I defined a nested Node class inside a class, and
class HNode<T extends ArrayList> {
private T _datum;
private HNode<T> _prev;
private HNode<T> _next;
public HNode(T datum, HNode<T> prev, HNode<T> next) {
_datum = datum;
_prev = prev;
_next = next;
}
public HNode<T> getNext() {
return _next;
}
and I designated _datum, _prev, and _next fields manually when I created this Node class.
_r0 = new HNode<ArrayList<String>>(_al.get(3), _r0, _r1);
_r1 = new HNode<ArrayList<String>>(_al.get(4), _r0, _r2);
_r2 = new HNode<ArrayList<String>>(_al.get(5), _r1, _pos3);
_pos3 = new HNode<ArrayList<String>>(_al.get(6), _r2, _pos4);
_pos4 = new HNode<ArrayList<String>>(_al.get(7), _pos3, _pos5);
(this is a part of the initialization.)
Hi. I defined a nested Node class inside a class named 'Structure', and
class HNode<T extends ArrayList> {
private T _datum;
private HNode<T> _prev;
private HNode<T> _next;
public HNode(T datum, HNode<T> prev, HNode<T> next) {
_datum = datum;
_prev = prev;
_next = next;
}
public HNode<T> getNext() {
return _next;
}
and I designated _datum, _prev, and _next fields manually when I created this Node class.
_r0 = new HNode<ArrayList<String>>(_al.get(3), _r0, _r1);
_r1 = new HNode<ArrayList<String>>(_al.get(4), _r0, _r2);
_r2 = new HNode<ArrayList<String>>(_al.get(5), _r1, _pos3);
_pos3 = new HNode<ArrayList<String>>(_al.get(6), _r2, _pos4);
_pos4 = new HNode<ArrayList<String>>(_al.get(7), _pos3, _pos5);
(this is a part of the initialization. _al is a ArrayList of ArrayList, for which I used to call Collections.shuffle(_al) method.)
Then I wanted to define a getPosition method inside 'Structure' class. When I tested this method, it throws a NullPointerException.
The reason I partitioned this method with if statements is that the 2th and 3rd, 16th and 17th Nodes are not connected to each other.
I guess the for loops of pos.getNext(); might be the cause of the problem, not the conditional statements, though.
public HNode<ArrayList<String>> getPosition(int index) {
HNode<ArrayList<String>> pos = null;
if(index<3) {
pos = _l0;
for(int i=0; i<index; i++) {
pos = pos.getNext();
}
}
else if(3<=index && index <= 16) {
pos = _r0;
for(int i=0; i<(index-3); i++) {
pos = pos.getNext();
}
}
else if(index <= 17 && index <= 24) {
pos = _w1;
for(int i=0; i<(index-17); i++) {
pos = pos.getNext();
}
}
return pos;
}
Do you know why this gives a NullPointerException? I am trying to figure this out for a while but could not. Please give me an advice. Thank you!
The error occurs when I call getNext() method. Is there any problem?
From personal experiance of null references, it seems likely that
_r0 = new HNode
_pos3 = new HNode etc
Is null. Have you attempted using the debugger?
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