猜測遊戲二叉樹Java

Question

我正在製作一個基本上知道各種海洋動物的程序，要求用戶想一個動物，然後詢問用戶的問題，直到它準備猜測。我用二叉樹來做到這一點。這裏是我的代碼截至目前：

import java.util.Scanner; 
public class TwentyQuestions 
{ 
    private static Scanner stdin = new Scanner(System.in); 
    public static void main(String[ ] args) 
    { 
     BTNode<String> root; 

     instruct(); 
     root = beginningTree(); 
     do 
     play(root); 
     while (query("Shall we play again?")); 

     System.out.println("Thanks for teaching me a thing or two."); 
     System.out.println ("Here is the tree:"); 
     root.print(1); 
    } 

    public static void instruct() 
    { 
     System.out.println("Please think of an ocean animal."); 
     System.out.println("I will ask some yes/no questions to try to figure out which animal you're thinking of."); 
    } 


    public static void play(BTNode<String> current) 
    { 
     while (!current.isLeaf()) 
     { 
     if (query(current.getData())) 
      current = current.getLeft(); 
     else 
      current = current.getRight(); 
     } 

     System.out.print("My guess is " + current.getData() + ". "); 
     if (!query("Am I right?")) 
     learn(current); 
     else 
     System.out.println("I knew it all along!"); 
    } 

    public static BTNode<String> beginningTree() 
    { 
     BTNode<String> root; 
     BTNode<String> child; 
     BTNode<String> child1; 
     BTNode<String> child2; 
     BTNode<String> child3; 
     BTNode<String> child4; 
     BTNode<String> child5; 
     BTNode<String> child6; 
     BTNode<String> child7; 
     BTNode<String> child8; 
     BTNode<String> child9; 
     BTNode<String> child10; 
     BTNode<String> child11; 
     BTNode<String> child12; 
     BTNode<String> child13; 
     BTNode<String> child14; 

     final String ROOT_QUESTION = "Is it a mammal?"; 
     final String LEFT_QUESTION = "Is it able to move on land?"; 
     final String LEFT_QUESTION2 = "Is it a solitary animal?"; 
     final String RIGHT_QUESTION2 = "Is it larger than a truck?"; 
     final String RIGHT_QUESTION3 = "Does it have tusks?"; 
     final String RIGHT_QUESTION = "Does it have any limbs/tentacles?"; 
     final String LEFT_QUESTION4 = "Does it have more than four limbs/tentacles?"; 
     final String LEFT_QUESTION5 = "Does it have an exoskeleton?"; 
     final String LEFT_QUESTION6 = "Does it have claws?"; 
     final String LEFT_QUESTION7 = "Does it have a long tail?"; 
     final String RIGHT_QUESTION7 = "Does it have 8 arms?"; 
     final String RIGHT_QUESTION5 = "Does it have a shell?"; 
     final String RIGHT_QUESTION4 = "Can it sting?"; 
     final String LEFT_QUESTION8 = "Is it long and snakelike?"; 
     final String RIGHT_QUESTION8 = "Is it generally smaller than a car?"; 
     final String ANIMAL1 = "Seal"; 
     final String ANIMAL2 = "Sea Lion"; 
     final String ANIMAL3 = "Walrus"; 
     final String ANIMAL4 = "Whale"; 
     final String ANIMAL5 = "Dolphin"; 
     final String ANIMAL6 = "Shrimp"; 
     final String ANIMAL7 = "Lobster"; 
     final String ANIMAL8 = "Crab"; 
     final String ANIMAL9 = "Jellyfish"; 
     final String ANIMAL10 = "Octopus"; 
     final String ANIMAL11 = "Squid"; 
     final String ANIMAL12 = "Turtle"; 
     final String ANIMAL13 = "Alligator"; 
     final String ANIMAL14 = "Eel"; 
     final String ANIMAL15 = "Stingray"; 
     final String ANIMAL16 = "Shark"; 
     final String ANIMAL17 = "Fish"; 

