Autoboxing + Unboxing

Autoboxing is an automatic conversion of a primitive to its wrapper class.

int x  = 20;
Integer y = x; // autoboxing

Unboxing is an automatic conversion of a wrapper object back to its primitive form.

Integer a = 20;
int b = a; // unboxing

Warning

Autoboxing/unboxing can cause performance hits in tight loops or large data structures.

Primitive ↔ Wrapper Types

Primitive	Wrapper Class
byte	Byte
short	Short
int	Integer
long	Long
float	Float
double	Double
char	Character
boolean	Boolean

Example

List<Integer> nums = new ArrayList<>();
nums.add(5);          // int → Integer (autoboxing)
int n = nums.get(0);  // Integer → int (unboxing)

Tip

Primitives are more performant, but wrappers are required in collections like List, Map, etc.

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Parent: _Casting & Types