pointers - Fix a java.lang.NullPointerException error in a queue -

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• what nullpointerexception, , how fix it? 12 answers

so implemented queue in java , trying implement procedure inserts element @ end of queue.but error message "java.lang.nullpointerexception".i new @ , don't know means or how fix it.

class node {  public int ielement; public string selement; public node next;     public node(int iel, string sel) {      ielement = iel;     selement = sel; } class implement {   private node head; private node tail;   public implement() {     head = null;     tail=null; } public void insertattheend(int iel, string sel){     node newnode=new node(iel,sel);     if(!empty())//if queue empty{     tail.next=newnode;     tail=newnode;}     else{     newnode.next=head;     head=newnode;         }  }     public static void main(string[] args) {     implement k = new implement();    k.insertattheend(1, "b");    k.insertattheend(3, "bes");    k.insertattheend(4, "kes");} 

you never initialize tail. if queue not empty, tail.next should point newly created node, have done. if is, though, both head , tail should point newly added node.

public void insertattheend (int iel, string sel) {     node newnode = new node(iel,sel);     if (!empty()) {         tail.next = newnode;         tail = newnode;     } else {         head = newnode;         tail = newnode;     } }