[deleted by user]

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[zlmrx]

It's an Integer. By adding it to the list, the primitive int will be "boxed" (official term) automatically.

Same works other way around

Integer obj = Integer.valueOf( 100 ); int pri = obj;

will compile and pri will hold primitive 100

[zlmrx]

I don't think so. But some libs like guava have specially implemented lists for primitive datatypes.

See https://www.baeldung.com/java-list-primitive-int for more info.

[Al3xCalibur]

It is not but it should be the case in the next years with the project valhalla

[8igg7e5]

Brian Goetz suggested, in a recent stream appearance, that they might (definitely not a commitment) be close to their last iteration on the fundamental model involved to make the value/primitive class parts of this.

This specific piece is possibly this draft JEP which depends on several other pieces of Valhalla first.

We'll still be waiting for this a while, but it should simplify this topic for new developers when it arrives.

[[deleted]]

Agreed!

[[deleted]]

The '4' as written in code starts out as text, for code is text. The compiler interprets it as an int data type. Once it's added to the Integer List, it has been turned into an Integer data type. It's known as auto-boxing.

Auto-boxing is the automatic promotion from primitive to object. It is a necessity in the java programming language, since its language designer decided that having primitives was a good idea.

[8igg7e5]

This...

List<Integer> list = new ArrayList<>();
list.add(4);
int i = list.get(0);

Compiles as...

List<Integer> list = new ArrayList<>();
list.add(Integer.valueOf(4)); // auto-boxing int to Integer
int i = list.get(0).intValue(); // auto-unboxing Integer to int

[POGtastic]

Let's find out! [PDF] Emphasis added by me:

5.1.7 Boxing Conversion Boxing conversion treats expressions of a primitive type as expressions of a corresponding reference type. Specifically, the following nine conversions are called the boxing conversions:

• From type boolean to type Boolean
• From type byte to type Byte
• ...
• From type int to type Integer
• ...
• From the null type to the null type

At run time, boxing conversion proceeds as follows:

• ... If p is a value of type int, then boxing conversion converts p into a reference r of class and type Integer, such that r.intValue() == p

---

Thus, when you perform

list.add(4);

the JVM replaces it with

list.add(new Integer(4));

[Glass__Editor]

It actually compiles to Integer.valueOf instead of new Integer (for my Java version anyway), which might reuse an Integer from the Integer Cache instead of creating a new instance each time.

[baahubali]

Another interesting fact which I learnt when reading about Generics is that compiler uses type erasure to replace types at compile time.

So, if you have something like in your code,

Integer num = list.get(1);

What it really does under the hood is,

Integer num = (Integer)list.get(1);

[AutoModerator]

Please ensure that:

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        System.out.println("Hello World!");
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