java.lang.nullponiterException的问题

[biange2019]

前面的程序应该没有问题，问题主要应该在 Example7_1.java和LinkBinarySearchTree.java上
Example7_1.java

public class Example7_1
{
  public static void main(String[] args)
   {
	BinarySearchTree bst=new LinkBinarySearchTree();
	Integer []a={23,45,89,40,73,12,49,72,20,44};
	for(int i=0;i<a.length;i++)
	bst.insert(a[i]);
	System.out.println("前序：");bst.traverseBTree("preOrder");
	System.out.println("中序：");bst.traverseBTree("inOrder");
	System.out.println("后序：");bst.traverseBTree("postOrder");
	System.out.println("按层：");bst.traverseBTree("levelOrder");
	System.out.println("按节点值升序：");bst.ascend();
	System.out.println("深度："+bst.depthBTree());
	System.out.println("节点数："+bst.countBTree());
	System.out.println("查找38的结果："+bst.find(38));
	System.out.println("删除48的结果："+bst.delete(48));
	System.out.println("删除73的结果："+bst.delete(70));
	System.out.println("删除40的结果："+bst.delete(40));
	System.out.println("删除23的结果："+bst.delete(23));
	System.out.println("运算后的形式：");bst.printBTree();
   }
}
 

LinkBinarySearchTree.java

public class LinkBinarySearchTree extends LinkBinaryTree implements BinarySearchTree
{
	public LinkBinarySearchTree()
	{  super();}  //初始为空树

	public LinkBinarySearchTree(BTreeNode st)
	{  super(st);}   //把已知二叉树赋值

	public Object find(Object obj) 
	{                                //查找值为OBJ的节点
	   if(root==null) return null;
	     BTreeNode st=root;
	   while(st!=null)
	   {
	   	if(((Comparable)obj).compareTo(st.element)==0)
		           return st.element;
	        if(((Comparable)obj).compareTo(st.element)<0)
		            st=st.left;	
                   else   st=st.right;
	       }                 //20
	  return null;
       }

	public Object update(Object obj)
	{                     //更新值为OBJ的节点
	    if(root==null) return null;
	       BTreeNode st=root;
	    while(st!=null)
	      { 
		 if(((Comparable)obj).compareTo(st.element)==0) 
		 {
		      Object x=st.element;
		      st.element=obj;
		      return x;
		 }
		else if(((Comparable)obj).compareTo(st.element)<0)
		             st=st.left;
		     else    st=st.right;
	       }
         return null;       
	}
	
	public boolean insert(Object obj)
	{                             //插入节点
	     BTreeNode st=root,pt=null;
	     while(st!=null)
	      { 
		  pt=st;
		 if(((Comparable)obj).compareTo(st.element)==0)
		           return false;  
		 else  if(((Comparable)obj).compareTo(st.element)<0)
	              		 st=st.left;
		       else 	 st=st.right;
	      }
	 BTreeNode s=new BTreeNode(obj);
	 if(pt==null) root=s;
	 else if(((Comparable)obj).compareTo(st.element)<0)
	             pt.left=s;
	      else   pt.right=s;
	 return true;         
	}
	
	public boolean delete(Object obj)
	{                         //删除节点
	    if(root==null) return false;
	     BTreeNode st=root,pt=null;
	     while(st!=null)
	    { 
	       if(((Comparable)obj).compareTo(st.element)==0)//
		           break;
	      else   if(((Comparable)obj).compareTo(st.element)<0)  
                           { pt=st;st=st.left; }
	            else  { pt=st;st=st.right; }
	    }
	    if(st==null) return false;

	    if(st.left==null&&st.right==null)//分三种情况删除节点
	    {
		 if(st==root) root=null;
	 	 else  if(pt.left==st) 
			     pt.left=null;
	 	       else   
			  pt.right=null;
 	       }
	   if(st.left==null||st.right==null)
	     {
	 	  if(st==root)
	    	      if(st.left==null)
			  root=st.right;
	      	      else 
		         root=st.left;
	     	 else if(pt.left==st&&st.left==null)
			 pt.left=st.right;
	   	 else if(pt.left==st&&st.right==null)
			 pt.left=st.left;
	  	 else if(pt.right==st&&st.left==null)
		        pt.right=st.right;
	   	else if(pt.right==st&&st.right==null)
			pt.right=st.left;
		 }
	else if(st.left!=null||st.right!=null)
	{
	   BTreeNode s1=st,s2=st.left;	
	   while(s2.right!=null)
	 	 {  s1=s2;s2=s2.right;}
	  st.element=s2.element;
	  if(s1==st)
	 	 st.left=s2.left;
	  else
	 	 s1.right=s2.left;
	}
	return true;
      } 
	
	public void ascend()
	{traverseBTree("inOrder");}
 }

LinkBinaryTree.java

public class LinkBinaryTree implements  BinaryTree
   {
        protected  BTreeNode root;

        public LinkBinaryTree()
          {  
            root=null;
           }

        public LinkBinaryTree(BTreeNode rt)  //10
          {
             root=rt;
           }

        private void preOrder(BTreeNode rt) // xian geng xu
          { 
             if(rt!=null)
                {
                   System.out.print(rt.element+" ");
                    preOrder(rt.left);     //20
                    preOrder(rt.right);
                  }
           }

        private void inOrder(BTreeNode rt) // zhong geng xu
         { 
           if(rt!=null)
           {
             inOrder(rt.left);
             System.out.print(rt.element+" ");     //30
             inOrder(rt.right);
            }
          }

