Question

在我的JUnitTest上，我收到了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;
        }
    }   
}

这是我的clear（）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：行上的错误“sb.append（p.data.toString（））;”

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提示null个对象，因此当您致电multipleItems时，null最有可能clear()。

Answer 2

我建议你有一张支票

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

为什么我在clear（）方法上得到NullPointerException？

2 个答案: