1.12 Wrapper Classes

What are wrapper classes?

Wrapper classes let primitive values be treated as objects. Each primitive type has a corresponding wrapper: `int` → Integer, `double` → Double, `boolean` → Boolean. Wrapper objects are needed when a program requires an object (e.g., storing values in generic collections like `ArrayList<Integer>`), since primitives alone cannot be used there.

Autoboxing and unboxing

Autoboxing is Java's automatic conversion of a primitive into its wrapper object (e.g., `int` → `Integer`) when needed. Unboxing is the reverse — converting a wrapper object back into a primitive. This happens automatically in assignments, method calls, and arithmetic, so code often looks the same whether primitives or wrappers are used.

Common wrapper methods & constants

Wrapper classes include useful static constants and methods, such as `Integer.MAX_VALUE` and `Integer.MIN_VALUE`, which give the largest and smallest values an `int` can hold. These are frequently tested for understanding integer overflow and boundary behavior.

Key terms

Wrapper class

A class that allows a primitive value to be treated as an object (e.g., Integer for int, Double for double).

Autoboxing

Automatic conversion of a primitive value into its corresponding wrapper object.

Unboxing

Automatic conversion of a wrapper object back into its primitive value.

Integer.MAX_VALUE

A constant representing the largest value an int can hold (2147483647).

Why use wrapper classes?

They let primitives be used where objects are required, such as in generic collections like ArrayList<Integer>.

Try an AP-style question

Why must ArrayList<Integer> be used instead of ArrayList<int>?

1. ArrayList can only store Strings
2. Generics need object types, not primitives
3. Integer is always faster than int
4. int is too small to store in a list