       private void postOrder(BTreeNode rt)
         {  
           if(rt!=null)
            {
               postOrder(rt.left);
               postOrder(rt.right);              // 40
               System.out.print(rt.element+" "); 
             }
          }

      private void levelOrder(BTreeNode rt)
         {    
           Quene que=new SequenceQuene();
           BTreeNode p=null;
           que.enter(rt);
           while(!que.isEmpty())        //50
            {   
              p=(BTreeNode)que.leave();
             System.out.print(p.element+" ");
             if(p.left!=null)
              que.enter(p.left);
             if(p.right!=null)
             que.enter(p.right);
           }
	}///////////////////////////
                   //60
public Object findBTree(BTreeNode rt, Object x)  //查找节点
      {
           if(rt==null) return null;
           else{
                  if(rt.element.equals(x))
                      {
                       return rt.element;
                       }      
                 else 
                     {
                       Object y;       //70
                      y= findBTree(rt.left, x);
                      if(y!=null) return y;
                       y= findBTree(rt.right, x);
                      if(y!=null) return y;
			return null;
                      }
              }
     }

 private int depthBTree(BTreeNode rt)
     {
         if(rt==null)
                  return 0;
        else{
                int dep1=depthBTree(rt.left);
                int dep2=depthBTree(rt.right);
              if(dep1>dep2)
                   return dep1+1;
              else
                   return dep2+1;
             }
       }
private int countBTree(BTreeNode rt)
       {
          if(rt==null)
                  return 0;
           else 
                return countBTree(rt.left)+countBTree(rt.right)+1;
       }
private void printBTree(BTreeNode rt)
    {   
        if(rt!=null)
          {
            System.out.print(rt.element);
            if(rt.left!=null||rt.right!=null)
              {
                System.out.print('(');
                printBTree(rt.left);
                if(rt.right!=null)
                System.out.print(',');  //160
                printBTree(rt.right);
                System.out.print(')');
              }
          }
   }
     public boolean createBTree(String str)
       {
         Stack sck=new SequenceStack();
         root=null;
         BTreeNode p=null;
         int k=1;
         char []a=str.toCharArray();
         for(int i=0;i<a.length;i++)
           {
             switch(a[i]){
               case ' ':
                   break;
               case '(':
                  sck.push(p);  k=1;
                   break;
               case ')':
                  if(sck.isEmpty()){
                  System.out.println("二叉树广义表字符串错");
                   return false;
                   }
                  sck.pop();break;
              case ',':
                  k=2;break;
              default:
               if((a[i]>='a'&&a[i]<='z'||a[i]>='A'&&a[i]<='Z'))
                    {
                      p=new BTreeNode(a[i]);
                      if(root==null)root=p;
                      else{
                         if(k==1)((BTreeNode)sck.peek()).left=p;
                         else  ((BTreeNode)sck.peek()).right=p;
                           }
                      }
               else 
                  {
                  System.out.println("二叉树广义表字符串错");
                  return false; 
                  }
               }  //switch
             } //for
         if(sck.isEmpty())return true;
         else  return false;
      }  // function

   public boolean isEmpty()
      {return root==null;}

  public void traverseBTree(String s)
        { 
          if(s.equals(mode[0])) preOrder(root);
          else if(s.equals(mode[1])) inOrder(root);
          else if(s.equals(mode[2])) postOrder(root);
          else if(s.equals(mode[3])) levelOrder(root);
          System.out.println();
        }
public int depthBTree()
{
   return  depthBTree(root);
}
  public int countBTree()
{
   return  countBTree(root);
} 
public Object  findBTree(Object obj)
{
   return  findBTree(root,obj);
}
public void printBTree()
{
  printBTree(root);
  System.out.println();
}
 public void clearBTree()
  {root=null;}    
  }

[biange2019]

为方便测试，我把相关的类也贴出来 二叉树接口BinaryTree.tree

public interface BinaryTree
  {
    final String[] mode={"preOrder","inOrder","postOrder","levelOrder"};
    boolean createBTree(String gt);
    boolean isEmpty();
    void traverseBTree(String s);
    Object findBTree(Object obj);
    int depthBTree();
    int countBTree();
    void printBTree();
    void clearBTree();
  }

二叉搜索树接口BinarySearchTree.tree

public interface BinarySearchTree extends BinaryTree
{
	Object find(Object obj);
	Object update(Object obj);
	boolean  insert(Object obj);
	boolean delete(Object obj);
	void ascend();
}

树的节点 BTreeNode.tree

public class BTreeNode
{
	Object element;
	BTreeNode left,right;
	public BTreeNode(Object obj)
	{
	 element=obj;left=right=null;
	 }
	public BTreeNode(Object obj,BTreeNode lt,BTreeNode  rt)
	{
	  element=obj;left=lt;right=rt;
	 }
}

[wula0010]

LinkBinarySearchTree的57行有空值啊，................... else if(((Comparable)obj).compareTo(st.element)<0) 断点调试看看哪个是空值，找出原因来

[sunandmoxi]

st = null

[村儿里人]

看到贴长代码的 帖子感觉好头痛

[biange2019]

else if(((Comparable)obj).compareTo(st.element)<0) 确实是st==null 把st应该是pt就可以了

[biange2019]

3年前的问题，一直耿耿于怀，今天终于有时间解决了