ISSUE-4:fix SimpleKeyValueDeserializationSchema and SimpleKeyValueDeserializationSchema class field Accessibility. (#5)

Co-authored-by: gongzhongqiang <gongzhongqiang@oristand.com>
diff --git a/src/main/java/org/apache/rocketmq/flink/legacy/common/serialization/SimpleKeyValueDeserializationSchema.java b/src/main/java/org/apache/rocketmq/flink/legacy/common/serialization/SimpleKeyValueDeserializationSchema.java
index 456f477..3c9ae95 100644
--- a/src/main/java/org/apache/rocketmq/flink/legacy/common/serialization/SimpleKeyValueDeserializationSchema.java
+++ b/src/main/java/org/apache/rocketmq/flink/legacy/common/serialization/SimpleKeyValueDeserializationSchema.java
@@ -26,8 +26,8 @@
     public static final String DEFAULT_KEY_FIELD = "key";
     public static final String DEFAULT_VALUE_FIELD = "value";
 
-    public String keyField;
-    public String valueField;
+    private final String keyField;
+    private final String valueField;
 
     public SimpleKeyValueDeserializationSchema() {
         this(DEFAULT_KEY_FIELD, DEFAULT_VALUE_FIELD);
diff --git a/src/main/java/org/apache/rocketmq/flink/legacy/common/serialization/SimpleKeyValueSerializationSchema.java b/src/main/java/org/apache/rocketmq/flink/legacy/common/serialization/SimpleKeyValueSerializationSchema.java
index c3ae600..de01cbc 100644
--- a/src/main/java/org/apache/rocketmq/flink/legacy/common/serialization/SimpleKeyValueSerializationSchema.java
+++ b/src/main/java/org/apache/rocketmq/flink/legacy/common/serialization/SimpleKeyValueSerializationSchema.java
@@ -23,8 +23,8 @@
     public static final String DEFAULT_KEY_FIELD = "key";
     public static final String DEFAULT_VALUE_FIELD = "value";
 
-    public String keyField;
-    public String valueField;
+    private final String keyField;
+    private final String valueField;
 
     public SimpleKeyValueSerializationSchema() {
         this(DEFAULT_KEY_FIELD, DEFAULT_VALUE_FIELD);