Java String Methods Cheat Sheet

Based on common DSA patterns, here’s a complete reference of String methods frequently used in coding interviews and competitive programming.

1. Character Access & Information

Method

Return Type

Description

Example

charAt(int index)

char

Get character at index

"Java".charAt(0) → ‘J’

length()

int

Get string length

"Hello".length() → 5

toCharArray()

char[]

Convert to char array

"abc".toCharArray() → [‘a’,‘b’,‘c’]

codePointAt(int index)

int

Get Unicode value

"A".codePointAt(0) → 65

isEmpty()

boolean

Check if empty

"".isEmpty() → true

isBlank()

boolean

Check if empty/whitespace (Java 11+)

" ".isBlank() → true

DSA Usage:

// Character frequency counting
for (char c : str.toCharArray()) {
  freq[c - 'a']++;
}
// Palindrome check
for (int i = 0; i < str.length() / 2; i++) {
  if (str.charAt(i) != str.charAt(str.length() - 1 - i)) {
    return false;
  }
}

2. Search & Index Methods

Method

Return Type

Description

Example

indexOf(String str)

int

First occurrence index

"hello".indexOf("ll") → 2

indexOf(char ch)

int

First char occurrence

"hello".indexOf('e') → 1

indexOf(String str, int from)

int

Index from position

"hello".indexOf("l", 3) → 3

lastIndexOf(String str)

int

Last occurrence index

"hello".lastIndexOf("l") → 3

contains(CharSequence s)

boolean

Check if contains substring

"hello".contains("ell") → true

startsWith(String prefix)

boolean

Check prefix

"hello".startsWith("he") → true

endsWith(String suffix)

boolean

Check suffix

"hello".endsWith("lo") → true

DSA Usage:

// Pattern matching
if (text.indexOf(pattern) != -1) {
  // Pattern found
}
// File extension validation
if (fileName.endsWith(".java")) {
  // Process Java file
}

3. Comparison Methods

Method

Return Type

Description

Example

equals(Object obj)

boolean

Content equality

"abc".equals("abc") → true

equalsIgnoreCase(String str)

boolean

Case-insensitive equals

"ABC".equalsIgnoreCase("abc") → true

compareTo(String str)

int

Lexicographic comparison

"abc".compareTo("abd") → -1

compareToIgnoreCase(String str)

int

Case-insensitive compare

"ABC".compareToIgnoreCase("abc") → 0

contentEquals(CharSequence cs)

boolean

Compare with CharSequence

"test".contentEquals("test") → true

DSA Usage:

// Sorting strings
Arrays.sort(strings, (a, b) -> a.compareTo(b));
// Case-insensitive comparison
if ("Admin".equalsIgnoreCase(userRole)) {
  // Grant access
}

4. Substring & Extraction Methods

Method

Return Type

Description

Example

substring(int begin)

String

Substring from begin to end

"Hello".substring(2) → “llo”

substring(int begin, int end)

String

Substring

DSA Usage:

// Generate all substrings
for (int i = 0; i < str.length(); i++) {
  for (int j = i + 1; j <= str.length(); j++) {
    String sub = str.substring(i, j);
    // Process substring
  }
}
// Sliding window
String window = str.substring(left, right + 1);

5. Modification Methods (Return New String)

Method

Return Type

Description

Example

toLowerCase()

String

Convert to lowercase

"HELLO".toLowerCase() → “hello”

toUpperCase()

String

Convert to uppercase

"hello".toUpperCase() → “HELLO”

trim()

String

Remove leading/trailing spaces

" hi ".trim() → “hi”

strip()

String

Remove Unicode whitespace (Java 11+)

" hi ".strip() → “hi”

stripLeading()

String

Remove leading whitespace

" hi".stripLeading() → “hi”

stripTrailing()

String

Remove trailing whitespace

"hi ".stripTrailing() → “hi”

replace(char old, char new)

String

Replace all chars

"hello".replace('l', 'L') → “heLLo”

replace(CharSequence target, CharSequence replacement)

String

Replace all occurrences

"hello".replace("ll", "LL") → “heLLo”

replaceAll(String regex, String replacement)

String

Replace using regex

"a1b2".replaceAll("\\d", "X") → “aXbX”

replaceFirst(String regex, String replacement)

String

Replace first match

"a1b2".replaceFirst("\\d", "X") → “aXb2”

concat(String str)

