[ISSUE #6198] BUG nacos-common>utils>StringUtils.join method NullPointerException #6198 (#6213)

* [ISSUE #6198] fix bug

* format code

* Modify Comment.Remove toString()

common/src/main/java/com/alibaba/nacos/common/utils/StringUtils.java

@@ -125,11 +125,12 @@ public class StringUtils {
     }
     
     /**
-     * Join object with input separator.
+     * <p>Joins the elements of the provided array into a single String
+     * containing the provided list of elements.</p>
      *
-     * @param collection collection of objects need to join
-     * @param separator  separator
-     * @return joined string
+     * @param collection  the Collection of values to join together, may be null
+     * @param separator  the separator string to use
+     * @return the joined String, {@code null} if null array input
      */
     public static String join(Collection collection, String separator) {
         if (collection == null) {
@@ -139,12 +140,15 @@ public class StringUtils {
         StringBuilder stringBuilder = new StringBuilder();
         Object[] objects = collection.toArray();
         
-        for (int i = 0; i < collection.size() - 1; i++) {
-            stringBuilder.append(objects[i].toString()).append(separator);
-        }
-        
-        if (collection.size() > 0) {
-            stringBuilder.append(objects[collection.size() - 1]);
+        for (int i = 0; i < collection.size(); i++) {
+            if (objects[i] != null) {
+                stringBuilder.append(objects[i]);
+                if (i != collection.size() - 1 && separator != null) {
+                    if (separator != null) {
+                        stringBuilder.append(separator);
+                    }
+                }
+            }
         }
         
         return stringBuilder.toString();

common/src/test/java/com/alibaba/nacos/common/utils/StringUtilsTest.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 1999-2018 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.alibaba.nacos.common.utils;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+
+/**
+ * String utils.
+ * @author zzq
+ */
+public class StringUtilsTest {
+    
+    @Test
+    public void testJoin() {
+        ArrayList<Object> objects = new ArrayList<>();
+        objects.add(null);
+        Assert.assertNull(StringUtils.join(null, "a"));
+        Assert.assertEquals(StringUtils.EMPTY, StringUtils.join(Arrays.asList(), "a"));
+        Assert.assertEquals(StringUtils.EMPTY, StringUtils.join(objects, "a"));
+        Assert.assertEquals("a;b;c", StringUtils.join(Arrays.asList("a", "b", "c"), ";"));
+        Assert.assertEquals("abc", StringUtils.join(Arrays.asList("a", "b", "c"),  null));
+    }
+    
+}