Wrapper Classes

Vocabulary

English	Meaning
wrapper class/ˈræpə klæs/	In Java, a class whose objects represent values of a corresponding primitive type, such as Integer for int.
autoboxing/ˌɔːtəʊˈbɒksɪŋ/	Java's automatic conversion of a primitive value to a reference to an object of its corresponding wrapper class.
unboxing/ʌnˈbɒksɪŋ/	Java’s automatic conversion from a wrapper reference, such as Integer, to its primitive value; null cannot be unboxed successfully.

Boxing a primitive

Some Java structures (like ArrayList) hold objects, not primitives.
A wrapper class 包装类 wraps a primitive in an object: Integer wraps int, Double wraps double.
Integer boxed = Integer.valueOf(5); puts the int 5 inside an object.
Wrappers let primitives live where only objects are allowed.

Autoboxing and unboxing

Java converts automatically — autoboxing 自动装箱 and unboxing 自动拆箱.
Integer x = 5; autoboxes 5; int y = x; unboxes it back.
So list.add(5) works even though the list holds Integer objects.
You rarely write Integer.valueOf by hand — autoboxing does it.

Autoboxing and unboxing between int and Integer

Why ArrayList needs them

ArrayList<Integer> holds Integer objects, because generics require a reference type.
You can't write ArrayList<int> — primitives aren't objects.
Autoboxing lets you add and read plain ints conveniently.
Double, Boolean, etc. play the same role for their primitives.

Compare wrappers with equals

Two Integer objects should be compared with .equals(), not ==.
== compares references; .equals() compares the wrapped values.
(Java caches small Integers, so == sometimes seems to work — don't rely on it.)
The same "use .equals() for objects" rule from String applies here.

Watch out

Wrappers are objects, so compare their values with .equals(), not ==. == tests reference identity; for two Integer objects with the same value it can be false. And you can't parameterize a collection with a primitive — it's ArrayList<Integer>, never ArrayList<int>. Autoboxing bridges the gap so you can still add plain ints.

Example

Wrappers with an ArrayList:

ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(5); — the int 5 is autoboxed into an Integer.
int first = nums.get(0); — the Integer is unboxed back to int.

Key idea

A wrapper class (Integer, Double, Boolean) wraps a primitive in an object so it can go where objects are required — like an ArrayList<Integer> (you can't write ArrayList<int>). Autoboxing and unboxing convert automatically. Compare wrapper objects with .equals(), not ==.

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