String

Concatenate strings

"Hello".concat(" World") → “Hello World”

DSA Usage:

// Remove non-alphanumeric
String cleaned = str.replaceAll("[^a-zA-Z0-9]", "");
// Case-insensitive operations
String lower = str.toLowerCase();
// Remove all spaces
String noSpaces = str.replaceAll("\\s+", "");

6. Split & Join Methods

Method

Return Type

Description

Example

split(String regex)

String[]

Split by delimiter

"a,b,c".split(",") → [“a”,“b”,“c”]

split(String regex, int limit)

String[]

Split with limit

"a,b,c".split(",", 2) → [“a”,“b,c”]

String.join(CharSequence delimiter, CharSequence... elements)

String

Join with delimiter

String.join(",", "a", "b") → “a,b”

DSA Usage:

// Reverse words
String[] words = str.split("\\s+");
Collections.reverse(Arrays.asList(words));
String reversed = String.join(" ", words);
// CSV parsing
String[] fields = line.split(",");

7. Pattern Matching Methods

Method

Return Type

Description

Example

matches(String regex)

boolean

Match entire string

"123".matches("\\d+") → true

regionMatches(int toffset, String other, int ooffset, int len)

boolean

Compare regions

Check if regions match

DSA Usage:

// Validation
if (email.matches("^[\\w.-]+@[\\w.-]+\\.[a-zA-Z]{2,}$")) {
  // Valid email
}
// Pattern checking
if (str.matches("[a-zA-Z]+")) {
  // Only letters
}

8. Conversion Methods

Method

Return Type

Description

Example

String.valueOf(int i)

String

Convert to String

String.valueOf(123) → “123”

String.valueOf(char[] data)

String

Array to String

String.valueOf(new char[]{'a','b'}) → “ab”

String.valueOf(Object obj)

String

Object to String

Handles null safely

Integer.parseInt(String s)

int

String to int

Integer.parseInt("123") → 123

toString()

String

Return string itself

"hello".toString() → “hello”

getBytes()

byte[]

Convert to bytes

Get byte array

DSA Usage:

// Number to string
String numStr = String.valueOf(number);
// String to number (atoi)
int num = Integer.parseInt(str);
// Character array to string
String s = String.valueOf(charArray);

9. Format & Builder Methods

Method

Return Type

Description

Example

String.format(String format, Object... args)

String

Formatted string

String.format("Age: %d", 25) → “Age: 25”

formatted(Object... args)

String

Format (Java 15+)

"Name: %s".formatted("John")

repeat(int count)

String

Repeat string (Java 11+)

"ab".repeat(3) → “ababab”

DSA Usage:

// Formatted output
String result = String.format("Result: %d/%d = %.2f", a, b, (double)a/b);
// String repetition
String pattern = "01".repeat(5); // "0101010101"

10. Advanced Methods

Method

Return Type

Description

Example

intern()

String

Canonical string

Get from String Pool

hashCode()

int

Hash code

For hashing

lines()

Stream

Stream of lines (Java 11+)

Process lines

chars()

IntStream

Character stream

str.chars().forEach(...)

codePoints()

IntStream

Code point stream

Unicode processing

DSA Usage:

// String interning for comparison optimization
String s1 = new String("test").intern();
String s2 = "test";
// s1 == s2 → true
// Stream processing
long count = str.chars().filter(c -> c == 'a').count();

Quick Reference: Most Used in DSA

Top 15 for Interviews:

length() - Almost every problem
charAt(i) - Array-like access
substring(i, j) - Subproblems
toCharArray() - Manipulation
indexOf()/lastIndexOf() - Pattern search
equals()/equalsIgnoreCase() - Comparison
compareTo() - Sorting
toLowerCase()/toUpperCase() - Normalization
trim()/strip() - Cleanup
split() - Tokenization
replace()/replaceAll() - Modification
String.valueOf() - Conversion
contains() - Substring check
startsWith()/endsWith() - Prefix/suffix
String.join() - Concatenation

Time Complexity Reference:

charAt(), length(), isEmpty(): O(1)
indexOf(), contains(): O(n)
substring(): O(n) where n = substring length
replace(), replaceAll(): O(n)
split(): O(n)
toLowerCase(), toUpperCase(): O(n)
compareTo(), equals(): O(n)