Работа со строками

Strings and indexing

• A String is a sequence of characters, numbered from 0.
• In "HELLO", index 0 is 'H', index 1 is 'E', … index 4 is 'O'.
• s.length() gives the count of characters; the last index is length() - 1.
• Off-by-one errors here are the most common String bug.

substring

• s.substring(start, end) returns the characters from index start up to but not including end.
• "HELLO".substring(1, 3) → "EL" (indices 1 and 2, not 3).
• s.substring(start) (one argument) goes from start to the end.
• The result's length is end - start.

String indices start at 0

indexOf and equals

• s.indexOf("lo") returns the first index where the text appears, or -1 if not found.
• s.equals(t) tests whether two Strings have the same characters — returns a boolean.
• Use .equals(), never ==, to compare String contents.
• == compares references (addresses), not the characters.

Strings are immutable

• A String is immutable 不可变: its characters can never be changed once created.
• Methods like substring and toUpperCase return a new String — they don't edit the original.
• s.toUpperCase(); alone does nothing visible; you must store the result: s = s.toUpperCase();
• Immutability is why String methods always hand back a fresh value.