The String class represents character strings. All string literals in Java programs, such as "abc", are implemented as instances of this class.
Java maintains a pool of string literals to help save memory. When a new string literal is created, Java checks the Pool for a matching string. If found, the new variable references the pooled string. If not, the new string is added to the Pool.
| Method | Return | Runtime |
| charAt(int index) | char | O(1) |
| codePointAt(int index) | int | O(1) |
| equals(Object anObject) | boolean | O(n) |
| equalsIgnoreCase(String str) | boolean | O(n) |
| length() | int | O(1) |
| split(String regex) | String[] | O(n) |
| substring(int beginIndex) | String | O(n) |
| substring(int beginIndex, int endIndex) | String | O(n) |
| toCharArray() | char[] | O(n) |
| toLowerCase() | String | O(n) |
| toUpperCase() | String | O(n) |
| trim() | String | O(n) |
| indexOf(String str) | int | O(n*m) |
Examples:
String str = "Hello world";
String str2 = "Hello World";
int len = str.length();
String lower = str.toLowerCase();
String upper = str.toUpperCase();
String trimmed = str.trim();
String subString = str.substring(0, 3);
boolean isContains = str.contains("world");
char ch = str.charAt(0);
int code = str.codePointAt(0);
boolean equal = str.equals("hello");
boolean equalsIgnoreCase = str.equalsIgnoreCase(str2);
int compare = str.compareTo(str2);
String[] splitted = str.split(" ");
// static methods
String joined = String.join("-", splitted);
System.out.println(joined);StringBuilder:
Java StringBuilder class is used to create mutable (modifiable) String.
Constructors:
| Method | Summary |
| StringBuilder() | Constructs a string builder with no characters in it and an initial capacity of 16 characters. |
| StringBuilder(String str) | Constructs a string builder initialized to the contents of the specified string. |
Methods:
| Method | Return Type |
| append(String str) | StringBuilder |
| reverse() | StringBuilder |
| toString() | String |
| substring(int start) | String |
| substring(int start, int end) | String |
| length() | int |
| indexOf(String str) | int |
| charAt(int index) | char |
| delete(int start, int end) | StringBuilder |
| insert(int offset, String str) | StringBuilder |
Example:
// Initialize a new StringBuilder with some text
StringBuilder sb = new StringBuilder("Hello, StringBuilder!");
// 1. Append text to the end
sb.append(" It's a powerful tool.");
System.out.println("After append: " + sb);
// 2. Insert text at a specified index
sb.insert(7, "Java ");
System.out.println("After insert: " + sb);
// 3. Replace part of the text between start and end index
sb.replace(7, 11, "Awesome");
System.out.println("After replace: " + sb);
// 4. Delete part of the text between start and end index
sb.delete(7, 15);
System.out.println("After delete: " + sb);
// 5. Delete a single character at a specific index
sb.deleteCharAt(5);
System.out.println("After deleteCharAt: " + sb);
// 6. Reverse the entire text
sb.reverse();
System.out.println("After reverse: " + sb);
sb.reverse(); // Reverse again to get back to original order
// 7. Set a character at a specific index
sb.setCharAt(0, 'h');
System.out.println("After setCharAt: " + sb);
// 8. Get the current length and capacity of StringBuilder
System.out.println("Length: " + sb.length());
System.out.println("Capacity: " + sb.capacity());
// 9. Convert StringBuilder to String
String result = sb.toString();
System.out.println("StringBuilder to String: " + result);