Strings and StringBuilder
Strings: length, charAt, substring, immutability and StringBuilder.
A String is a sequence of characters. The main methods:
• s.length() — the length;
• s.charAt(i) — the character at index i (indices start at zero);
• s.substring(a, b) — the substring from index a up to b, excluding b;
• s.toLowerCase() / s.toUpperCase() — case;
• s.equals(t) — content comparison (not ==, as you remember from the conditions lesson).
Strings in Java are immutable: every "edit" creates a new string.
Java
Enter a word, for example: hello
Because strings are immutable, concatenating with + in a loop is slow: every operation copies the whole string. To build strings piece by piece there is StringBuilder — and it even has a ready-made reverse method.
Below — a palindrome check done two ways: by comparing characters and via StringBuilder.
Java
Palindrome check. Enter: racecar
Task: read a word and count how many times the letter "a" occurs in it (a loop + charAt).
The problems "Reverse a String", "Palindrome Check" and "Count the Vowels" are attached to this lesson — solve all three and mark the lesson as completed.