Wrapper Classes

Từ vựng

English	Ý nghĩa
wrapper class/ˈræpə klæs/	lớp bao封装
autoboxing/ˌɔːtəʊˈbɒksɪŋ/	tự động đóng gói
unboxing/ʌnˈbɒksɪŋ/	tự động mở gói

English

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.

English

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

English

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.

English

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.

English

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.

English

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.

English

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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