData());
      t = t.getNext();
    }
    System.out.println();
  }

  public void deleteAltNode()
  {
    Node node;
    node = start;
    while ( node != null )
    {
      Node next = node.getNext();
      if ( next != null)
      {
        node.setNext (next.getNext());
        --size;
      }
      node = node.getNext();
    }
  }
}

public class Delete_alternate_node
{
  public static void main(String[] args)
  {
    Scanner sc = new Scanner(System.in);

    Linkedlist l = new Linkedlist();

    System.out.println("Enter the no of node ");
    int k = sc.nextInt();

    for(int i=1;i<=k;i++)
    {
      System.out.println("Enter the elements for linkedlist");
      int val = sc.nextInt();
      l.insertAtfirst(val);
    }
    System.out.print("list before alternate nodes deletion: ");
    l.viewlist();

    System.out.print("list after alternate nodes deletion: ");
    l.deleteAltNode();

    l.viewlist();
  }
}

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