     // Create the root node with the question 「Are you a mammal?」 
     root = new BTNode<String>(ROOT_QUESTION, null, null); 

     child = new BTNode<String>(LEFT_QUESTION, child2, child14); 
     root.setLeft(child); 

     child2 = new BTNode<String>(LEFT_QUESTION2,null,child3); 
     child2.setLeft(new BTNode<String>(ANIMAL1, null, null)); 
     child.setLeft(child2); 

     child14 = new BTNode<String>(RIGHT_QUESTION2,null,null); 
     child14.setLeft(new BTNode<String>(ANIMAL4,null,null)); 
     child14.setRight(new BTNode<String>(ANIMAL5,null,null)); 
     child.setRight(child14); 

     child3 = new BTNode<String>(RIGHT_QUESTION3, null, null); 
     child3.setLeft(new BTNode<String>(ANIMAL3, null, null)); 
     child3.setRight(new BTNode<String>(ANIMAL2, null, null)); 
     child.setRight(child3); 

     child1 = new BTNode<String>(RIGHT_QUESTION, child4, child8); 
     root.setRight(child1); 

     child4 = new BTNode<String>(LEFT_QUESTION4,child5,child10); 
     child1.setLeft(child4); 

     child5 = new BTNode<String>(LEFT_QUESTION5,child6,child8); 
     child4.setLeft(child5); 

     child6 = new BTNode<String>(LEFT_QUESTION6,child7, null); 
     child6.setRight(new BTNode<String>(ANIMAL6,null,null)); 
     child5.setLeft(child6); 

     child7 = new BTNode<String>(LEFT_QUESTION7, null, null); 
     child7.setLeft(new BTNode<String>(ANIMAL7,null,null)); 
     child7.setRight(new BTNode<String>(ANIMAL8,null,null)); 
     child6.setLeft(child7); 

     child8 = new BTNode<String>(RIGHT_QUESTION4,null,child9); 
     child8.setLeft(new BTNode<String>(ANIMAL9,null,null)); 
     child5.setRight(child8); 

     child9 = new BTNode<String>(RIGHT_QUESTION7,null,null); 
     child9.setLeft(new BTNode<String>(ANIMAL10,null,null)); 
     child9.setRight(new BTNode<String>(ANIMAL11,null,null)); 
    child8.setRight(child9); 

     child10 = new BTNode<String>(RIGHT_QUESTION5,null,null); 
     child10.setLeft(new BTNode<String>(ANIMAL12,null,null)); 
     child10.setRight(new BTNode<String>(ANIMAL13,null,null)); 
     child4.setRight(child10); 

     child11 = new BTNode<String>(RIGHT_QUESTION4,child12,child13); 
     child1.setRight(child11); 

     child12 = new BTNode<String>(LEFT_QUESTION8,null,null); 
     child12.setLeft(new BTNode<String>(ANIMAL14,null,null)); 
     child12.setRight(new BTNode<String>(ANIMAL15,null,null)); 
    child11.setLeft(child12); 

     child13 = new BTNode<String>(RIGHT_QUESTION8,null,null); 
     child13.setLeft(new BTNode<String>(ANIMAL17,null,null)); 
     child13.setRight(new BTNode<String>(ANIMAL16,null,null)); 
     child11.setRight(child13); 

     return root; 
    } 


    public static void learn(BTNode<String> current) 

    { 
     String guessAnimal; // The animal that was just guessed 
     String correctAnimal; // The animal that the user was thinking of 
     String newQuestion; // A question to distinguish the two animals 

     // Set Strings for the guessed animal, correct animal and a new question. 
     guessAnimal = current.getData(); 
     System.out.println("I give up. What are you? "); 
     correctAnimal = stdin.nextLine(); 
     System.out.println("Please type a yes/no question that will distinguish a"); 
     System.out.println(correctAnimal + " from a " + guessAnimal + "."); 
     newQuestion = stdin.nextLine(); 

     // Put the new question in the current node, and add two new children. 
     current.setData(newQuestion); 
     System.out.println("As a " + correctAnimal + ", " + newQuestion); 
     if (query("Please answer")) 
     { 
     current.setLeft(new BTNode<String>(correctAnimal, null, null)); 
     current.setRight(new BTNode<String>(guessAnimal, null, null)); 
     } 
     else 
     { 
     current.setLeft(new BTNode<String>(guessAnimal, null, null)); 
     current.setRight(new BTNode<String>(correctAnimal, null, null)); 
     }   
    } 

    public static boolean query(String prompt) 
    { 
     String answer; 

