### Question:
ApiOAuth2TokenManager extends AbstractOAuthManager implements IApiOAuth2TokenManager { @Override public Collection<OAuth2AccessToken> findTokensByClientId(String clientId) { return this.getOAuth2TokenDAO().findTokensByClientId(clientId); }  @Override void init(); @Override void destroy(); @Override OAuth2Authentication readAuthentication(OAuth2AccessToken token); @Override OAuth2Authentication readAuthentication(String tokenValue); @Override void storeAccessToken(OAuth2AccessToken accessToken, OAuth2Authentication authentication); @Override OAuth2AccessToken readAccessToken(String tokenValue); @Override void removeAccessToken(OAuth2AccessToken token); @Override void storeRefreshToken(OAuth2RefreshToken refreshToken, OAuth2Authentication authentication); @Override OAuth2RefreshToken readRefreshToken(String tokenValue); @Override OAuth2Authentication readAuthenticationForRefreshToken(OAuth2RefreshToken refreshToken); @Override void removeRefreshToken(OAuth2RefreshToken refreshToken); @Override void removeAccessTokenUsingRefreshToken(OAuth2RefreshToken refreshToken); @Override OAuth2AccessToken createAccessTokenForLocalUser(String username); @Override OAuth2AccessToken getAccessToken(OAuth2Authentication authentication); @Override Collection<OAuth2AccessToken> findTokensByClientIdAndUserName(String clientId, String username); @Override Collection<OAuth2AccessToken> findTokensByUserName(String username); @Override Collection<OAuth2AccessToken> findTokensByClientId(String clientId); void setOAuth2TokenDAO(IOAuth2TokenDAO oAuth2TokenDAO);  }### Answer:
@Test public void findTokensByClientId() { when(tokenDAO.findTokensByClientIdAndUserName(Mockito.anyString(), Mockito.anyString())).thenReturn(new ArrayList<>()); Collection<OAuth2AccessToken> tokens = tokenManager.findTokensByClientId("clientId"); Assert.assertNotNull(tokens); }