lucene4.2 java.lang.NullPointerException at org.apache.lucene.analysis.standard.StandardTokenizerIm

public class AnalyzerUtil {

public void displayToken(String str, Analyzer a) {
try {
TokenStream stream = a.tokenStream("content", new StringReader(str));
//            PositionIncrementAttribute pia = stream.addAttribute(PositionIncrementAttribute.class);
CharTermAttribute cta = stream.addAttribute(CharTermAttribute.class);
//            TypeAttribute ta = stream.addAttribute(TypeAttribute.class);
while(stream.incrementToken()) {
System.out.println(cta);
}
System.out.println("---------------------------------------------");
} catch (IOException e) {
e.printStackTrace();
}
}
}

public class AnalyzerTest {
private AnalyzerUtil au = null;

@Before
public void before() {
au = new AnalyzerUtil();
}

@Test
public void AnalyzerTest() {
String str = "this is my book,thank you";
StandardAnalyzer a1 = new StandardAnalyzer(Version.LUCENE_42);
au.displayToken(str, a1);
}

}

java.lang.NullPointerException
at org.apache.lucene.analysis.standard.StandardTokenizerImpl.zzRefill(StandardTokenizerImpl.java:923)
at org.apache.lucene.analysis.standard.StandardTokenizerImpl.getNextToken(StandardTokenizerImpl.java:1133)
at org.apache.lucene.analysis.standard.StandardTokenizer.incrementToken(StandardTokenizer.java:180)
at org.apache.lucene.analysis.standard.StandardFilter.incrementToken(StandardFilter.java:49)
at org.apache.lucene.analysis.core.LowerCaseFilter.incrementToken(LowerCaseFilter.java:54)
at org.apache.lucene.analysis.util.FilteringTokenFilter.incrementToken(FilteringTokenFilter.java:50)
at com.my.test.analyzer.AnalyzerUtil.displayToken(AnalyzerUtil.java:23)
at com.my.test.analyzer.AnalyzerTest.AnalyzerTest(AnalyzerTest.java:24)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at org.junit.internal.runners.TestMethodRunner.executeMethodBody(TestMethodRunner.java:99)
at org.junit.internal.runners.TestMethodRunner.runUnprotected(TestMethodRunner.java:81)
at org.junit.internal.runners.BeforeAndAfterRunner.runProtected(BeforeAndAfterRunner.java:34)
at org.junit.internal.runners.TestMethodRunner.runMethod(TestMethodRunner.java:75)
at org.junit.internal.runners.TestMethodRunner.run(TestMethodRunner.java:45)
at org.junit.internal.runners.TestClassMethodsRunner.invokeTestMethod(TestClassMethodsRunner.java:75)
at org.junit.internal.runners.TestClassMethodsRunner.run(TestClassMethodsRunner.java:36)
at org.junit.internal.runners.TestClassRunner$1.runUnprotected(TestClassRunner.java:42)
at org.junit.internal.runners.BeforeAndAfterRunner.runProtected(BeforeAndAfterRunner.java:34)
at org.junit.internal.runners.TestClassRunner.run(TestClassRunner.java:52)
at org.junit.runner.JUnitCore.run(JUnitCore.java:130)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:78)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:212)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:68)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)