Looking for Interesting/Tricky Java questions

[vtqanh]

I'm looking for interesting/tricky java questions to make quizzes for students (small quizzes at the beginning of labs). Anyone has some on top of your head or knows some place that has them, can you share it with me? Any Java level is welcome

 

[AFB]

What level?


I do remember a polymorphism question on the AP test that I thought was good.

 

[ArmchairAthlete]

Do something relating to comparing two floats or doubles for equality and the possibility that two you think should be equal will not be (because of how computers approximate).

Where you have to use something like subtracting the floats and then making sure what's left is less than 0.0000001 or so.

 

[Kilrsat]

Amdfanboy's question about stack overflow might be a good warmup

Stack Overflow Blues

It would really demonstrate an understanding of inheritence and the problems you can run into when overriding some functions but not others.

Or why this code prints:
lalala
lalala
lalala
S1!=S2
S1=S3
S2!=S3

 

[vtqanh]

I assume that you will then call myMethod(o) in main. I'm pretty sure the result would be "You passed an Object"
It was a good question.
Thanks

 

[Barnaby W. Füi]

Questions about references could be quite tricky and good.

 

[AFB]

Another one

 

[vtqanh]

Originally posted by: Kilrsat
Amdfanboy's question about stack overflow might be a good warmup

<a target=_blank class=ftalternatingbarlinklarge href="http://forums.anandtech.com/me...eyword1=java">Stack Overflow Blues</a>

It would really demonstrate an understanding of inheritence and the problems you can run into when overriding some functions but not others.

Or why this code prints:
lalala
lalala
lalala
S1!=S2
S1=S3
S2!=S3

Interesting one. I was thinking that S1 would not equal to S3 as well, since the operation was checking if the two pointers were pointing to the same object.

S1 = new String("lalala");
S3 = new String("lalala");
S1 == S3 would yield false.

 

[AFB]

Originally posted by: vtqanh
<blockquote>quote:
<hr><i>Originally posted by: <b>Kilrsat</b></i>
Amdfanboy's question about stack overflow might be a good warmup

<a target=_blank class=ftalternatingbarlinklarge href="<a target=_blank class=ftalternatingbarlinklarge href="http://forums.anandtech.com/me...va">Stack</a> Overflow Blues</a>

It would really demonstrate an understanding of inheritence and the problems you can run into when overriding some functions but not others.

Or why this code prints:
lalala
lalala
lalala
S1!=S2
S1=S3
S2!=S3<hr></blockquote>

Interesting one. I was thinking that S1 would not equal to S3 as well, since the operation was checking if the two pointers were pointing to the same object.

S1 = new String("lalala");
S3 = new String("lalala");
S1 == S3 would yield false.

Exactly, == check if they are the same object (of course they are equal) and .equals method from object checks for equality. It is important to note that the defualt implementation of equal is to check if they are the same object. You need to override it with your own way to check if they are equal.

 

[Kilrsat]

Originally posted by: vtqanh
<blockquote>quote:
<hr><i>Originally posted by: <b>Kilrsat</b></i>
Amdfanboy's question about stack overflow might be a good warmup

<a target=_blank class=ftalternatingbarlinklarge href="<a target=_blank class=ftalternatingbarlinklarge href="http://forums.anandtech.com/me...va">Stack</a> Overflow Blues</a>

It would really demonstrate an understanding of inheritence and the problems you can run into when overriding some functions but not others.

Or why this code prints:
lalala
lalala
lalala
S1!=S2
S1=S3
S2!=S3<hr></blockquote>

Interesting one. I was thinking that S1 would not equal to S3 as well, since the operation was checking if the two pointers were pointing to the same object.

S1 = new String("lalala");
S3 = new String("lalala");
S1 == S3 would yield false.

It gets into the Java String pool. Even though S1 and S3 are different objects according to the code both S1 and S3 actually point to the same object in the String pool. Its the String pool that creates issues with comparing strings because sometimes == does produce the expected results.

 

[vtqanh]

Originally posted by: Kilrsat
<blockquote>quote:
<hr><i>Originally posted by: <b>vtqanh</b></i>
<blockquote>quote:
<hr><i>Originally posted by: <b>Kilrsat</b></i>
Amdfanboy's question about stack overflow might be a good warmup

<a target=_blank class=ftalternatingbarlinklarge href="<a target=_blank class=ftalternatingbarlinklarge href="<a target=_blank class=ftalternatingbarlinklarge href="http://forums.anandtech.com/me...ck</a></a> Overflow Blues</a>

It would really demonstrate an understanding of inheritence and the problems you can run into when overriding some functions but not others.

