Java char data type. A 16-bit unsigned integer.

• Supports original 16-bit Unicode.
• Supports 21-bit Unicode 3.0

String:

• substring is O(1) in Java 6, but O(n) since Java 7 update 6
• concat is O(n)

StringBuilder:

• Underlying implementation. Resizing char[] array and length
• substring is O(n)
• concat is O(1)

StringBuffer is threadsafe and slower.

Radix. Number of digits R in alphabet. e.g. radix of English alphabet is 26.