我正在使用二元决策树开发调查。用户会收到一个问题，他们回答“是”或“否”，答案确定接下来会问什么问题。
我的问题是代码只在左边工作，这意味着只允许用户回答“否”以使代码正常工作。如果用户给出答案是，则抛出NullPointerException。这是我做错了吗
import java.util.Scanner;
public class MusicGenreExpert {
private static LinkedBinaryTree<String> tree;
static Scanner scan = new Scanner(System.in);
public MusicGenreExpert() throws ElementNotFoundException
{
String e1 = "Are you eco-friendly?";
String e2 = "Do you consider yourself to be outgoing?";
String e3 = "Do you consider yourself to be an introvert?";
String e4 = "On an average day, do you think that you approach and solve" +
"problems differently than others?";
String e5 = "Are you often uneasy (nervous)?";
String e6 = "If the person in front of you dropped money without knowing," +
"would you pick it up and keep it?";
String e7 = "Is gentle a good word that describes you?";
String e8 = "Do you often worry about what other people may think?";
String e9 = "Do you think highly of yourself, with little to no care of" +
"what others may think?";
String e10 = "Are you happy with the accomplishments you've had in life" +
"thus far?";
String e11 = "Would you consider yourself to be old fashion?";
String e12 = "Do you put your all in everything that you do?";
String e13 = "Is your idea of an epic night going to a rave with thousands" +
"of people?";
String e14 = "Do you believe that Bob Marley is one of the greatest artist" +
"of all time?";
String e15 = "Would you call yourself an extravert?";
String e16 = "Do you know remember when you fell in love with" +
"Hip Hop?";
String e17 = "You are the music genre classical. You are a creative introvert"+
"with a good sense of self esteem. You are at ease with yourself as well as"+
"the world around you.";
String e18 = "The indie and punk music genre best fits you. You are an intellectual" +
"and creative introvert. You prefer not to work as hard as others and are less gentle." +
"Your self esteem is a constant battle.";
String e19 = "Rock/Heavy Metal best describes you. You are a creative and gentle" +
"introvert with a constant battle with self esteem. However you are no stranger to" +
"hard work.";
String e20 = "You are a high self esteem, honest and convetional" +
"introvert. The Pop culture is who you are inside. You may lack in creativity," +
"but you make up for in hard work.";
String e21 = " You remember the exact day you fell in love with Hip Hop." +
"\n With your high self esteem and outgoing personality, you are sure to be" +
"\n the life of the party. However, you could be more eco friendly and less" +
" aggressive.";
String e22 = "You are jazz, blues, and soul. You are a lot more extraverted" +
"than others. Your at ease personality, high self esteem, and creativity" +
"makes you the person that eveyone wants to know.";
String e23 = "The electronic/dance genre describes exactly who you are." +
"Assertive, outgoing, creative, yet aggressive gives everyone that comes" +
"in contact with you something to love. You aren't as kind as others, but" +
"hey, they love you anyway.";
String e24 = "Music is suppose to tell a story, and country music does" +
"just that. You are hardworking, conventional and outgoing. Oh and contrary" +
"to what others may believe your emotional stability is just fine.";
String e25 = "There is no other genre better to describe you than Reggae." +
"You are a creative outgoing individual, with high self esteem. Where you" +
"lack in hardwork you make up for in your kind and generous ways.";
LinkedBinaryTree<String> n2, n3, n4, n5, n6, n7, n8, n9, n10, n11, n12, n13, n14,
n15, n16, n17, n18, n19, n20, n21, n22, n23, n24, n25, n26, n27, n28, n29, n30,
n31, n32, n33, n34, n35, n36, n37, n38, n39, n40, n41, n42, n43, n44, n45, n46,
n47, n48, n49, n50, n51, n52, n53;
n50 = new LinkedBinaryTree<String> (e16);
n51 = new LinkedBinaryTree<String> (e17);
n33 = new LinkedBinaryTree<String> (e8, n50, n51);
n52 = new LinkedBinaryTree<String> (e16);
n53 = new LinkedBinaryTree<String> (e18);
n49 = new LinkedBinaryTree<String> (e8, n52, n53);
n32 = new LinkedBinaryTree<String> (e21);
n16 = new LinkedBinaryTree<String> (e15, n32, n33);
n34 = new LinkedBinaryTree<String> (e17);
n35 = new LinkedBinaryTree<String> (e18);
n18 = new LinkedBinaryTree<String> (e7, n34, n35);