Or why this code prints:
lalala
lalala
lalala
S1!=S2
S1=S3
S2!=S3<hr></blockquote>

Interesting one. I was thinking that S1 would not equal to S3 as well, since the operation was checking if the two pointers were pointing to the same object.

S1 = new String("lalala");
S3 = new String("lalala");
S1 == S3 would yield false.<hr></blockquote>
It gets into the Java String pool. Even though S1 and S3 are different objects according to the code both S1 and S3 actually point to the same object in the String pool. Its the String pool that creates issues with comparing strings because sometimes == does produce the expected results.

This makes sense now. So i guess if we insert this statement s2 = s2.intern(), then the check (s2==s3) would yield 'true'.

Keep them coming guys, everything posted in this thread has been very interesting.

 

[kamper]

Here's a tricky little sob that I ran into the other day at work. :beer: to anyone who can predict it correctly without actually running it.

 

[Barnaby W. Füi]

null and null? DescendantData needs to call its init() in its constructor. Better yet, just ditch init() and use the constructor to initialize; that's its purpose.

 

[dighn]

Originally posted by: BingBongWongFooey
null and null? DescendantData needs to call its init() in its constructor. Better yet, just ditch init() and use the constructor to initialize; that's its purpose.

1 null

the base's constructor still gets called implicitly

 

[Barnaby W. Füi]

Originally posted by: dighn
<blockquote>quote:
<hr><i>Originally posted by: <b>BingBongWongFooey</b></i>
null and null? DescendantData needs to call its init() in its constructor. Better yet, just ditch init() and use the constructor to initialize; that's its purpose.<hr></blockquote>

1 null

the base's constructor still gets called implicitly

Aha. Tricky indeed!

 

[kamper]

Originally posted by: BingBongWongFooey
<blockquote>quote:
<hr><i>Originally posted by: <b>dighn</b></i>
<blockquote>quote:
<hr><i>Originally posted by: <b>BingBongWongFooey</b></i>
null and null? DescendantData needs to call its init() in its constructor. Better yet, just ditch init() and use the constructor to initialize; that's its purpose.<hr></blockquote>

1 null

the base's constructor still gets called implicitly<hr></blockquote>

Aha. Tricky indeed! <img src="i/expressions/face-icon-small-smile.gif" border="0">

Ok, you've got the first trick but you're still wrong Well, which one do you mean to be null?



Edit: crap, the quote functionality blows... did that change since they ditched JRun?

 

[kamper]

Whoops, dighn, you are correct, sorry. Can you explain why it's not:
1
2

Hint: DescendantData.init() does get called.

BingBong: I know the constructor should be used for initialization. There was a reason why we had a situation like this that is not apparent in this example because it's been stripped down to just demonstrate the tricky part.

 

[vtqanh]

Originally posted by: kamper
Whoops, dighn, you are correct, sorry. Can you explain why it's not:
1
2

Hint: DescendantData.init() <b>does</b> get called.

BingBong: I know the constructor should be used for initialization. There was a reason why we had a situation like this that is not apparent in this example because it's been stripped down to just demonstrate the tricky part.

Here's the reason: (and yes, DescentdantData.init() DOES get called)
When this line DescendantData data = new DescendantData(); is excecuted, these things will occur (in this order):
the constructor of the BASE class gets called, which then calls init(); However, the function init() has been overriden by DescendantData, this function will set descendantInteger to 2 (however, note that at this point, descendantInteger is not initialized yet!!!)
After this, the initialization takes place, which means this line private Integer descentdantInteger = null will be executed, this will set descendantInteger to null.
After this, the constructor of DescentdantData will be executed.

 

[kamper]

:beer::beer: Good work. That seemed so counter-intuitive to me the first time I heard it. I had to go write the code and do a step-through.

 

[kamper]

On the non-technical side you could ask them design things like why you shouldn't expose data members of a class publicly (more of a beginner question).

You could also maybe ask something about how protected actually means package-access.

 

[AFB]

Originally posted by: kamper
On the non-technical side you could ask them design things like why you shouldn't expose data members of a class publicly (more of a beginner question).

You could also maybe ask something about how protected actually means package-access.

Yes, ask them what classes they would use to make a web server with a GUI. Pass the results on, I would really like to detangle my code