我正在用java创建一个基本上了解各种海洋动物的程序，要求用户考虑动物，然后询问用户问题，直到它准备好猜测。我使用二叉树来做到这一点。这是我现在的代码：
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");
}
}
不断出现的错误是“变量子（+任何数字）可能尚未启动。”我该如何解决这个问题？
哦，这是BTNode的代码：
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);
}
}
答案 0 :(得分：1)
声明变量时，请确保在使用变量之前对其进行初始化。也就是说，而不是写
BTNode<String> root;
BTNode<String> child;
BTNode<String> child1;
...
写
BTNode<String> root = null;
BTNode<String> child = null;
...
这是因为你以后继续写下声明
child = new BTNode<String>(LEFT_QUESTION, child2, child14);
child2和child14会被初始化（而在你的情况下，它们只被声明，而不是被初始化）