java: boolean instanceOf Boolean?

Question

I'm a bit confused: I have a function, that takes an Object as argument. But the compiler does not complain if I just pass a primitive and even recognizes a boolean primitive as Boolean Object. Why is that so?

public String test(Object value)
{
   if (! (value instanceof Boolean) ) return "invalid";
   if (((Boolean) value).booleanValue() == true ) return "yes";
   if (((Boolean) value).booleanValue() == false ) return "no";
   return "dunno";
}

String result = test(true);  // will result in "yes"

Note a boolean is NEVER an instance of Boolean. You can never pass a primitive to the instanceof operator. Your boolean primitive never entered this method, boxing happened when the invocation happened and method invocation conversion occurred! — Mishax, Mar 19 '13 at 06:10

jmj · Accepted Answer · 2010-09-16T10:40:47.220

33

Because primitive 'true' will be Autoboxed to Boolean and which is a Object.

edited Sep 16 '10 at 10:40

answered Aug 30 '10 at 13:20

jmj

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(+1) And here's some [documentation to go along with it](http://download.oracle.com/javase/1.5.0/docs/guide/language/autoboxing.html) – Tim Stone Aug 30 '10 at 13:22
Further reading: http://download.oracle.com/javase/1.5.0/docs/guide/language/autoboxing.html @jpegzz, the code would not compile if you ran with 1.4.x – NG. Aug 30 '10 at 13:22
interesting :) Well, the docs suggest only to use autoboxing if really neccessary so I wont. But it's nice to know that this is not a bug but a feature :) – epegzz Aug 30 '10 at 13:27
what is the meaning of ------------ if (object instanceof Boolean ) -----------------then what happends? Means what it things? – alishaik786 Nov 09 '11 at 13:57
`instanceof` operator checks if the object is instance of Boolean Class – jmj Nov 09 '11 at 14:48

score 3 · Answer 2 · edited Apr 05 '18 at 10:31

3

Like previous answers says, it's called autoboxing.

In fact, at compile-time, javac will transform your boolean primitve value into a Boolean object. Notice that typically, reverse transformation may generate very strange NullPointerException due, as an example, to the following code

Boolean b = null;
if(b==true) <<< Exception here !

You can take a look at JDK documentation for more infos.

edited Apr 05 '18 at 10:31

Jozef Dúc

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answered Aug 30 '10 at 13:24

Riduidel

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Did you mean `Boolean b = null;`?? – mlvljr Nov 13 '10 at 21:45
1

I think you meant `Boolean b = null;` instead of `boolean b = null;`. Note the capital. – Thomas Vos Jun 11 '17 at 18:40

score 2 · Answer 3 · answered Aug 30 '10 at 13:23

2

This part of the method:

  if (((Boolean) value).booleanValue() == true ) return "yes";
  if (((Boolean) value).booleanValue() == false ) return "no";
  return "dunno";

Could be replaced with

  if (value == null) return "dunno";
  return value ? "yes" : "no";

answered Aug 30 '10 at 13:23

Paul Tomblin

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not the same as OP's code behavior. `null` would return "invalid" since `null` is not an instance of `Boolean` (you're missing that line from OP's code in your first code above); "dunno" would never be returned at all (by original code). Without that line your first code would throw a NPE when value is `null`. – user85421 Aug 30 '10 at 13:33
@Carlos, I can never remember if `instanceof` returns true or false with all nulls, so I usually avoid the case by checking for null beforehand. – Paul Tomblin Aug 30 '10 at 14:07

score 1 · Answer 4 · answered Aug 30 '10 at 13:21

1

its called autoboxing - new with java 1.5

http://download.oracle.com/javase/1.5.0/docs/guide/language/autoboxing.html

answered Aug 30 '10 at 13:21

Adam Butler

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java: boolean instanceOf Boolean?

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