### Question:
JavassistUtils { public static List<CtConstructor> extractValidConstructors(final CtClass classToTransform, CtClassFilter filter) throws NotFoundException { List<CtConstructor> constructors = new ArrayList<CtConstructor>(); CtConstructor[] declaredConstructors = classToTransform.getDeclaredConstructors(); for (CtConstructor constructor : declaredConstructors) { CtClass[] paramClasses = constructor.getParameterTypes(); if (paramClasses.length >= 1) { int indexValidParam = findValidParamIndex(paramClasses, filter); if (indexValidParam >= 0) { constructors.add(constructor); } } } return constructors; } private  JavassistUtils(); static boolean isArrayOf(CtField field, CtClass typeOfArrayElements); static boolean isArrayOf(CtField field, ClassPool classPool, Class<?> superClass); static boolean isBooleanArray(CtField field); static boolean isByteArray(CtField field); static boolean isCharArray(CtField field); static boolean isDoubleArray(CtField field); static boolean isFloatArray(CtField field); static boolean isIntArray(CtField field); static boolean isLongArray(CtField field); static boolean isShortArray(CtField field); static boolean isCharSequenceArray(CtField field, ClassPool classPool); static boolean isStringArray(CtField field, ClassPool classPool); static boolean isArrayList(CtField field, ClassPool classPool); static boolean isParcelableArray(CtField field, ClassPool classPool); static boolean isSerializableArray(CtField field, ClassPool classPool); static boolean isActivity(CtClass clazz); static boolean isService(CtClass clazz); static boolean isContext(CtClass clazz); static boolean isBroadCastReceiver(CtClass clazz); static boolean isContentProvider(CtClass clazz); static boolean isApplication(CtClass clazz); static boolean isFragment(CtClass clazz); static boolean isSupportFragment(CtClass clazz); static boolean isView(CtClass clazz); static boolean isSubType(ClassPool classPool, CtClass clazz, Class<?> superClass); static boolean isSubClass(ClassPool classPool, CtClass clazz, Class<?> superClass); static boolean isSubClass(ClassPool classPool, CtClass clazz, String superClassName); static List<CtField> getAllInjectedFieldsForAnnotation(CtClass clazz,
      Class<? extends Annotation> annotationClazz); static List<CtConstructor> extractValidConstructors(final CtClass classToTransform,
      CtClassFilter filter); static int findValidParamIndex(CtClass[] parameterTypes, CtClassFilter filter);  }### Answer:
@Test public void testExtractValidConstructors() throws Exception { Class<TestClassWithConstructors> aClass = TestClassWithConstructors.class; List<CtConstructor> constructorList1 = JavassistUtils.extractValidConstructors(ClassPool.getDefault().get(aClass.getName()), new FalseCtClassFilter()); List<CtConstructor> constructorList2 = JavassistUtils.extractValidConstructors(ClassPool.getDefault().get(aClass.getName()), new SingleCtClassFilter(Foo.class)); List<CtConstructor> constructorList3 = JavassistUtils.extractValidConstructors(ClassPool.getDefault().get(aClass.getName()), new SingleCtClassFilter(Foo.class)); assertThat(constructorList1.size(), is(0)); assertThat(constructorList2.size(), is(1)); assertThat(constructorList3.size(), is(1)); }