为什么我的clear()方法上会出现NullPointerException?

Question

在我的JUnitTest中，我收到Java.Lang.NullPointerException错误，这是什么意思？我的 clear() 方法错了吗？

以下是我的字段：

private static class Node<E> {
/** The data value. */
private E data;
/** The link */
private Node<E> next = null;
/**
* Construct a node with the given data value and link
* @param data - The data value 
* @param next - The link
*/
public Node(E data, Node<E> next) {
this.data = data;
this.next = next;
}
/**
* Construct a node with the given data value
* @param data - The data value 
*/
public Node(E data) {
this(data, null);
}
}
/*</listing>*/
// Data fields
/** A reference to the head of the list */
private Node<E> head = null;
/** The size of the list */
private int size = 0;    

这是我的clear()方法：

/**
* Removes all of the elements from this list (optional operation). 
* The list will be empty after this call returns
* @throws UnsupportedOperationException if the clear operation is not supported by this list
*/
public void clear() {
Node<E> temp = head;
for(int i=0; i < size; i++){
if(temp != null){
temp.data = null;
temp = temp.next;
}
}   
}

这是我对clear()的测试方法：

public void testClear() {
multipleItems.clear();
assertEquals("", multipleItems.toString());
}

堆栈跟踪？

java.lang.NullPointerException
at SingleLinkedList.toString(SingleLinkedList.java:196)
at SingleLinkedListTest.testClear(SingleLinkedListTest.java:93)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
java.lang.NullPointerException
at SingleLinkedList.toString(SingleLinkedList.java:196)
at SingleLinkedListTest.testClear(SingleLinkedListTest.java:93)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

toString 方法：行"sb.append(p.data.toString());"上的错误

public String toString() {
StringBuilder sb = new StringBuilder("[");
Node p = head;
if (p != null) {
while (p.next != null) {
sb.append(p.data.toString());
//sb.append(" ==> ");
sb.append(", ");
p = p.next;
}
sb.append(p.data.toString());
}
sb.append("]");
return sb.toString();
}

Answer 1

NullPointerException

对null对象的提示，因此当您对其调用clear()时，multipleItems最有可能null。

Answer 2

我建议你做一个像

if(temp.hasNext) {
temp = temp.next
}