Here are the 20 most important Java String methods in an interview-friendly cheat sheet.

Method Syntax	Return Type	Primary Purpose	Example Usage	Result
`str.length()`	`int`	Returns the number of characters in the string	`"Java".length()`	`4`
`str.charAt(index)`	`char`	Returns the character at the specified index	`"Java".charAt(2)`	`'v'`
`str.substring(beginIndex)`	`String`	Returns substring from the given index to the end	`"Java".substring(2)`	`"va"`
`str.substring(beginIndex, endIndex)`	`String`	Returns substring between two indexes (`endIndex` excluded)	`"Java".substring(1,3)`	`"av"`
`str.indexOf(value)`	`int`	Returns the index of the first occurrence	`"Java".indexOf("a")`	`1`
`str.lastIndexOf(value)`	`int`	Returns the index of the last occurrence	`"Java".lastIndexOf("a")`	`3`
`str.contains(sequence)`	`boolean`	Checks whether the string contains a sequence of characters	`"Java".contains("av")`	`true`
`str.equals(str2)`	`boolean`	Compares two strings for exact equality (case-sensitive)	`"Java".equals("java")`	`false`
`str.equalsIgnoreCase(str2)`	`boolean`	Compares two strings ignoring case	`"Java".equalsIgnoreCase("java")`	`true`
`str.compareTo(str2)`	`int`	Lexicographically compares two strings	`"Apple".compareTo("Banana")`	Negative value
`str.split(regex)`	`String[]`	Splits the string into an array using a delimiter	`"A,B,C".split(",")`	`{"A","B","C"}`
`str.replace(oldValue, newValue)`	`String`	Replaces all occurrences of a character or string	`"Java".replace("a","o")`	`"Jovo"`
`str.replaceAll(regex, replacement)`	`String`	Replaces all substrings matching a regular expression	`"a1b2".replaceAll("\\d","")`	`"ab"`
`str.startsWith(prefix)`	`boolean`	Checks whether the string starts with the specified prefix	`"Java".startsWith("Ja")`	`true`
`str.endsWith(suffix)`	`boolean`	Checks whether the string ends with the specified suffix	`"Java".endsWith("va")`	`true`
`str.trim()`	`String`	Removes leading and trailing whitespace	`" Java ".trim()`	`"Java"`
`str.toUpperCase()`	`String`	Converts all characters to uppercase	`"java".toUpperCase()`	`"JAVA"`
`str.toLowerCase()`	`String`	Converts all characters to lowercase	`"JAVA".toLowerCase()`	`"java"`
`str.toCharArray()`	`char[]`	Converts the string into a character array	`"Java".toCharArray()`	`{'J','a','v','a'}`
`str.isEmpty()`	`boolean`	Checks whether the string length is `0`	`"".isEmpty()`	`true`
`str.concat(str2)`	`String`	Appends one string to another	`"Java".concat("8")`	`"Java8"`

⭐ Interview Quick Notes¶

Method	Time Complexity	Interview Tip
`length()`	O(1)	Most frequently used method.
`charAt()`	O(1)	Used in almost every string coding problem.
`substring()`	O(n)	`endIndex` is exclusive.
`indexOf()`	O(n)	Returns `-1` if not found.
`lastIndexOf()`	O(n)	Searches from the end.
`contains()`	O(n)	Returns `true` if the substring exists.
`equals()`	O(n)	Always use for string comparison, not `==`.
`equalsIgnoreCase()`	O(n)	Ignores uppercase/lowercase differences.
`compareTo()`	O(n)	Returns `0`, positive, or negative.
`split()`	O(n)	Returns a `String[]`; uses regular expressions.
`replace()`	O(n)	Does not use regex.
`replaceAll()`	O(n)	Uses regex; slower than `replace()`.
`startsWith()`	O(m)	`m` = prefix length.
`endsWith()`	O(m)	`m` = suffix length.
`trim()`	O(n)	Removes only leading and trailing spaces.
`toUpperCase()`	O(n)	Returns a new string.
`toLowerCase()`	O(n)	Returns a new string.
`toCharArray()`	O(n)	Common in DSA questions.
`isEmpty()`	O(1)	Equivalent to `length() == 0`.
`concat()`	O(n + m)	Creates a new string since `String` is immutable.

These 20 methods cover the vast majority of String operations asked in Java coding interviews and day-to-day development.