Package: java.lang
public final class String implements Serializable, Comparable, CharSequence
Key fact: String is immutable — every method that “modifies” a string returns a new String object, the original never changes.
1️⃣ Basic Information & Access Methods
int length()
Use: Get total number of characters
Common use: input validation, loops
String s = "Hello"; System.out.println(s.length()); // 5
char charAt(int index)
Use: Access a character at a specific index
System.out.println("Java".charAt(2)); // 'v'
boolean isEmpty()
Use: Check if string length is zero
"".isEmpty(); // true
boolean isBlank() (Java 11+)
Use: True for empty or whitespace‑only strings
" ".isBlank(); // true
2️⃣ Comparison Methods (Very Important)
boolean equals(Object obj)
Use: Compare content (NOT reference)
"Java".equals("Java"); // true
boolean equalsIgnoreCase(String s)
Use: Case‑insensitive comparison
"Java".equalsIgnoreCase("java"); // true
int compareTo(String s)
Use: Lexicographical comparison (sorting)
"apple".compareTo("banana"); // negative
int compareToIgnoreCase(String s)
Use: Sorting ignoring case
boolean contains(CharSequence s)
Use: Check substring presence
"Java Developer".contains("Dev"); // true
int indexOf(String s)
Use: First occurrence position
"hello".indexOf("l"); // 2
int lastIndexOf(String s)
Use: Last occurrence position
"hello".lastIndexOf("l"); // 3
boolean startsWith(String prefix)
boolean endsWith(String suffix)
Use: Prefix/suffix checks
"file.txt".endsWith(".txt"); // true
String substring(int beginIndex)
String substring(int beginIndex, int endIndex)
Use: Extract part of string
"HelloWorld".substring(5); // World "HelloWorld".substring(0, 5); // Hello
CharSequence subSequence(int start, int end)
Use: Similar to substring, returns CharSequence
5️⃣ Modification (Returns New String)
String toLowerCase()
String toUpperCase()
Use: Case normalization
"Java".toUpperCase(); // JAVA
String trim()
Use: Remove leading & trailing spaces
" hi ".trim(); // "hi"
String strip() (Java 11+)
Use: Unicode‑aware trim (better than trim)
String replace(char old, char new)
String replace(CharSequence old, CharSequence new)
Use: Replace characters or substrings
"banana".replace("a", "o"); // bonono
String replaceAll(String regex, String replacement)
String replaceFirst(String regex, String replacement)
Use: Regex‑based replacements
6️⃣ Splitting & Joining
String[] split(String regex)
Use: Tokenize string
"one,two,three".split(",");
static String join(CharSequence delimiter, CharSequence… elements)
Use: Join strings
String.join("-", "2026", "04", "05"); // 2026-04-05
7️⃣ Conversion Methods
char[] toCharArray()
Use: Character‑level processing
byte[] getBytes()
Use: Encoding / I/O operations
static String valueOf(anyType)
Use: Convert primitives/objects to string
String.valueOf(123); // "123"
8️⃣ Unicode‑Aware Methods (Advanced)
int codePointAt(int index)
int codePointCount(int begin, int end)
Use: Emoji / surrogate‑pair safe processing
✅ MOST COMMONLY USED STRING METHODS (Interview + Projects)
Used daily (90% of code)
length()
isEmpty(), isBlank()
equals(), equalsIgnoreCase()
contains()
substring()
split()
replace()
toUpperCase(), toLowerCase()
trim() / strip()
indexOf(), lastIndexOf()
Used often in backend / automation
startsWith(), endsWith()
compareTo()
join()
toCharArray()
valueOf()
Advanced / Unicode handling
codePointAt()
codePointCount()
