diff --git a/build.gradle b/build.gradle
index 66ef630..9bddcf3 100644
--- a/build.gradle
+++ b/build.gradle
@@ -3,8 +3,8 @@ version '1.0-SNAPSHOT'
 
 apply plugin: 'java'
 
-sourceCompatibility = 1.8
-targetCompatibility = 1.8
+sourceCompatibility = 1.7
+targetCompatibility = 1.7
 
 repositories {
     mavenCentral()
diff --git a/src/main/java/com/kavenegar/sdk/KavenegarApi.java b/src/main/java/com/kavenegar/sdk/KavenegarApi.java
index f99c271..a80462c 100644
--- a/src/main/java/com/kavenegar/sdk/KavenegarApi.java
+++ b/src/main/java/com/kavenegar/sdk/KavenegarApi.java
@@ -10,7 +10,7 @@
 import com.kavenegar.sdk.excepctions.BaseException;
 import com.kavenegar.sdk.excepctions.HttpException;
 import com.kavenegar.sdk.models.*;
-import org.apache.commons.lang3.StringUtils;
+import com.kavenegar.sdk.utils.StringUtils;
 import org.apache.http.NameValuePair;
 import org.apache.http.client.ClientProtocolException;
 import org.apache.http.client.entity.UrlEncodedFormEntity;
@@ -32,6 +32,7 @@
  */
 public class KavenegarApi {
 
+
     static final String API_PATH = "https://api.kavenegar.com/v1/%s/%s.json";
 
     private String apiKey;
@@ -99,11 +100,11 @@ public List<SendResult> send(String sender, List<String> receptors, String messa
 
         JsonArray entry = execute(getApiPath("sms/send"),
                 "sender", sender,
-                "receptor", String.join(",", receptors),
+                "receptor", StringUtils.join(",", receptors),
                 "message", message,
                 "type", type.getValue(),
                 "date", date,
-                "localid", localIds == null ? "" : String.join(",", localIds)
+                "localid", localIds == null ? "" : StringUtils.join(",", localIds)
         ).getAsJsonArray();
 
         List<SendResult> list = new ArrayList<>();
diff --git a/src/main/java/com/kavenegar/sdk/utils/StringUtils.java b/src/main/java/com/kavenegar/sdk/utils/StringUtils.java
new file mode 100644
index 0000000..7dd12a7
--- /dev/null
+++ b/src/main/java/com/kavenegar/sdk/utils/StringUtils.java
@@ -0,0 +1,23 @@
+package com.kavenegar.sdk.utils;
+
+
+import java.util.List;
+
+/**
+ * Created by mohsenk on 2/23/17.
+ */
+public class StringUtils {
+
+    public static String join(CharSequence delimiter, List<?> elements) {
+        if (elements == null || elements.isEmpty()) {
+            return "";
+        }
+        // Number of elements not likely worth Arrays.stream overhead.
+        String result = elements.get(0).toString();
+        for (int i = 1; i < elements.size(); i++) {
+            result += delimiter + elements.get(i).toString();
+        }
+        return result;
+    }
+
+}