     System.out.print(prompt + " [Y or N]: "); 
     answer = stdin.nextLine().toUpperCase(); 
     while (!answer.startsWith("Y") && !answer.startsWith("N")) 
     { 
    System.out.print("Invalid response. Please type Y or N: "); 
    answer = stdin.nextLine().toUpperCase(); 
     } 

     return answer.startsWith("Y"); 
    } 

} 

---

，保持上來的錯誤是「變量名兒童（+任意數字）可能沒有inititalized。」我該如何解決？

哦，這裏是爲BTNode代碼：

public class BTNode<E> 
    { 
     private E data; 
     private BTNode<E> left, right; 
     public BTNode(E initialData, BTNode<E> initialLeft, BTNode<E> initialRight) 
     { 
      data = initialData; 
      left = initialLeft; 
      right = initialRight; 
     }  

     public E getData() 
     { 
      return data; 
     } 



    public BTNode<E> getLeft() 
    { 
     return left;            
    } 



    public E getLeftmostData() 
    { 
     if (left == null) 
     return data; 
     else 
     return left.getLeftmostData(); 
    } 


    public BTNode<E> getRight() 
    { 
     return right;            
    } 


    public E getRightmostData() 
    { 
     if (left == null) 
     return data; 
     else 
     return left.getRightmostData(); 
    } 

    public void inorderPrint() 
    { 
     if (left != null) 
     left.inorderPrint(); 
     System.out.println(data); 
     if (right != null) 
     right.inorderPrint(); 
    } 


    public boolean isLeaf() 
    { 
     return (left == null) && (right == null);            
    } 


    public void preorderPrint() 
    { 
     System.out.println(data); 
     if (left != null) 
     left.preorderPrint(); 
     if (right != null) 
     right.preorderPrint(); 
    } 

    public void postorderPrint() 
    { 
     if (left != null) 
     left.postorderPrint(); 
     if (right != null) 
     right.postorderPrint(); 
     System.out.println(data); 
    } 

    public void print(int depth) 
    { 
     int i; 

     // Print the indentation and the data from the current node: 
     for (i = 1; i <= depth; i++) 
     System.out.print(" "); 
     System.out.println(data); 

     if (left != null) 
     left.print(depth+1); 
     else if (right != null) 
     { 
     for (i = 1; i <= depth+1; i++) 
      System.out.print(" "); 
     System.out.println("--"); 
     } 

     if (right != null) 
     right.print(depth+1); 
     else if (left != null) 
     { 
     for (i = 1; i <= depth+1; i++) 
      System.out.print(" "); 
     System.out.println("--"); 
     } 
    } 

    public BTNode<E> removeLeftmost() 
    { 
     if (left == null) 
     return right; 
     else 
     { 
     left = left.removeLeftmost(); 
     return this; 
     } 
    } 

    public BTNode<E> removeRightmost() 
    { 
     if (right == null) 
     return left; 
     else 
     { 
     right = right.removeRightmost(); 
     return this; 
     } 
    } 

    public void setData(E newData) 
    { 
     data = newData; 
    }                


    public void setLeft(BTNode<E> newLeft) 
    {      
     left = newLeft; 
    } 



    public void setRight(BTNode<E> newRight) 
    {      
     right = newRight; 
    } 

    public static <E> BTNode<E> treeCopy(BTNode<E> source) 
    { 
     BTNode<E> leftCopy, rightCopy; 

     if (source == null) 
     return null; 
     else 
     { 
     leftCopy = treeCopy(source.left); 
     rightCopy = treeCopy(source.right); 
     return new BTNode<E>(source.data, leftCopy, rightCopy); 
     } 
    } 

    public static <E> long treeSize(BTNode<E> root) 
    { 
     if (root == null) 
     return 0; 
     else 
     return 1 + treeSize(root.left) + treeSize(root.right); 
    } 

} 

Answer 1

當你聲明變量，一定要在使用前也對它們進行初始化。也就是說，不是寫

BTNode<String> root; 
BTNode<String> child; 
BTNode<String> child1; 
... 

寫

BTNode<String> root = null; 
BTNode<String> child = null; 
... 

這是因爲當你以後去寫聲明

child = new BTNode<String>(LEFT_QUESTION, child2, child14); 

的child2和child14將被初始化（而在，你的情況，他們只被宣佈，沒有初始化）