Cosmic Module

1String s = "Hello, Deepak!";
2
3s.length()                    // 14 — number of chars
4s.charAt(7)                   // 'D' — char at index
5s.indexOf('e')                // 1 — first occurrence
6s.indexOf('e', 2)             // -1 — first occurrence after index 2
7s.lastIndexOf('e')            // 3 — last occurrence
8s.isEmpty()                   // false — length == 0
9s.isBlank()                   // false — empty or only whitespace (Java 11+)
10s.contains("Deepak")          // true — substring check
11s.startsWith("Hello")         // true
12s.endsWith("!")               // true

1s.substring(7)                // "Deepak!" — from index to end
2s.substring(7, 13)            // "Deepak" — from 7 to 12 (end exclusive)
3s.split(", ")                 // ["Hello", "Deepak!"] — split by delimiter
4s.split("")                   // each char as separate string

1s.toLowerCase()               // "hello, deepak!"
2s.toUpperCase()               // "HELLO, DEEPAK!"
3s.trim()                      // remove leading/trailing whitespace
4s.strip()                     // trim() but Unicode aware (Java 11+)
5s.replace('l', 'r')           // "Herro, Deepak!"
6s.replace("Deepak", "World")  // "Hello, World!"
7s.replaceAll("[aeiou]", "*")  // replace regex matches
8String.valueOf(123)           // "123" — int to String

1s.equals("Hello, Deepak!")           // true — exact match
2s.equalsIgnoreCase("hello, deepak!") // true — case insensitive
3s.compareTo("Hello, World!")          // negative/0/positive
4s.compareToIgnoreCase("HELLO")        // case insensitive compare

1s.toCharArray()               // char[] {'H','e','l','l','o'...}
2new String(charArray)         // char[] back to String
3String.valueOf(42)            // int → String
4Integer.parseInt("42")        // String → int
5Double.parseDouble("3.14")    // String → double
6String.join("-", "a","b","c") // "a-b-c"
7String.join(", ", list)       // join list elements

1// Character codes
2'A' = 65, 'Z' = 90
3'a' = 97, 'z' = 122
4'0' = 48, '9' = 57
5
6// Alphabet index (0 to 25)
7int index = c - 'a';          // 'a'=0, 'b'=1, 'c'=2...
8
9// Digit value from char
10int digit = c - '0';          // '5' → 5
11
12// Upper to lower
13char lower = (char)(c + 32);  // 'A' → 'a'
14
15// Lower to upper
16char upper = (char)(c - 32);  // 'a' → 'A'
17
18// Next character
19char next = (char)(c + 1);    // 'A' → 'B'
20
21// Check ranges
22c >= 'A' && c <= 'Z'  // uppercase
23c >= 'a' && c <= 'z'  // lowercase
24c >= '0' && c <= '9'  // digit

1Character.isLetter('A')       // true
2Character.isDigit('5')        // true
3Character.isLetterOrDigit('_') // false
4Character.isUpperCase('A')    // true
5Character.isLowerCase('a')    // true
6Character.isWhitespace(' ')   // true
7Character.toLowerCase('A')    // 'a'
8Character.toUpperCase('a')    // 'A'

1StringBuilder sb = new StringBuilder();
2
3sb.append("Hello")            // add to end
4sb.insert(0, "Hi ")           // insert at index
5sb.delete(0, 3)               // delete from 0 to 2
6sb.deleteCharAt(0)            // delete at index
7sb.replace(0, 2, "Hey")       // replace range
8sb.reverse()                  // reverse entire string
9sb.charAt(0)                  // get char at index
10sb.setCharAt(0, 'h')          // set char at index
11sb.length()                   // current length
12sb.toString()                 // convert to String (mandatory!)

1// Trap 1 — substring end is exclusive
2"Hello".substring(1, 3)  // "el" (index 1 and 2, NOT 3)
3
4// Trap 2 — split with regex special chars
5"a.b.c".split(".")    // ❌ splits on every char (. is regex wildcard)
6"a.b.c".split("\\.")  // ✅ ["a","b","c"]
7
8// Trap 3 — String is immutable
9String s = "hello";
10s.toUpperCase();              // result discarded!
11String upper = s.toUpperCase(); // ✅ assign result
12
13// Trap 4 — indexOf returns -1 if not found
14if (s.indexOf('z') != -1) { }  // always check!
15
16// Trap 5 — charAt throws StringIndexOutOfBoundsException
17s.charAt(s.length());   // ❌ valid range: 0 to length-1

1Top 10 most-used String methods in DSA:
21.  s.toCharArray()           → iterate chars
32.  s.charAt(i)               → access char by index
43.  s.length()                → size
54.  c - 'a'                   → alphabet index
65.  sb.append()               → build result
76.  sb.reverse()              → reverse
87.  sb.toString()             → convert back
98.  s.equals()                → compare
109.  s.substring()             → extract
1110. Character.isLetterOrDigit() → alphanumeric check