Create a string with n characters

I highly suggest not to write the loop by hand. You will do that over and over again during the course of your programming career. People reading your code - that includes you - always have to invest time, even if it are just some seconds, to digest the meaning of the loop.

Instead reuse one of the available libraries providing code that does just that like StringUtils.repeatfrom Apache Commons Lang:

StringUtils.repeat(' ', length);

That way you also do not have to bother about performance, thus all the gory details of StringBuilder, Compiler optimisations etc. are hidden. If the function would turn out as slow it would be a bug of the library.

With Java 11 it becomes even easier:

" ".repeat(length);

Likely the shortest code using the String API, exclusively:

String space10 = new String(new char[10]).replace('\0', ' ');

System.out.println("[" + space10 + "]");
// prints "[          ]"

---

As a method, without directly instantiating char:

import java.nio.CharBuffer;

/**
 * Creates a string of spaces that is 'spaces' spaces long.
 *
 * @param spaces The number of spaces to add to the string.
 */
public String spaces( int spaces ) {
  return CharBuffer.allocate( spaces ).toString().replace( '\0', ' ' );
}

Invoke using:

System.out.printf( "[%s]%n", spaces( 10 ) );