diff --git a/native/jni/src/bigram_dictionary.cpp b/native/jni/src/bigram_dictionary.cpp
index 8dc21220..bc34e4e 100644
--- a/native/jni/src/bigram_dictionary.cpp
+++ b/native/jni/src/bigram_dictionary.cpp
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-#include <string.h>
+#include <cstring>
 
 #define LOG_TAG "LatinIME: bigram_dictionary.cpp"
 
diff --git a/native/jni/src/bigram_dictionary.h b/native/jni/src/bigram_dictionary.h
index f66e27b..d676cca 100644
--- a/native/jni/src/bigram_dictionary.h
+++ b/native/jni/src/bigram_dictionary.h
@@ -24,7 +24,6 @@
 
 namespace latinime {
 
-class Dictionary;
 class BigramDictionary {
  public:
     BigramDictionary(const unsigned char *dict, int maxWordLength, int maxPredictions);
diff --git a/native/jni/src/char_utils.cpp b/native/jni/src/char_utils.cpp
index 9d2bc85..88803cb 100644
--- a/native/jni/src/char_utils.cpp
+++ b/native/jni/src/char_utils.cpp
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-#include <stdlib.h>
+#include <cstdlib>
 
 namespace latinime {
 
diff --git a/native/jni/src/correction.cpp b/native/jni/src/correction.cpp
index 74e256f..921ab0a 100644
--- a/native/jni/src/correction.cpp
+++ b/native/jni/src/correction.cpp
@@ -14,11 +14,10 @@
  * limitations under the License.
  */
 
-#include <assert.h>
-#include <ctype.h>
-#include <math.h>
-#include <stdio.h>
-#include <string.h>
+#include <cassert>
+#include <cctype>
+#include <cmath>
+#include <cstring>
 
 #define LOG_TAG "LatinIME: correction.cpp"
 
diff --git a/native/jni/src/correction.h b/native/jni/src/correction.h
index 13f951a..a7028ac 100644
--- a/native/jni/src/correction.h
+++ b/native/jni/src/correction.h
@@ -17,7 +17,7 @@
 #ifndef LATINIME_CORRECTION_H
 #define LATINIME_CORRECTION_H
 
-#include <assert.h>
+#include <cassert>
 #include <stdint.h>
 
 #include "correction_state.h"
diff --git a/native/jni/src/dictionary.cpp b/native/jni/src/dictionary.cpp
index dabd98b..b9f6534 100644
--- a/native/jni/src/dictionary.cpp
+++ b/native/jni/src/dictionary.cpp
@@ -14,8 +14,6 @@
  * limitations under the License.
  */
 
-#include <stdio.h>
-
 #define LOG_TAG "LatinIME: dictionary.cpp"
 
 #include "binary_format.h"
diff --git a/native/jni/src/proximity_info.cpp b/native/jni/src/proximity_info.cpp
index 393791f..53ae1ea 100644
--- a/native/jni/src/proximity_info.cpp
+++ b/native/jni/src/proximity_info.cpp
@@ -14,9 +14,9 @@
  * limitations under the License.
  */
 
-#include <assert.h>
-#include <math.h>
-#include <stdio.h>
+#include <cassert>
+#include <cmath>
+#include <cstring>
 #include <string>
 
 #define LOG_TAG "LatinIME: proximity_info.cpp"
diff --git a/native/jni/src/proximity_info_state.cpp b/native/jni/src/proximity_info_state.cpp
index d481a3b..04a2017 100644
--- a/native/jni/src/proximity_info_state.cpp
+++ b/native/jni/src/proximity_info_state.cpp
@@ -14,9 +14,8 @@
  * limitations under the License.
  */
 
-#include <assert.h>
+#include <cassert>
 #include <stdint.h>
-#include <string>
 
 #define LOG_TAG "LatinIME: proximity_info_state.cpp"
 
diff --git a/native/jni/src/proximity_info_state.h b/native/jni/src/proximity_info_state.h
index a5fe528..456e3ec 100644
--- a/native/jni/src/proximity_info_state.h
+++ b/native/jni/src/proximity_info_state.h
@@ -17,7 +17,7 @@
 #ifndef LATINIME_PROXIMITY_INFO_STATE_H
 #define LATINIME_PROXIMITY_INFO_STATE_H
 
-#include <assert.h>
+#include <cassert>
 #include <stdint.h>
 #include <string>
 
diff --git a/native/jni/src/unigram_dictionary.cpp b/native/jni/src/unigram_dictionary.cpp
index ce3108b..b2fc870 100644
--- a/native/jni/src/unigram_dictionary.cpp
+++ b/native/jni/src/unigram_dictionary.cpp
@@ -14,8 +14,8 @@
  * limitations under the License.
  */
 
-#include <assert.h>
-#include <string.h>
+#include <cassert>
+#include <cstring>
 
 #define LOG_TAG "LatinIME: unigram_dictionary.cpp"
 
diff --git a/native/jni/src/unigram_dictionary.h b/native/jni/src/unigram_dictionary.h
index f6aef2b..ac17f50 100644
--- a/native/jni/src/unigram_dictionary.h
+++ b/native/jni/src/unigram_dictionary.h
@@ -19,16 +19,15 @@
 
 #include <map>
 #include <stdint.h>
-#include "correction.h"
-#include "correction_state.h"
 #include "defines.h"
-#include "proximity_info.h"
-#include "words_priority_queue.h"
-#include "words_priority_queue_pool.h"
 
 namespace latinime {
 
+class Correction;
+class ProximityInfo;
 class TerminalAttributes;
+class WordsPriorityQueuePool;
+
 class UnigramDictionary {
     typedef struct { int first; int second; int replacement; } digraph_t;
 
diff --git a/native/jni/src/words_priority_queue.h b/native/jni/src/words_priority_queue.h
index fc9c55e..b24cf84 100644
--- a/native/jni/src/words_priority_queue.h
+++ b/native/jni/src/words_priority_queue.h
@@ -20,6 +20,8 @@
 #include <cstring> // for memcpy()
 #include <iostream>
 #include <queue>
+
+#include "correction.h"
 #include "defines.h"
 
 namespace latinime {
diff --git a/native/jni/src/words_priority_queue_pool.h b/native/jni/src/words_priority_queue_pool.h
index aff3074..3888729 100644
--- a/native/jni/src/words_priority_queue_pool.h
+++ b/native/jni/src/words_priority_queue_pool.h
@@ -17,8 +17,7 @@
 #ifndef LATINIME_WORDS_PRIORITY_QUEUE_POOL_H
 #define LATINIME_WORDS_PRIORITY_QUEUE_POOL_H
 
-#include <assert.h>
-#include <new>
+#include <cassert>
 #include "words_priority_queue.h"
 
 namespace latinime {