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PONullPointerException with while loop using compareTo
 

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  1. PONullPointerException with while loop using compareTo
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    <p>For our homework, I have to take in <code>Chair</code> objects and add them to the <code>DoublyLinkedList</code> that we made; it has to be sorted by alphabetical order, if style is the same alphabetically, we sort by color</p> <p>When I try to go through the loop, I keep getting a <code>NullPointerException</code>.</p> <pre><code>public void add(Chair element){ if(isEmpty() || first.object.style.compareTo(element.style) &gt; 0 || (first.object.style.compareTo(element.style) == 0 &amp;&amp; first.object.color.compareTo(element.color) &gt;= 0){ addFirst(element); }else if(first.object.style.compareTo(element.style) &lt;= 0){ Node temp = first; Node insert = new Node(); insert.object = element; while(temp.object.style.compareTo(element.style) &lt;= 0) //This is where the nullPointerException occurs if(temp.hasNext()) temp = temp.next; while(temp.object.style.compareTo(element.style) == 0 &amp;&amp; temp.object.color.compareTo(element.color) &lt;= 0) if(temp.hasNext()) temp = temp.next; insert.prev = temp.prev; insert.next = temp; temp.prev.next = insert; temp.prev = insert; } } </code></pre> <p>This is the code where I put the information into the DoublyLinkedList</p> <pre><code>try{ FileReader fr = new FileReader(filename); Scanner sc = new Scanner(fr); String[] temp; while(sc.hasNext()){ temp = sc.nextLine().split(" "); if(temp[0].equals("Bed")){} else if(temp[0].equals("Table")){ // tables.add(new Table(Integer.parseInt(temp[1]), Integer.parseInt(temp[2]), Integer.parseInt(temp[3]), temp[4])); }else if(temp[0].equals("Desk")){} else if(temp[0].equals("Chair")){ chairs.add(new Chair(temp[1], temp[2])); }else if(temp[0].equals("Bookshelves")){} else{ color = temp[0]; } } while(!chairs.isEmpty()) System.out.println(chairs.removeFirst().info()); System.out.println(); //while(!tables.isEmpty()) // System.out.println(tables.removeFirst().info()); }catch(Exception e){e.printStackTrace();} </code></pre> <p>This is the DoublyLinkedList class that I've made: class CDoublyLinkedList{ Node first, last;</p> <pre><code>public CDoublyLinkedList(){ first = new Node(); last = new Node(); first.prev = last.next = null; first.object = last.object = null; first.next = last; last.prev = first; } public boolean isEmpty(){ return first.object == null; } public void addFirst(Chair element){ Node insert = new Node(); insert.object = element; insert.prev = null; insert.next = first; first.prev = insert; first = insert; } public void add(Chair element){ if(isEmpty() || first.object.style.compareTo(element.style) &gt; 0 || (first.object.style.compareTo(element.style) == 0 &amp;&amp; first.object.color.compareTo(element.color) &gt;= 0){ addFirst(element); }else if(first.object.style.compareTo(element.style) &lt;= 0){ Node temp = first; Node insert = new Node(); insert.object = element; while(first.object.style.compareTo(element.style) &lt;= 0) if(temp.hasNext()) temp = temp.next; while(first.object.style.compareTo(element.style) == 0 &amp;&amp; first.object.color.compareTo(element.color) &lt;= 0) if(temp.hasNext()) temp = temp.next; insert.prev = temp.prev; insert.next = temp; temp.prev.next = insert; temp.prev = insert; } } public Chair removeFirst(){ Chair tobedeleted = first.object; Node temp = first.next; first = temp; first.prev = null; return tobedeleted; } private class Node{ Node next, prev; Chair object; public boolean hasNext(){ return next != null; } } </code></pre> <p>}</p> <p>The Chair class:</p> <pre><code>class Chair extends Furniture{ public String style, color; public Chair(String s, String c){ style = s; color = c; } public String toString(){ return color; } public String getType(){ return "Chair"; } public String info(){ return (color+", "+style); } } </code></pre> <p>Can someone please explain to me why I keep getting this error? Thank you!</p> <p>EDIT: </p> <pre><code>while(temp.object.style.compareTo(element.style) &lt;= 0) //This is where the nullPointerException occurs chairs.add(new Chair(temp[1], temp[2])); java.lang.NullPointerException at CDoublyLinkedList.add(Furnish2SS.java:119) at Furnish2SS.main(Furnish2SS.java:23) java.lang.NullPointerException at CDoublyLinkedList.add(Furnish2SS.java:119) at Furnish2SS.main(Furnish2SS.java:23) </code></pre> <p>EDIT2: SOLVED!</p> <p>I changed my while loop to:</p> <pre><code>while(temp.object != null &amp;&amp; element != null &amp;&amp; (temp.object.compareTo(element) == 0 || temp.object.compareTo(element) == -1)) </code></pre> <p>The reason I got the error was because I wasn't checking for <code>null</code> every iteration.</p>
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    1. COone or many of your inner objects seem to be null first or object or style try putting the loop in a try catch to get the inner exception
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    2. COYou're getting the error because something is null. To find out what, look at the stacktrace and see what line it says the exception occurred on.
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      1. UStrutheality
    3. COThere is an object that is being referred to in the project that hasn't been instantiated or is set to null. Debug the program and please tell us which line your error is found in. This might even help YOU solve your own problem.
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