This is an alternate solution for reversing Strings in java

The algorithm:

• Split the String into an array with all the characters
• Iterate throught the array, starting from the last position and adding into a new string

public static String reverse(String s1) {
    if(isNull(s1)){
        return null;
    }
    StringBuilder stringBuilder = new StringBuilder();
    String[] splitted = s1.split("");
    for (int i = splitted.length - 1; i >= 0; i--) {
        stringBuilder.append(splitted[i]);
    }
    return stringBuilder.toString();
}

Tests:

@Test
public void reverse1(){
    String entry = "ABCDEF";
    String expected = "FEDCBA";
    String result = StringReverserByConcat.reverse(entry);
    assertEquals(expected, result);
}

@Test
public void reverseNull(){
    String entry = null;
    String expected = null;
    String result = StringReverserByConcat.reverse(entry);
    assertEquals(expected, result);
}

@Test
public void reverseEmpty(){
    String entry = "";
    String expected = "";
    String result = StringReverserByConcat.reverse(entry);
    assertEquals(expected, result);
}

Source code: Reverse Class Source code: Reverse Test