第二个编程课程 因此，我们的任务是链接列表，从头开始构建每个方法。 好吧，我在昨天前一天开始并且有一个空指针异常，我想他以后继续使用它。 好吧，把我的程序切割成什么都没有找到罪魁祸首我留下的代码应该从我们的实验室复制的工作（有效）。 如果你们认为你可以弄清楚为什么我在我的add方法id上得到一个空指针异常非常欣赏它，看看我是否正确地做了第二个构造函数。如果我能在这方面获得一些牵引力，那么它就会变得更容易，但我甚至无法开始。
你会注意到一些空白的方法，一旦我可以让我的构造函数+添加方法工作就生病了
我的代码：
import java.util.Collection;
import java.util.Iterator;
/**
* Created by hhhh on 11/2/2014.
*/
public class Lset<R> implements Set151Interface<R> {
private Node head;
private int length;
/**In the first (following) constructor im trying to re use code and call my clear method.
*Should save space and make code look cleaner.
*/
public Lset(){
clear();
}
public Lset(Collection<? extends R> list){
this();
for (R object : list) {
add(object);
}
}
/**
* Copied from Lab7, this add method checks to see if there are more nodes than just the head.
* After the check if false, creates a new node and adds it to the end of the list.
* @param entry
* @return
*/
@Override
public boolean add(R entry) {
Node newNode = new Node(entry);
// empty list is handled differently from a non-empty list
if (head.next == null) {
head = newNode;
} else {
Node lastNode = getNodeAt(length - 1);
lastNode.next = newNode;
}
length++;
return true;
}
@Override
public void clear() {
this.length = 0;
this.head = null;
}
@Override
public boolean contains(Object o) {
return false;
}
@Override
public Iterator<R> iterator() {
return null;
}
@Override
public boolean containsAll(Collection<?> c) {
return false;
}
@Override
public boolean isEmpty() {
return false;
}
@Override
public boolean remove(Object o) {
return false;
}
@Override
public boolean addAll(Collection<? extends R> c) {
return false;
}
@Override
public boolean removeAll(Collection<?> c) {
return false;
}
@Override
public boolean retainAll(Collection<?> c) {
return false;
}
@Override
public int size() {
return length;
}
@Override
public Object[] toArray() {
return null;
}
@Override
public <T> T[] toArray(T[] array) {
return null;
}
/**
* Code used in Lab 7, getNodeAt uses the length field and starts at head to traverse array and arrive at the
* position desired.
* @param position
* @return
*/
private Node getNodeAt(int position) {
assert !isEmpty() && (position >= 0) && position < length;
Node cNode = head;
for (int i = 0; i < position; i++)
cNode = cNode.next;
assert cNode != null;
return cNode;
}
public String toString(){
String arrayString = "<";
for(int i = 0; i < length; i++){
String two = getNodeAt(i).toString();
arrayString += two;
if(i <= (length - 2)){
two = ", ";
arrayString += two;
}
}
arrayString += ">";
return arrayString;
}
//TODO comment better
public class Node {
/** Reference to the data */
public R data;
/** Reference to the next node is in the list */
public Node next;
/**
* Sets the data for this node.
* @param data data to be carried by this node.
*/
public Node(R data) {
this.data = data;
this.next = null;
}
/**
* Sets the data for the node and assigns the next node in the list.
* @param data data to be carried by this node.
* @param nextNode next node in the list.
*/
public Node(R data, Node nextNode) {
this.data = data;
this.next = nextNode;
}
/**
* Returns just the data portion of the node.
* @return The data portion of the node.
*/
public R getData() {
return this.data;
}
/**
* Modified just the data portion of the node.
* @param data new data to be contained within the node.
*/
public void setData(R data) {
this.data = data;
}
/**
* What node does this node point to.
* @return the node that this node points to or null if it does not
* point anywhere.
*/
public Node getNextNode() {
return this.next;
}
/**
* Change the node that this node points to.
* @param nextNode a new node for this node to point to.
*/
public void setNextNode(Node nextNode) {
this.next = nextNode;
}
/**
* Display the state of just the data portion of the node.
*/
public String toString() {
return this.data.toString();
}
}
}
这是主要杀死它的方法
private void testConstruction() {
System.out.println("\nTesting Constructor");
System.out.print("----------------------------------------");
System.out.println("----------------------------------------");
Set151Interface s = makeSet();
//added
s.add("Butterfinger");
test(s.size() == 0,
"size() should return 0: " + s.size());
test(s.toString().equals("<>"),
"toString returns \"<>\": " + s.toString());
ArrayList<String> temp = new ArrayList<String>();
temp.add("Butterfinger");
temp.add("Milky Way");
temp.add("Kit Kat");
temp.add("Three Muskateers");
Set151Interface s3 = makeSet(temp);
test(s3.size() == 4,
"size should return 4: " + s3.size());
test(s3.toString().equals("<Butterfinger, Milky Way, Kit Kat, Three Muskateers>"),
"toString should return\n "+
"\"<Butterfinger, Milky Way, Kit Kat, Three Muskateers>\":\n "
+ s3.toString());
}
一旦butterfinger尝试添加，我就会得到指向此行的空指针异常
if (head.next == null) {
答案 0 :(得分：1)
您刚刚声明了private Node head;并且没有分配任何值。所以编译器会抛出NPE
答案 1 :(得分：0)
感谢帮助人员，我明白了:)。 在第一天我编辑了我的驱动程序（并忘记了）一旦我复制了驱动程序一切正常（到目前为止），再次感谢你们！