diff --git a/src/algo/string/ReplaceString.java b/src/algo/string/ReplaceString.java
index 8daec87..f726952 100644
--- a/src/algo/string/ReplaceString.java
+++ b/src/algo/string/ReplaceString.java
@@ -7,7 +7,7 @@ public class ReplaceString {
     public static void main(String[] args) {
         System.out.println(replaceAll("x%20x%20", "%2", "c"));
     }
-    static String replaceAll(String s, String s1, String s2){
+    public static String replaceAll(String s, String s1, String s2){
         StringBuilder sb= new StringBuilder();
         char[] a= s.toCharArray();
         for (int i = 0; i < a.length; i++) {
diff --git a/src/algo/string/ReplaceStringTest.java b/src/algo/string/ReplaceStringTest.java
new file mode 100644
index 0000000..b4bc24d
--- /dev/null
+++ b/src/algo/string/ReplaceStringTest.java
@@ -0,0 +1,63 @@
+
+import org.junit.jupiter.api.Test;
+
+        import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class ReplaceStringTest {
+    /*-------------------Simple Unit Test------------------------*/
+    @Test
+    void unitTest1(){
+        String s = "aaa";
+        assertEquals("111", algo.string.ReplaceString.replaceAll(s,"a","1"));
+    }
+    @Test
+    void unitTest2(){
+        String s = "aaa";
+        assertEquals("   ", algo.string.ReplaceString.replaceAll(s,"a"," "));
+    }
+    @Test
+    void unitTest3(){
+        String s = "aaa";
+        assertEquals("aaa", algo.string.ReplaceString.replaceAll(s,"a","a"));
+    }
+    /*-------------------Some MCDC Test------------------------*/
+    @Test
+    void MCDCTest1(){
+        String s = "abcabc";
+        assertEquals("a1ca1c", algo.string.ReplaceString.replaceAll(s,"b","1"));
+        // TH: If(A)=T; If(B)=F; If(C) = T
+    }
+    @Test
+    void MCDCTest2(){
+        String s = "abcabc";
+        assertEquals("", algo.string.ReplaceString.replaceAll(s,"abc",""));
+        // TH: If(A)=f; If(B)=F; If(C) = T
+    }
+    @Test
+    void MCDCTest3(){
+        String s = "abcabc";
+        assertEquals("123123", algo.string.ReplaceString.replaceAll(s,"abc","123"));
+        // TH: If(A)=T; If(B)=T; If(C) = T
+    }
+    @Test
+        /*-------------------Some All DU path Test------------------------*/
+    void allDUpathTest1(){
+        String s = "abcabc";
+        assertEquals("1bc1bc", algo.string.ReplaceString.replaceAll(s,"a","1"));
+        // DU path cho s
+    }
+    @Test
+    void allDUpathTest2(){
+        String s = "abcabc";
+        assertEquals("123123", algo.string.ReplaceString.replaceAll(s,"abc","123"));
+        // DU path cho 1
+    }
+    @Test
+    void allDUpathTest3(){
+        String s = "abcabc";
+        assertEquals("", algo.string.ReplaceString.replaceAll(s,"abc",""));
+        // DU path cho 2
+    }
+
+
+}