Which method on the wrapper class is getting called when boxing a primitive in Java

Question

Which method in the Integer class will be used when you do Integer i = 1;

I'm pretty sure that it's not the constructor, it might be the valueOf() method.

Possible duplicate of [What code does the compiler generate for autoboxing?](http://stackoverflow.com/q/408661/851811) — Xavi López, Oct 26 '11 at 08:04

Peter Lawrey · Answer 1 · 2011-10-26T08:08:54.533

5

It is Integer.valueOf(int) similarly for Boolean, Byte, Character, Long, Float and Double.

Note: for Boolean and Byte all possible values are cached. For Character, the values 0 to 127 are cached. For Short and Long values -128 to 127 are cached. For Integer -128 to 127 are cached by default, however the maximum can increase using a number of options.

This can lead to surprising behaviour with

System.out.println((Integer) (int) -128 == (Integer) (int) -128);
System.out.println((Integer) (int) -129 == (Integer) (int) -129);

prints

true
false

Not sure what you need to cast -128 with (int) -128 for this to compile in Java 7.

edited Oct 26 '11 at 08:08

answered Oct 26 '11 at 08:00

Peter Lawrey

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I was looking into the Integer source for the cache stuff, that's why I was wondering with method was used while boxing because the constructor doesn't use the '+-127 cache'. – Stijn Vanpoucke Oct 26 '11 at 08:27
The constructor has to create a new object every time, it has no choice. One of my pet hates is people using `new Boolean(flag)` ;) – Peter Lawrey Oct 26 '11 at 08:29

score 5 · Accepted Answer · answered Oct 26 '11 at 08:01

Yes, it's valueOf:

Here's the output of javap:

public static void main(java.lang.String[]);
  Code:
   Stack=1, Locals=2, Args_size=1
   0:   iconst_1
   1:   invokestatic    #16; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
   4:   astore_1
   5:   return
  LineNumberTable:
   line 5: 0
   line 6: 5

score 3 · Answer 3 · edited May 23 '17 at 09:59

3

It actually is valueOf(). Take a look at this possible duplicate question: What code does the compiler generate for autoboxing?.

And particulary for integers in the range -128, 127, you'll never see the constructor invoked when using valueOf because Integer has these instances cached.

edited May 23 '17 at 09:59

Community

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1

answered Oct 26 '11 at 08:02

Xavi López

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Which method on the wrapper class is getting called when boxing a primitive in Java

3 Answers3