Fixed issue #30. Same behavour as the default implementation(javascri…

…pt). Array length check moved up.

src/main/java/org/hashids/Hashids.java

@@ -166,15 +166,18 @@ public String decryptHex(String hash) {
    * @return the encrypt string
    */
   public String encode(long... numbers) {
-    for (long number : numbers) {
+	if (numbers.length == 0) {
+	  return "";
+	}
+
+	for (long number : numbers) {
+      if (number < 0) {
+        return "";
+      }
       if (number > MAX_NUMBER) {
         throw new IllegalArgumentException("number can not be greater than " + MAX_NUMBER + "L");
       }
     }
-
-    if (numbers.length == 0) {
-      return "";
-    }
     return this._encode(numbers);
   }
 

src/test/java/org/hashids/HashidsTest.java

@@ -145,4 +145,13 @@ public void test_issue23() {
     res2 = a.decode(expected);
     Assert.assertEquals(res2[0], num_to_hash);
   }
+
+  @Test
+  public void test_issue30() {
+	  String expected = "", res;
+	  long num_to_hash = -1L;
+	  Hashids a = new Hashids("this is my salt");
+	  res = a.encode(num_to_hash);
+	  Assert.assertEquals(expected, res);
+  }
 }