A problem about PsiClass.getSuperClass in Junit

I want to write some test cases about PsiClass, but I cann't get a psiClass's superClass in my test case.
Is there something I made mistakes?

I try to wrtie Test Case as follow
1) Create a local Java File "DemoTest.Java"

package myTest;
import myTest.MyBaseClass;
public class myBaseClassImpl extends MyBaseClass {
   <caret>
}

2) Wrtie a TestCase extends AndroidTestCase

3) Add a Class by text and Configure File in myFixture

myFixture.setTestDataPath(MyBaseDataPath);
myFixture.addClass( "package myTest; public class MyBaseClass{}");
myFixture.configureByFiles("DemoTest.java");

4) Try to get superClass, but it returns null

PsiElement element = myFixture.getFile().findElementAt(myFixture.getEditor().getCaretModel().getOffset());

while (element != null) {
     if (element instanceof PsiClass) {
          PsiClass psiClass = (PsiClas)element;
          PsiClass superClass = psiClass.getSuperClass(); // it returns null
     }
     element = element.parent();
}