空指針異常

Question

我一直在使用一個驅動程序來測試我的一個數據結構（二叉搜索樹），我遇到過這個問題。 - 當我向bst插入2個以上的對象時發生 - 我正在嘗試做的事情是：我將4個對象插入樹中，然後刪除2個對象，然後打印出我的find方法，以顯示是否不是它找到我要求的對象。例如：

public class Driver5 { 

public static void main(String[] args) { 
    Integer c1 = new Integer(1); 
    Integer c2 = new Integer(2); 
    Integer c3 = new Integer(3); 
    Integer c4 = new Integer(4); 
    Integer c5 = new Integer(5); 
    Integer c6 = new Integer(6); 
    Integer c7 = new Integer(7); 
    Integer c8 = new Integer(8); 
    Integer c9 = new Integer(9); 
    Integer c10 = new Integer(10); 
    Integer c11 = new Integer(11); 
    Integer c12 = new Integer(23); 
    Integer c13 = new Integer(65); 
    Integer c14 = new Integer(45); 
    Integer c15 = new Integer(18); 
    Integer c16 = new Integer(33); 
    Integer c17 = new Integer(54); 

    DataStructure1<Integer> theData = new DataStructure1<Integer>(); 
    System.out.println("DS1");  
    long start = System.currentTimeMillis(); 
    theData.insert(c1); 
    theData.insert(c2); 
    theData.insert(c3); 
    theData.insert(c4); 
    theData.insert(c5); 
    theData.insert(c6); 
    theData.insert(c7); 
    theData.insert(c8); 
    theData.insert(c9); 
    theData.insert(c10); 
    theData.insert(c11); 
    theData.insert(c12); 
    theData.insert(c13); 
    theData.insert(c14); 
    theData.insert(c15); 
    theData.insert(c16); 
    theData.insert(c17); 
    theData.delete(c2); 
    theData.delete(c4); 
    System.out.println(theData.find(c1)); 
    System.out.println(theData.find(c2)); 
    System.out.println(theData.find(c3)); 
    System.out.println(theData.find(c4)); 
    theData.delete(c2); 
    theData.insert(c3); 
    System.out.println(theData.find(c2)); 
    System.out.println(theData.find(c3)); 
    System.out.println(theData.find(c4)); 
    System.out.println(theData.find(c5)); 
    System.out.println(theData.find(c6)); 
    System.out.println(theData.find(c7)); 
    System.out.println(theData.find(c8)); 
    System.out.println(theData.find(c9)); 
    System.out.println(theData.find(c10)); 
    System.out.println(theData.find(c11)); 
    System.out.println(theData.find(c12)); 
    System.out.println(theData.find(c13)); 
    System.out.println(theData.find(c14)); 
    System.out.println(theData.find(c15)); 
    System.out.println(theData.find(c16)); 
    System.out.println(theData.find(c17)); 

    long elapsed = System.currentTimeMillis() - start; 
    System.out.println("The time elapsed was: " + elapsed + " ms."); 
    System.out.println(); 

BinarySearchTree<Integer> theData1 = new BinarySearchTree<Integer>(); 
    System.out.println("BST");  
    long start1 = System.currentTimeMillis(); 
    theData1.insert(c1); 
    theData1.insert(c2); 
    theData1.insert(c3); 
    theData1.insert(c4); 
    theData1.insert(c5); 
    theData1.insert(c6); 
    theData1.insert(c7); 
    theData1.insert(c8); 
    theData1.insert(c9); 
    theData1.insert(c10); 
    theData1.insert(c11); 
    theData1.insert(c12); 
    theData1.insert(c13); 
    theData1.insert(c14); 
    theData1.insert(c15); 
    theData1.insert(c16); 
    theData1.insert(c17); 
    theData1.delete(c2); 
    theData1.delete(c4); 
    System.out.println(theData1.find(c1)); 
    System.out.println(theData1.find(c2)); 
    System.out.println(theData1.find(c3)); 
    System.out.println(theData1.find(c4)); 
    theData1.delete(c2); 
    theData1.insert(c3); 
    System.out.println(theData1.find(c2)); 
    System.out.println(theData1.find(c3)); 
    System.out.println(theData1.find(c4)); 
    System.out.println(theData1.find(c5)); 
    System.out.println(theData1.find(c6)); 
    System.out.println(theData1.find(c7)); 
    System.out.println(theData1.find(c8)); 
    System.out.println(theData1.find(c9)); 
    System.out.println(theData1.find(c10)); 
    System.out.println(theData1.find(c11)); 
    System.out.println(theData1.find(c12)); 
    System.out.println(theData1.find(c13)); 
    System.out.println(theData1.find(c14)); 
    System.out.println(theData1.find(c15)); 
    System.out.println(theData1.find(c16)); 
    System.out.println(theData1.find(c17)); 

    long elapsed1 = System.currentTimeMillis() - start1; 
    System.out.println("The time elapsed was: " + elapsed1 + " ms."); 




} 


} 

我在BST得到一個錯誤的位置：

if(nd.getLeft() == null && nd.getRight() == null) 
     { 

      nd = null; 
     } 

全刪除是：

public void delete(E item) { 

     TreeNode<E> nd = root; 

     while(nd != null && nd.getItem().compareTo(item) != 0) 
     { 
      if(nd.getItem().compareTo(item) < 0) 
       nd = nd.getRight(); 

      else 
       nd = nd.getLeft(); 
     } 

     if(nd.getLeft() == null && nd.getRight() == null) 
     { 

      nd = null; 
     } 

     else if(nd.getLeft() != null && nd.getRight() == null) 

     { 
      nd.setItem((E)nd.getLeft().getItem()); 

     } 
     else if(nd.getLeft() == null && nd.getRight() != null) 
     {  
      nd.setItem((E)nd.getRight().getItem()); 

     } 

     else if(nd.getLeft() != null && nd.getRight() != null) 
     { 

      nd.setItem((E)findsucc(nd)); 
     }  


} 

Answer 1

如果nd是null,那麼你不能打電話給getLeft()就可以了。因此，請您檢查

if (nd != null && nd.getLeft() == null && nd.getRight() == null) 

見all causes of a NullPointerException，以便下次你遇到它的時候，你知道該怎麼做，立即。

Answer 2

nd必須是空的某些原因。在打電話給getLeft()或getRight()之前檢查它。