Enthuware

Hello Paul,
Sorry if my question is silly but I wanna know whether in real exam the difficulty level be specified or not?
Like easy,real brainer Just wanted to know

No, the real exam does not specify the difficulty level and the exam objective. Our simulator lets you hide both.
HTH,
Paul.

I'm still a bit confused about Boolean.TRUE. Does this return a wrapper object or a primitive?
This is where my confusion stems: This returns "false" which implies that both sides of the expression represent Boolean objects in which case no unboxing would occur.

However, the following returns "true": This would seem to imply that unboxing is occurring. If Boolean.valueOf("true") returns a wrapper object and Boolean.TRUE returns a wrapper object, how is it that their equality comparisons return true?

Boolean.TRUE is a Boolean object. Please see this: https://docs.oracle.com/javase/8/docs/a ... .html#TRUE

Boolean.valueOf also returns a Boolean object.

So there is no reason for unboxing.

HTH,
Paul.

Hello, I was wondering if the case of Boolean.valueOf(Boolean boolean) creates a different object type from new Boolean(boolean) is unique to the Boolean class. For instance, does Integer.valueOf(Integer integer) create a different object type from new Integer(Integer integer) or are the two Integer methods substitutable? I did some research online but can't seem to find the answer. Thank you.

There is no valueOf method in Boolean that takes a Boolean. There is one that takes a boolean, though. The Javadoc API description clearly says that it returns Boolean.TRUE or Boolean.FALSE object.


Now, regarding the Integer objects. Check out the JavaDoc. It clearly says the following:

This method will always cache values in the range -128 to 127, inclusive, and may cache other values outside of this range.

The new operator always creates a new object. So that means in both the cases above, the object created using new and the one received using valueOf are different objects.


You should try creating one Integer object using new and one using valueOf and compare them using ==. That will validate your understanding.

Paul.

Why does new Boolean(null); return false? I thought it should be compile time error. Why null in this case considered like string?

Sergey wrote:Why does new Boolean(null); return false? I thought it should be compile time error. Why null in this case considered like string?

So if you have some constructor or a method that takes a String (or any other class ) as an argument, can't you pass null to it? Yes, right?
It is the same thing.

However, the following returns "true":

Code: Select all

Boolean.valueOf("true") == Boolean.TRUE

This would seem to imply that unboxing is occurring. If Boolean.valueOf("true") returns a wrapper object and Boolean.TRUE returns a wrapper object, how is it that their equality comparisons return true?

The reason this expression is true is that both sides of the expression point to static final Boolean.TRUE. That is because Boolean.valueOf(param) returns either Boolean.TRUE or Boolean.FALSE

new Boolean("no") == false;

Does left side of "==" unbox to boolean before the comparison? thanks

Yes, as per 15.21.2 of JLS, if one operand of == is a Boolean wrapper and another is a boolean primitive, then the wrapper is unboxed.

You can test this easily by printing out the value of your expression new Boolean("no") == false;