n36 = new LinkedBinaryTree<String> (e18);
n37 = new LinkedBinaryTree<String> (e19);
n19 = new LinkedBinaryTree<String> (e12, n36, n37);
n38 = new LinkedBinaryTree<String> (e16);
n39 = new LinkedBinaryTree<String> (e20);
n23 = new LinkedBinaryTree<String> (e16, n38, n39);
n40 = new LinkedBinaryTree<String> (e21);
n41 = new LinkedBinaryTree<String> (e23);
n25 = new LinkedBinaryTree<String> (e8, n40, n41);
n42 = new LinkedBinaryTree<String> (e22);
n43 = new LinkedBinaryTree<String> (e20);
n26 = new LinkedBinaryTree<String> (e16, n42, n43);
n44 = new LinkedBinaryTree<String> (e23);
n45 = new LinkedBinaryTree<String> (e19);
n27 = new LinkedBinaryTree<String> (e6, n44, n45);
n46 = new LinkedBinaryTree<String> (e17);
n47 = new LinkedBinaryTree<String> (e24);
n28 = new LinkedBinaryTree<String> (e14, n46, n47);
n48 = new LinkedBinaryTree<String> (e17);
n30 = new LinkedBinaryTree<String> (e7, n48, n49);
n17 = new LinkedBinaryTree<String> (e18);
n8 = new LinkedBinaryTree<String> (e7, n16, n17);
n9 = new LinkedBinaryTree<String> (e9, n18, n19);
n20 = new LinkedBinaryTree<String> (e17);
n21 = new LinkedBinaryTree<String> (e18);
n10 = new LinkedBinaryTree<String> (e7, n20, n21);
n22 = new LinkedBinaryTree<String> (e19);
n11 = new LinkedBinaryTree<String> (e5, n22, n23);
n24 = new LinkedBinaryTree<String> (e19);
n12 = new LinkedBinaryTree<String> (e10, n24, n25);
n13 = new LinkedBinaryTree<String> (e11, n26, n27);
n29 = new LinkedBinaryTree<String> (e19);
n14 = new LinkedBinaryTree<String> (e12, n28, n29);
n31 = new LinkedBinaryTree<String> (e22);
n15 = new LinkedBinaryTree<String> (e13, n30, n31);
n4 = new LinkedBinaryTree<String> (e6, n8, n9);
n5 = new LinkedBinaryTree<String> (e9, n10, n11);
n6 = new LinkedBinaryTree<String> (e4, n12, n13);
n7 = new LinkedBinaryTree<String> (e3, n14, n15);
n2 = new LinkedBinaryTree<String> (e2, n4, n5);
n3 = new LinkedBinaryTree<String> (e3, n6, n7);
tree = new LinkedBinaryTree<String> (e1, n2, n3);
}
public static void diagnose(){
BinaryTree<String> current = tree;
System.out.println(" What music genre are you? ");
while(current.size() > 1)
{
System.out.println (current.getRootElement());
if (scan.nextLine().equalsIgnoreCase("N"))
current = current.getLeft();
else
current = current.getRight();
}
System.out.println (current.getRootElement());
}
}
这是实现BinaryTree的LinkedBinaryTree：
import java.util.Iterator;
public class LinkedBinaryTree<T> implements BinaryTree<T>
{
protected BTNode<T> root;
public LinkedBinaryTree(){
root = null;
}
public LinkedBinaryTree (T element)
{
root = new BTNode<T>(element);
}
public LinkedBinaryTree (T element, LinkedBinaryTree<T> left,
LinkedBinaryTree<T> right)
{
root = new BTNode<T>(element);
root.setLeft(left.root);
root.setRight(right.root);
}
public T getRootElement()
{
if (root == null)
throw new EmptyCollectionException ("Get root operation "
+ "failed. The tree is empty.");
return root.getElement();
}
public LinkedBinaryTree<T> getLeft()
{
if (root == null)
throw new EmptyCollectionException ("Get left operation "
+ "failed. The tree is empty.");
LinkedBinaryTree<T> result = new LinkedBinaryTree<T>();
result.root = root.getLeft();
return result;
}
public T find (T target)
{
BTNode<T> node = null;
if (root != null)
node = root.find(target);
if (node == null)
throw new ElementNotFoundException("Find operation failed. "
+ "No such element in tree.");
return node.getElement();
}
public int size()
{
int result = 0;
if (root != null)
result = root.count();
return result;
}
public Iterator<T> iterator()
{
return new ArrayIterator<T>();
}
public int height()
{
int result = 0;
if (root == null)
return result;
if (root != null)
result = root.height();
return result;
}
public BinaryTree<T> getRight() {
// TODO Auto-generated method stub
return null;
}
public boolean contains(T target) {
// TODO Auto-generated method stub
return false;
}
public boolean isEmpty() {
// TODO Auto-generated method stub
return false;
}
}
答案 0 :(得分：0)
现在您已提供LinkedBinaryTree的来源，问题很明显。调用getRight()的唯一时间是当您对其中一个执行此代码的决策回答Y或是时：
public BinaryTree<T> getRight() {
// TODO Auto-generated method stub
return null;
}
由于它正在返回null，下次您尝试取消引用current时 - 代码while(current.size() > 1)中会发生这种情况 - 会导致NullPointerException
getRight()方法的实施需要与getLeft()类似。