Wrapper classes (Integer, Double, Boolean, etc.) and autoboxing/unboxing

Wrapper classes and autoboxing/unboxing

Java provides wrapper classes for each primitive type, allowing primitives to be treated as objects.

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

Why wrappers?

• Collections (like ArrayList) can only store objects, not primitives.
• Methods that require objects (e.g., reflection, generics).
• Utility methods: parsing, converting, etc.

Autoboxing

Automatic conversion from a primitive to its wrapper object.

Integer i = 10;            // autoboxing: int → Integer
Double d = 3.14;           // autoboxing: double → Double

Unboxing

Automatic conversion from a wrapper object to its primitive.

int num = i;               // unboxing: Integer → int
double pi = d;             // unboxing: Double → double

Example with collections

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

Caching

Wrapper classes cache frequently used values for performance:

• Integer caches values from -128 to 127 by default (can be extended via system property).
• Similar caching for Byte, Short, Long, Character (0–127).

Integer a = 100;
Integer b = 100;
System.out.println(a == b); // true (both from cache)

Integer c = 200;
Integer d = 200;
System.out.println(c == d); // false (different objects)

Comparison

Use .equals() for value comparison, not == (which compares references).

Integer x = 1000;
Integer y = 1000;
System.out.println(x.equals(y)); // true
System.out.println(x == y);       // false

Performance considerations

Autoboxing/unboxing can create extra objects, so be mindful in performance‑critical loops.

Integer sum = 0;
for (int i = 0; i < 1000; i++) {
    sum += i;   // creates many Integer objects
}

Prefer primitives for calculations.