Country selector fixes

auth/src/main/java/com/firebase/ui/auth/AuthUI.java

@@ -668,8 +668,8 @@ public PhoneBuilder setDefaultCountryIso(@NonNull String iso) {
              * https://en.wikipedia.org/wiki/ISO_3166-1
              * and e-164 codes here: https://en.wikipedia.org/wiki/List_of_country_calling_codes
              *
-             * @param whitelistedCountries a case insensitive list of country codes and/or isos to
-             *                             be whitelisted
+             * @param whitelistedCountries a non empty case insensitive list of country codes
+             *                             and/or isos to be whitelisted
              */
             public PhoneBuilder setWhitelistedCountries(
                     @NonNull List<String> whitelistedCountries) {
@@ -678,6 +678,12 @@ public PhoneBuilder setWhitelistedCountries(
                             "You can either whitelist or blacklist country codes for phone " +
                                     "authentication.");
                 }
+
+                String message = "Invalid argument: The whitelist is %s. For this case, " +
+                        "don't use this method.";
+                Preconditions.checkNotNull(whitelistedCountries, String.format(message, "null"));
+                Preconditions.checkArgument(!whitelistedCountries.isEmpty(), String.format(message, "empty"));
+
                 addCountriesToBundle(whitelistedCountries, ExtraConstants.WHITELISTED_COUNTRIES);
                 return this;
             }
@@ -696,8 +702,8 @@ public PhoneBuilder setWhitelistedCountries(
              * https://en.wikipedia.org/wiki/ISO_3166-1
              * and e-164 codes here: https://en.wikipedia.org/wiki/List_of_country_calling_codes
              *
-             * @param blacklistedCountries a case insensitive list of country codes and/or isos to
-             *                             be blacklisted
+             * @param blacklistedCountries a non empty case insensitive list of country codes
+             *                             and/or isos to be blacklisted
              */
             public PhoneBuilder setBlacklistedCountries(
                     @NonNull List<String> blacklistedCountries) {
@@ -706,6 +712,12 @@ public PhoneBuilder setBlacklistedCountries(
                             "You can either whitelist or blacklist country codes for phone " +
                                     "authentication.");
                 }
+
+                String message = "Invalid argument: The blacklist is %s. For this case, " +
+                        "don't use this method.";
+                Preconditions.checkNotNull(blacklistedCountries, String.format(message, "null"));
+                Preconditions.checkArgument(!blacklistedCountries.isEmpty(), String.format(message, "empty"));
+
                 addCountriesToBundle(blacklistedCountries, ExtraConstants.BLACKLISTED_COUNTRIES);
                 return this;
             }

auth/src/main/java/com/firebase/ui/auth/util/Preconditions.java

@@ -91,4 +91,17 @@ public static void checkConfigured(@NonNull Context context,
             }
         }
     }
+
+    /**
+     * Ensures the truth of an expression involving parameters to the calling method.
+     *
+     * @param expression a boolean expression
+     * @param errorMessage the exception message to use if the check fails
+     * @throws IllegalArgumentException if {@code expression} is false
+     */
+    public static void checkArgument(boolean expression, String errorMessage) {
+        if (!expression) {
+            throw new IllegalArgumentException(errorMessage);
+        }
+    }
 }

auth/src/test/java/com/firebase/ui/auth/AuthUITest.java

@@ -29,6 +29,7 @@
 import org.junit.runner.RunWith;
 import org.robolectric.RobolectricTestRunner;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 
@@ -175,4 +176,32 @@ public void testPhoneBuilder_setBothBlacklistedAndWhitelistedCountries_expectIll
                 .build();
     }
 
+    @Test(expected = IllegalArgumentException.class)
+    public void testPhoneBuilder_passEmptyListForWhitelistedCountries_expectIllegalArgumentException() {
+        new IdpConfig.PhoneBuilder()
+                .setWhitelistedCountries(new ArrayList<String>())
+                .build();
+    }
+
+    @Test(expected = NullPointerException.class)
+    public void testPhoneBuilder_passNullForWhitelistedCountries_expectNullPointerException() {
+        new IdpConfig.PhoneBuilder()
+                .setWhitelistedCountries(null)
+                .build();
+    }
+
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testPhoneBuilder_passEmptyListForBlacklistedCountries_expectIllegalArgumentException() {
+        new IdpConfig.PhoneBuilder()
+                .setBlacklistedCountries(new ArrayList<String>())
+                .build();
+    }
+
+    @Test(expected = NullPointerException.class)
+    public void testPhoneBuilder_passNullForBlacklistedCountries_expectNullPointerException() {
+        new IdpConfig.PhoneBuilder()
+                .setBlacklistedCountries(null)
+                .build();
+    }
 }