Thread: How to fix error java.lang.NullPointerException in array of LinkedList?

How to fix error java.lang.NullPointerException in array of LinkedList?

/* LinkedList MergeSort */
package layout;
import java.util.Scanner;
 
/* Class Node */
class Node {
	protected int data;
	protected Node next, prev;
	/* Constructor */
	public Node() {
		data = 0;
		next = null;
	}
	/* Constructor */
	public Node( int d, Node n ) {
		data = d;
		next = n;
	}
	/* Function to set link to next node */
	public void setLinkNext( Node n ) {
		next = n;
	}
	/* Funtion to get link to next node */
	public Node getLinkNext() {
		return next;
	}
	/* Function to set data to node */
	public void setData( int d ) {
		data = d;
	}
	/* Function to get data from node */
	public int getData() {
		return data;
	}
}
 
/* Class linkedList */
class linkedList {
	protected Node start, end;
	public int size;
	/* Constructor */
	public linkedList() {
		start = null;
		end = null;
		size = 0;
	}
	/* Function to check if list is empty */
	public boolean isEmpty() {
		return start == null;
	}
	/* Function to get size of list */
	public int getSize() {
		return size;
	}
	public Node getNodeStart() {
		return start;
	}
	public Node getNodeEnd() {
		return end;
	}
	/* Function to insert element */
	public void insertHead( int val ) {
		Node p = new Node( val, null );
		if ( start == null ) {
			start = p;
			end = start;
		}
		else {
			p.setLinkNext( start );
			start = p;
		}
		size++;
	}
	void insertTail( int val ) {
		Node p = new Node( val, null );;
		if ( start == null ) {
			start = p;
			end = start;
		}
		else {
			end.setLinkNext( p );
			end = p;
		}
		p.setLinkNext( null );
		size++;
	}
	void Output() {
		Node p = start;
		while ( p != null ) {
			System.out.print( p.getData() + " <-> " );
			p = p.getLinkNext();
		}
	}
	/* Function to convert List to a String */
	public String display() {
		String ListString01 = "";
		Node ptr = start;
		ListString01 = ListString01 + ptr.getData() + ",  ";
		if ( size == 0 ) {
			ListString01 = "List is empty";
			return ListString01;
		}
		ptr = start.getLinkNext();
		while ( ptr.getLinkNext() != null ) {
			ListString01 = ListString01 + ptr.getData() + ",  ";
			ptr = ptr.getLinkNext();
		}
		return ListString01;
	}
	void initList( linkedList List ) {
		List.start = List.end = null;
	}
	/* Function to MergeSort */
	void appendList( linkedList List, linkedList List1 ) {
		if ( List.start == null )
			List = List1;
		else {
			List.end.setLinkNext( List1.start );
			if ( List1.start != null )
				List.end = List1.end;
		}
	}
	int getDigit( int n, int t ) {
		long temp = 1;
		for ( int i = 0; i < n; i++ )
			temp = temp * 10;
		return (int) ( ( t / temp ) % 10 );
	}
	int countDigit( int n ) {
		int count = 0;
		while ( n > 0 ) {
			n = n / 10;
			count++;
		}
		return count;
	}
	int findMax( linkedList list ) {
		Node p;
		p = new Node();
		p = list.start;
		int max = p.data;
		while ( p != null ) {
			if ( max < p.data )
				max = p.data;
			p = p.getLinkNext();
		}
		return max;
	}
	int getMaxDigit( linkedList list ) {
		int max = findMax( list );
		return countDigit( max );
	}
	void sendToBox( linkedList list, int n, linkedList temp[] ) {
		Node p;
		p = new Node();
		p = list.start;
		while ( p != null ) {
			temp[ getDigit( n, p.data ) ].insertTail( p.data );
			p = p.getLinkNext();
		}
	}
	void radixSort( linkedList list ) {
		linkedList[] temp = null;
		int i;
		if ( list.start == list.end )
			return;
		for ( i = 0; i < 10; i++ )
			temp[ i ] = new linkedList();
		int num = getMaxDigit( list );
		for ( i = 0; i < num; i++ ) {
			sendToBox( list, i, temp );
			for ( int j = 0; j < 10; j++ ) {
				appendList( list, temp[ j ] );
				initList( temp[ j ] );
			}
		}
	}
	@SuppressWarnings( "deprecation")
	public static void main( String arg[] ) {
		linkedList List01 = new linkedList();
		int k = 10;
		int a[] = { 12, 23, 34, 78, 91, 44, 52, 68, 79, 77 };
		for ( int i = 0; i < k; i++ ) {
			List01.insertHead( a[ i ] );
		}
		System.out.println( "oOo  m01.Output();  oOo" );
		List01.Output();
		System.out.println( " " );
		System.out.println( "oOo  m01.display( );  oOo" );
		String s01 = List01.display();
		System.out.println( s01 );
		List01.radixSort( List01 );
		System.out.println( "oOo  m01.display( );  oOo" );
		String s02 = List01.display();
		System.out.println( s02 );
	}
}

Please help me or take a look.