LinkedList NullPointerException（需要幫助實現一個LinkedList）

我需要實現一個LinkedList，並且到目前爲止我已經寫了我的方法來在我的列表中插入值。我有我的節點，前面，作爲我的類的實例數據，當創建我的第一個值，並試圖設置在我的新節點旁邊點的值時，我得到一個nullpointerexception。LinkedList NullPointerException（需要幫助實現一個LinkedList）

public class Class1 { 
private Node front = null;//Linked List pointer 
private int comparisons = 0; //# of comparisons in Lists 
//private LinkedList<Node> orderLink = new LinkedList<Node>(); 
public Class1() { 

} 

/*public Class1(int x){ 

}*/ 

public void insert (int num){ 
    Node current = null; 
    Node previous = null; 
    boolean placed = false; 
    if (front == null){ //generate first node of list 
     Node first = new Node(num, null); 
     front.setNext(first); 
     placed = true; 
    } 
    previous = front; 
    current = front.getNext(); 
    Node step = new Node(num, null); 
    if (placed == false){ 
    do{ 
     if (step.getData() < current.getData() && step.getData() > previous.getData() || step.getData() == current.getData() || step.getData() == previous.getData()){ //if the new data is between previous and current, place. If equals current, place. 
      //Insert into List 
      step.setNext(current); 
      previous.setNext(step); 
      placed = true; 
     } 
     if (previous == front && step.getData() < current.getData()){ //separate case for first node 
      step.setNext(current); 
      previous.setNext(step); 
      placed = true; 
     } 
     if (current.getNext() == null && step.getData() > current.getData()){ //case for last node 
      current.setNext(step); 
      placed = true; 
     } 
     //move a space up the list 
     previous = current; 
     current = current.getNext(); 

    }while(previous.getNext() != null || placed == false); 
    } 

} 

public int search(int num){ 
    int nodeIndex = 0; 

    return 1; //Return index of num 
} 
public void delete (int num){ 
    //delete num from the list 
} 
public void traverse(){ 
    System.out.println(front.getNext()); 
    System.out.println(front.getNext().getNext()); 
    System.out.println(front.getNext().getNext().getNext()); 
    System.out.println(front.getNext().getNext().getNext().getNext()); 
    System.out.println(front.getNext().getNext().getNext().getNext().getNext()); 
} 
public int getComparisons(){ 
    return comparisons; 
} 

public class Node { 
    private Node next; 
    private int data; 
    public Node(){ 
     next = null; 
    } 
    public Node(int data, Node next){ 
     this.next = next; 
     this.data = data; 
    } 
    public Node getNext(){ 
     return next; 
    } 
    public void setNext(Node nextNode){ 
     next = nextNode; 
    } 
    public int getData(){ 
     return data; 
    } 
    public void setData(int data){ 
     this.data = data; 
    } 
}

}

public class User { 
public static void main(String[] args){ 
    Class1 test = new Class1(); 
    test.insert(1); 
    test.insert(3); 
    test.insert(5); 
    test.insert(2); 
    test.insert(4); 

    test.traverse(); 
}

}

我剛開始學習LinkedLists，而且我不確定的是什麼，我實現錯了。任何幫助將不勝感激。你真的只需要看看我的插入方法和內部節點類。

來源

2014-12-03 Pds0303

您有問題在這裏：

front.setNext(first);

你不能讓空對象的任何操作（在條件你面前爲空）。也許這裏應該是：

front = first;

來源

2014-12-03 21:35:39

特別是因爲你已經檢查了前面是空之前，你就叫吧:) – realUser404 2014-12-03 21:37:15

的LinkedList使用三個參數 - >（leftNode，rightNode，數據）

來源

2017-01-30 01:47:02 user7487815

請擴展回答包含更多信息。 – 2017-01-30 02:01:15

LinkedList NullPointerException（需要幫助實現一個LinkedList）

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