Java - Most Efficent way to traverse a String

The first version is more efficient. The second version of the code will end up being slower and use more memory because of the cost of creating and filling the new char[] with toCharArray().

For long strings (more than 512 chars, approximately), the fastest way to inspect the string is to use reflection to access the backing char[] of the String (but only works up to Java 8, because of Compact Strings):

String data = "a really long string";
Field field = String.class.getDeclaredField("value");
field.setAccessible(true);
char[] chars = (char[]) field.get(data);

for (int i = 0, n = chars.length; i < n; i++)
    System.out.println(chars[i]);

By using the above approach, we were able to avoid entirely the need to create a new char[] and also to pay the cost of an extra method call to charAt() in each iteration.

Take a look at this post, the answer contains detailed benchmarks. The best of both worlds, but anyway it was a hack and it no longer works.