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OCP 11 - Complete Study Guide - Chapter 2 review questions

 
Greenhorn
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Hello everyone, I have started to study for OCP 11 certification using the OCP 11 complete study guide. In chapter 2 one question and its explanation confused me. The question says:

Which of the following Java operators can be used with boolean variables? (Choose all that apply.)
A. ==
B. +
C. --
D. !
E. %
F. <=
G. Cast with (boolean)

I chose A and D logically but the answers say A, D, G. In the answer, it explains like we can cast a boolean variable since boolean is a type but how can we cast other primitive types to boolean?


 
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Alex,
Welcome to CodeRanch!

You can't. The intent of saying cast with boolean is to rule out other types. The option means something like this:

 
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I hadn't seen this one before, I missed it.

I had falsely memorized that boolean can never be used in a cast, and got a question or two wrong on mock exams, because that is not QUITE true.

I later memorized that this is legal (trivial boolean cast):
jshell> boolean b = true
b ==> true

jshell> boolean another = (boolean)b
another ==> true


But still forgot the other dumb case where we combine un-boxing and the cast like Jeanne showed us here:

jshell> Object o = true
o ==> true

jshell> boolean yetAnother = (boolean)o
yetAnother ==> true


So the rule is nothing can be cast to or from a boolean except another boolean or an Object which has an actual type of Boolean

Lastly, something like this compiles but will throw a ClassCastException at Runtime:
jshell> Object oBad = 2468
oBad ==> 2468

jshell> boolean oneMoreBoolean = (boolean)oBad
|  Exception java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.Boolean (java.lang.Integer and java.lang.Boolean are in module java.base of loader 'bootstrap')
|        at (#9:1)


No, this is the last case, you always need to be careful of possible Null values using wrapper types:
jshell> Boolean bNull = null
bNull ==> null

jshell> boolean ohBoy = (boolean)bNull
|  Exception java.lang.NullPointerException
|        at (#11:1)


Auto-boxing and unboxing are fun, easy and cool and our friends until they bite us in the butt for carelessness.
 
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