easymock求助

现在我要对 TDDForInital类进行单体测试
代码如下：
public class TDDForInital {

public String returnStr(String str) {

if (null == str || str.length() < 3) {
return "";
}

return str.replaceAll(",", "-") + new A().a();
}
}

现在我想用easymock弄掉new A().a()方法

测试代码如下：
public class TestTDDForInital extends TestCase {

public void testReturnStr() {
A control = EasyMock.createMock(A.class);
control.a();
EasyMock.expectLastCall().andReturn("6");
EasyMock.replay(control);

TDDForInital t = new TDDForInital();
assertEquals("2006-03-036", t.returnStr("2006,03,03"));

EasyMock.verify(control);
}
}

为什么测试不通过啊？现象表明new A().a()方法根本没有被mock掉。
junit.framework.ComparisonFailure: expected:<2006-03-03[6]> but was:<2006-03-03[]>
at junit.framework.Assert.assertEquals(Assert.java:81)
at junit.framework.Assert.assertEquals(Assert.java:87)
at test_bj.TestTDDForInital.testReturnStr(TestTDDForInital.java:21)
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 junit.framework.TestCase.runTest(TestCase.java:164)
at junit.framework.TestCase.runBare(TestCase.java:130)
at junit.framework.TestResult$1.protect(TestResult.java:106)
at junit.framework.TestResult.runProtected(TestResult.java:124)
at junit.framework.TestResult.run(TestResult.java:109)
at junit.framework.TestCase.run(TestCase.java:120)
at org.eclipse.jdt.internal.junit.runner.junit3.JUnit3TestReference.run(JUnit3TestReference.java:130)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:460)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:673)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:386)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:196)