public void isFeatureFlagOn(String featureFlagId, TransactionContext transContext, boolean defaultReturnState)
    throws TAServiceException {
        boolean returnState = defaultReturnState;
        try {
            LDUser ldUser = new LDUser.Builder(transContext.getAuthId()).build();
            returnState = ldClient.boolVariation(featureFlagId, ldUser, defaultReturnState);
        } catch (Exception e) {
            returnState = defaultReturnState;
        }

        if (!returnState) {
            throw new TAServiceException("This service is not available", HttpStatus.NOT_FOUND);
        }
    }

// ...
when(mockLdUserBuilder.build()).thenReturn(mockLdUser);
classUnderTest.methodUnderTest(mockLdUserBuilder);
// ... assert/verify stuff

public void isFeatureFlagOn(...) throws TAServiceException {
     boolean returnState = defaultReturnState;
     try {
        getBuilder(transContext.getAuthId()).build();
        ...
     }
   ...
 }
 //package method
 Builder (int authId) {
     return new LDUser.Builder(authId);
 }

@Mock
LDUser.Builder builder;

@Test
public void test1() {
  MyClass myClass = new MyClass() {
    @Override
    Builder (int authId) {
      return builder;
    }
  }
  myClass.isFeatureFlagOn(...);
  ...
}