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我有一个项目是“以tree.java程序开始（代码清单8.1）并修改它以从一串字母（如A，B等等）创建一个二进制 树），每个 字母将显示在其自己的节点中，构建树使得包含字母的所有节点 都是叶子，父节点可以包含一些非字母 符号，如+，确保每个父母节点恰好有两个孩子 如果树不平衡，请不要担心。请注意，这不会是搜索树; 没有快速找到给定节点的方法。使用二叉树的空指针异常
import java.io.*;
import java.util.*;
class Node
{
public String iData; // data item (key)
public Node leftChild; // this node’s left child
public Node rightChild; // this node’s right child
public void displayNode() // display ourself
{
System.out.print('{');
System.out.print(iData);
System.out.print("} ");
}
} // end class Node
class Tree
{
private Node root; // first node of tree
public void setNode(Node newNode)
{root = newNode;}
public Node getNode()
{return root;}
// -------------------------------------------------------------
public Tree() // constructor
{ root = null; } // no nodes in tree yet
// -------------------------------------------------------------
public void traverse(int traverseType)
{
switch(traverseType)
{
case 1: System.out.print("\nPreorder traversal: ");
preOrder(root);
break;
case 2: System.out.print("\nInorder traversal: ");
inOrder(root);
break;
case 3: System.out.print("\nPostorder traversal: ");
postOrder(root);
break;
}
System.out.println();
}
private void preOrder(Node localRoot)
{
if(localRoot != null)
{
System.out.print(localRoot.iData + " ");
preOrder(localRoot.leftChild);
preOrder(localRoot.rightChild);
}
}
//A function I made to try and get the letters into leaves.
void preOrderLeaves(Node localRoot, Tree[] forest, int i)
{
if(localRoot != null)
{
localRoot.iData = "+";
localRoot.leftChild.iData = "+";
localRoot.rightChild = forest[i].getNode();
preOrderLeaves(localRoot.leftChild, forest, i + 1);
preOrderLeaves(localRoot.rightChild, forest, i + 1);
}
}
// -------------------------------------------------------------
private void inOrder(Node localRoot)
{
if(localRoot != null)
{
inOrder(localRoot.leftChild);
System.out.print(localRoot.iData + " ");
inOrder(localRoot.rightChild);
}
}
// -------------------------------------------------------------
private void postOrder(Node localRoot)
{
if(localRoot != null)
{
postOrder(localRoot.leftChild);
postOrder(localRoot.rightChild);
System.out.print(localRoot.iData + " ");
}
}
// -------------------------------------------------------------
public void displayTree()
{
Stack globalStack = new Stack();
globalStack.push(root);
int nBlanks = 32;
boolean isRowEmpty = false;
System.out.println(
"......................................................");
while(isRowEmpty==false)
{
Stack localStack = new Stack();
isRowEmpty = true;
for(int j=0; j<nBlanks; j++)
System.out.print(' ');
while(globalStack.isEmpty()==false)
{
Node temp = (Node)globalStack.pop();
if(temp != null)
{
System.out.print(temp.iData);
localStack.push(temp.leftChild);
localStack.push(temp.rightChild);
if(temp.leftChild != null ||
temp.rightChild != null)
isRowEmpty = false;
}
else
{
System.out.print("--");
localStack.push(null);
localStack.push(null);
}
for(int j=0; j<nBlanks*2-2; j++)
System.out.print(' ');
} // end while globalStack not empty
System.out.println();
nBlanks /= 2;
while(localStack.isEmpty()==false)
globalStack.push(localStack.pop());
} // end while isRowEmpty is false
System.out.println(
"......................................................");
} // end displayTree()
// -------------------------------------------------------------
}
public class Leaves
{
//I Tried to create an array of individual trees and then add them to a
//larger tree
public static void main(String[] args)
{
Tree[] forest = new Tree[10];
Scanner sc = new Scanner(System.in);
for(int i = 0; i < 10; i++)
{
String letter;
System.out.println("Enter a letter: ");
letter = sc.nextLine();
Node newNode = new Node();
newNode.iData = letter;
forest[i].setNode(newNode); //This line causes the null pointer exception
}
Tree letterTree = new Tree();
letterTree.preOrderLeaves(letterTree.getNode(), forest, 0);
letterTree.displayTree();
}
}
当我尝试将林设置为新节点时，我得到一个Null点异常。请帮忙。