Wrapper Classes

Wrapper Classes

1. Why Wrappers? (beginner)

Primitives (int) are fast but not objects — can't go in ArrayList, can't have methods/null. Wrappers wrap each primitive as object: int→Integer, double→Double, etc. Bridge for collections + parsing!

2. Eight Pairs (exam table!)

Primitive	Wrapper	Parse String→value
int	Integer	Integer.parseInt("42") → 42
double	Double	Double.parseDouble("3.14") → 3.14
char	Character	Character.isDigit('5') → true
boolean	Boolean	Boolean.parseBoolean("true") → true
byte/short/long/float	Byte/Short/Long/Float	same parseXxx pattern

3. Autoboxing Magic (Java 5+ — beginners love!)

Integer obj = 42;      // autoboxing: int → Integer (auto!)
int x = obj;           // unboxing: Integer → int (auto!)
ArrayList<Integer> list = new ArrayList<>(); // primitives FORBIDDEN here!
list.add(10);          // autoboxed to Integer (no new needed!)
list.add(Integer.valueOf(20)); // explicit (same!)
int sum = list.get(0) + list.get(1); // unboxed back!
String s = "123";
int n = Integer.parseInt(s); // THE args-parsing trick (see CLI topic!)

4. Traps (exam favourites!)

• Integer a=127, b=127 → a==b true (cached!), but 128==128 wrappers → false (different objects!). Always .equals() / intValue() for values!
• null Integer + unbox → NullPointerException: Integer o=null; int x=o; CRASH!
• Parse garbage → NumberFormatException: Integer.parseInt("abc") crashes!

5. Exam Points

• Table + autoboxing + parse program — 7 marks, very frequent.
• == cache trap (127 vs 128) + NPE — 5 marks, favourite traps.