Reverse a string is a common exercise and this is a simple Java solution

The algorithm is:

• Split the String into an array with all the characters
• Reverse the array
• Join the array again

public static String reverse(String s1){
        if(isNull(s1)){
            return null;
        }
        List<String> charList = asList(s1.split(""));
        Collections.reverse(charList);
        return String.join("", charList);
}

Tests:

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

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

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

Source code: Reverse Class Source code: Reverse Test