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"

Braces

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! – Nacred

Answer 1

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

Finley

(+1) And here's some documentation to go along with it – Montez

Further reading: download.oracle.com/javase/1.5.0/docs/guide/language/… @jpegzz, the code would not compile if you ran with 1.4.x – Hattie

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 :) – Braces

what is the meaning of ------------ if (object instanceof Boolean ) -----------------then what happends? Means what it things? – Airspace

instanceof operator checks if the object is instance of Boolean Class – Finley

Answer 2

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.

Cartelize

Did you mean Boolean b = null;?? – Velda

I think you meant Boolean b = null; instead of boolean b = null;. Note the capital. – Sisal

Answer 3

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";

Ungodly

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. – Vanderhoek

@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. – Ungodly

Answer 4

its called autoboxing - new with java 1.5

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

Maggs