Wrapper Classes in Java

What are wrapper classes

Wrapper classes turn a primitive type into an object. Each primitive has a matching class: int → Integer, double → Double, and so on. This is needed because some parts of Java (like collections) work only with objects, not primitives.

The wrapper classes

Each of the eight primitives has a wrapper class. Note that most just capitalise the name, but int becomes Integer and char becomes Character.

Autoboxing and unboxing

Integer boxed = 5;     // autoboxing: int -> Integer
int unboxed = boxed;   // unboxing: Integer -> int
System.out.println(boxed + " " + unboxed);

Java converts automatically: autoboxing wraps an int into an Integer, and unboxing takes the int back out. You usually do not convert by hand — Java handles it.

Why we need them

import java.util.ArrayList;

ArrayList<Integer> list = new ArrayList<>();
list.add(10);       // autoboxed to Integer
list.add(20);
System.out.println(list.get(0));

Collections store objects, so you write ArrayList<Integer>, not ArrayList<int>. Wrapper classes let numbers live in these object-based collections, with autoboxing doing the conversion.

Parsing strings to numbers

int n = Integer.parseInt("123");
double d = Double.parseDouble("9.5");
System.out.println(n + d);

Wrapper classes provide parseInt and parseDouble to turn text into numbers — essential when reading numeric input as strings.

Useful wrapper methods

System.out.println(Integer.MAX_VALUE);     // biggest int
System.out.println(Integer.toBinaryString(10)); // 1010
System.out.println(Character.isDigit('5'));     // true

Wrapper classes carry handy constants and methods, like Integer.MAX_VALUE, base conversions, and character checks — things a plain primitive cannot offer.

Common mistakes

• Using a primitive in a collection. Use the wrapper: ArrayList<Integer>, not <int>.
• Null unboxing. Unboxing a null Integer to int throws a NullPointerException.
• Comparing wrappers with ==. Use equals() for value comparison of Integer objects.
• Wrong wrapper name. It is Integer and Character, not Int or Char.

Summary

• Wrapper classes turn primitives into objects (int → Integer).
• Autoboxing and unboxing convert between them automatically.
• Collections need wrappers because they store objects.
• Use parseInt/parseDouble to convert strings to numbers.
• Wrappers add useful constants and methods primitives lack.

Next, store them in an ArrayList, or convert types with type casting.