String vs StringBuilder classes in Java

String vs StringBuilder Methods

Method	String	StringBuilder	Description
charAt(int index)	✓	✓	Returns the character at the specified index.
length()	✓	✓	Returns the length of the string.
substring(int start)	✓	✓	Returns a new string starting from the specified index to the end.
substring(int start, int end)	✓	✓	Returns a new string starting from start to end-1.
indexOf(String str)	✓	✓	Returns the index of the first occurrence of the specified string.
indexOf(String str, int fromIndex)	✓	✓	Returns the index of the specified string, starting from the given index.
lastIndexOf(String str)	✓	✓	Returns the index of the last occurrence of the specified string.
lastIndexOf(String str, int fromIndex)	✓	✓	Returns the last index of the specified string, searching backward.
append(String str)	✗	✓	Appends the specified string to the current string.
insert(int offset, String str)	✗	✓	Inserts the string at the specified offset.
replace(CharSequence old, CharSequence new)	✓	✓	Replaces occurrences of a substring with a new substring.
reverse()	✗	✓	Reverses the sequence of characters.
toString()	✓	✓	Returns the string representation of the object.
equals(Object obj)	✓	✗	Compares this string to the specified object.
compareTo(String anotherString)	✓	✗	Compares two strings lexicographically.
concat(String str)	✓	✗	Concatenates the specified string to the end of this string.
contains(CharSequence s)	✓	✗	Returns true if this string contains the specified sequence.
isEmpty()	✓	✗	Checks if the string is empty (length == 0).
trim()	✓	✗	Removes leading and trailing whitespace.
startsWith(String prefix)	✓	✗	Checks if the string starts with the specified prefix.
endsWith(String suffix)	✓	✗	Checks if the string ends with the specified suffix.
toLowerCase()	✓	✗	Converts all characters to lowercase.
toUpperCase()	✓	✗	Converts all characters to uppercase.
split(String regex)	✓	✗	Splits the string based on the given regular expression.
replaceAll(String regex, String replacement)	✓	✗	Replaces all substrings matching a regex with a new string.
delete(int start, int end)	✗	✓	Deletes characters from start to end.
deleteCharAt(int index)	✗	✓	Deletes the character at the specified index.
setCharAt(int index, char ch)	✗	✓	Sets the character at the specified index.
ensureCapacity(int minimumCapacity)	✗	✓	Ensures that the capacity of the StringBuilder is at least minimumCapacity.

Key Differences Between String and StringBuilder

• String: String is immutable, so it doesn't support methods like append(), insert(), reverse(), delete(), or setCharAt().
• StringBuilder: StringBuilder is mutable and optimized for string modifications with methods like append(), insert(), reverse(), delete(), and setCharAt().

Both classes share methods for accessing characters (charAt()), finding substrings (indexOf(), lastIndexOf()), and extracting substrings (substring()), but StringBuilder is designed for in-place modifications.