Implementing String Algorithms

Looping over characters

• To process a String, loop over its indices from 0 to length() - 1.
• for (int i = 0; i < s.length(); i++) { ... s.substring(i, i+1) ... }
• s.substring(i, i+1) gives the one-character String at index i.
• This visits every character in order — the basis of most String algorithms.

Counting characters

• Combine the loop with an if to count matching characters.
• "Count the vowels": loop, and if the current character is a vowel, count++.
• The accumulator pattern from 2.9 applies directly to Strings.
• Compare characters with .equals(), not ==.

Reversing a string by walking its indexes backwards

Building a new String

• To transform text, build up a new String with +, since Strings are immutable.
• String result = ""; then result = result + something; inside the loop.
• "Reverse a String": append each character from the end toward the start.
• The original never changes — you accumulate a fresh String.

Index bounds

• Valid indices run 0 to length() - 1; going outside throws a StringIndexOutOfBoundsException 越界异常.
• s.substring(i, i+1) is safe only while i < s.length().
• The loop condition i < s.length() keeps i in range.
• Off-by-one bounds are the most common String-loop bug.