1.10 Wrapper Classes

Learning Objectives

•Match each primitive type to its wrapper class.
•Convert between primitives and wrappers using autoboxing/unboxing.
•Parse Strings into numeric primitives using wrapper class methods.

Introduction

Every primitive type has a corresponding wrapper class in `java.lang` — `int`↔`Integer`, `double`↔`Double`, `boolean`↔`Boolean`, and so on. Wrapper classes let primitives be used where an object is required (like in collections such as `ArrayList`), and provide useful static utility methods for parsing and converting values.

Key Concepts

Autoboxing: the compiler automatically converts a primitive into its wrapper when an object is needed (`Integer i = 5;`).

Unboxing: the reverse conversion, wrapper back to primitive (`int x = i;`).

`Integer.parseInt(String)`, `Double.parseDouble(String)`, etc. convert text (e.g. user input) into primitive values.

Wrapper objects can be `null`; unboxing a `null` wrapper throws a `NullPointerException`.

Java Syntax

Code Examples

`42` is autoboxed into an `Integer` object, then unboxed back into an `int`. `Integer.parseInt` converts the text `"17"` into the primitive `int` 17.

Common Mistakes

⚠Unboxing a `null` wrapper reference, which throws `NullPointerException` at runtime instead of failing to compile.
⚠Comparing wrapper objects with `==` expecting value equality — like Strings, this compares references outside the small cached range Java pre-boxes (-128 to 127 for Integer).
⚠Calling `Integer.parseInt()` on non-numeric text, which throws `NumberFormatException`.

Practice

Practice 1

Short Answermedium

What exception is thrown if you call `Integer.parseInt("abc")`?

Summary

Wrapper classes (Integer, Double, Boolean, ...) box primitives as objects. Autoboxing/unboxing make conversion mostly automatic, but null unboxing and reference-equality pitfalls still require care.

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