JUnit中的奇異NullPointerException

Question

我正在運行一個簡單的JUnit測試。該測試是：

private HangmanModel model; 

private WordsToGuess word; 

public void setUp() 
{ 
    model = new HangmanModel(); 
    word = new WordsToGuess(); 
} 

@Test 
public void addWordAndChoose() 
{ 
    WordsToGuess testWord = new WordsToGuess("ahoy"); 
    model.addWord(testWord); <---- NullPointerException 

    String foundWord = model.randomWord(); 
    assertEquals("Not found the word", testWord, foundWord); 
} 

在WordsToGuess，構造函數是：

public WordsToGuess(String w) 
    { 
     word = w; 
    } 

在HangmanModel，該addWord方法是：

private ArrayList<WordsToGuess> words; 
words = new ArrayList<WordsToGuess>(); 

public void addWord(WordsToGuess w) 
    { 
     words.add(w); 
    } 

這是一個非常奇怪的NullPointerException異常，因爲一切都應該運行非常好。它幾乎是從一個類似的項目逐字複製。這裏是堆棧跟蹤：

java.lang.NullPointerException 
    at testing.HangmanModelTest.addWordAndChoose(HangmanModelTest.java:22) 
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) 
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) 
    at java.lang.reflect.Method.invoke(Method.java:606) 
    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.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) 

Answer 1

的@Before註釋安裝方法之前所需的JUnit 4

@Before 
public void setUp() { 
... 
}