Strange NullPointerException in JUnit

Question

I am running a simple JUnit test. The test is:

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);
}

In WordsToGuess, the constructor is:

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

In HangmanModel, the addWord method is:

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

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

This is a REALLY weird NullPointerException since everything should run perfectly fine. It is copied almost word for word from a similar project. Here is the Stack trace:

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

The @Before annotation is required before the setup method for JUnit 4

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