Java puzzles

Introduction - what this is about
Puzzles in java are those situations where the code is broken and
you are tricked by some API or the language itself.
Nikola Petrov<nikola.petrov@ontotext.com> Java puzzlers - Traps, Pitfalls

Those were started mostly by Joch Bloch and there even a
book(some of the examples are taken from there)

Those were started mostly by Joch Bloch and there even a
book(some of the examples are taken from there)
Want it to be interactive

Awards as I think you will be lazy

SplittingSizes
public class RegexSplit {
public static void main(String[] args) {
String[] nothing = "".split(":");
String[] bunchOfNothing = ":".split(":");
System.out.printf("%d|%d%n", nothing.length,
bunchOfNothing.length);
}
}

SplittingSizes
Answers
(a) 1j2
(b) 1j0
(c) 0j2
(d) Throws an exception
(e) None of the above
}
}

SplittingSizes
Answers
(a) 1j2
(b) 1j0 - Explanation follows. . .
(c) 0j2
}
}

If you navigate through the source for String.split, you’ll discover
that str.split(":") is effectively the same as
Pattern.compile(":").split(str, 0).
so. . . lets look there:
Pattern.split(Charsequence, int):
If this pattern does not match any subsequence of the input
then the resulting array has just one element, namely the
input sequence in string form.
If n is zero then the pattern will be applied as many times as
possible, the array can have any length, and trailing empty
strings will be discarded. [ emphasis added. ]

Joy of sets
import java.net.*;
public class UrlSet {
private static final String[] URL_NAMES = {
"http://javapuzzlers.com",
"http://www.summerartcircle.com",
"http://www.google.com",
"http://findbugs.sourceforge.net",
"http://www.cs.umd.edu"
};
public static void main(String[] args) throws Exception {
Set<URL> favourites = new HashSet<URL>();
for (String urlName : URL_NAMES){
favourites.add(new URL(urlName));
}
System.out.println(favourites.size());
}
}

Joy of sets
import java.net.*;
Answers
(a) 4
(b) 5
(c) Throws an exception
(d) None of the above
};
}
}
}

Joy of sets
import java.net.*;
Answers
(a) 4 (assuming you are connected
to the net)
(b) 5
(c) Throws an exception
(d) None of the above - it varies
from run to run
};
}
}
}

URL equals and hashCode are broken!
Two URL objects are equal if they have the same protocol, reference
equivalent hosts, have the same port number on the host, and the
same file and fragment of the file.
Two hosts are considered equivalent if both host names can be
resolved into the same IP addresses; else if either host name can’t be
resolved, the host names must be equal without regard to case; or
both host names equal to null.
Since hosts comparison requires name resolution, this operation is a
blocking operation.

Another look
public class Histogram {
private static final String[] words = { "I", "recommend",
"polygene", "lubricants" };
int[] histogram = new int[5];
for (String word1 : words) {
String pair = word1 + word2;
int bucket = Math.abs(pair.hashCode())
% histogram.length;
histogram[bucket]++;
}
}
int pairCount = 0;
for (int freq : histogram) {
pairCount += freq;
}
System.out.println(’C’ + pairCount);
}
}

Another look
Answers
(a) 83
(b) C16
(c) S
(d) None of the above
% histogram.length;
}
}
int pairCount = 0;
pairCount += freq;
}
}
}

Another look
Answers
(a) 83
(b) C16
(c) S
(d) None of the above - throws
ArrayOutOfBoundsException
% histogram.length;
}
}
int pairCount = 0;
pairCount += freq;
}
}
}

Math.abs(int) can return a negative number, and so can the %
operator
The hashcode of “polygenelubricants” is actually
Integer.MIN_VALUE i.e.
"polygenelubricants".hashCode() == Integer.MIN_VALUE

Long division
public class LongDivision {
final long MICROS_PER_DAY = 24 * 60 * 60 * 1000 * 1000;
final long MILLIS_PER_DAY = 24 * 60 * 60 * 1000;
System.out.println(MICROS_PER_DAY / MILLIS_PER_DAY);
}
}

Long division
final long MICROS_PER_DAY = 24 * 60 * 60 * 1000 * 1000;
final long MILLIS_PER_DAY = 24 * 60 * 60 * 1000;
}
}
Answers
(a) 0
(b) 5
(c) 1000

Long division answer
//Multiplication is not with long numbers!
final long MICROS_PER_DAY = 24L * 60 * 60 * 1000 * 1000;
final long MILLIS_PER_DAY = 24L * 60 * 60 * 1000;
}
}
Answers
(a) 0
(b) 5 - not actually long division
(c) 1000

The Dating Game
import java.util.*;
public class DatingGame {
Calendar cal = Calendar.getInstance();
cal.set(1999, 12, 31); // Year, Month, Day
System.out.print(cal.get(Calendar.YEAR) + " ");
Date d = cal.getTime();
System.out.print(d.getDay() + " ");
System.out.println(cal.get(Calendar.DAY_OF_MONTH));
}
}

The Dating Game
import java.util.*;
Answers
(a) 1999 31 31
(b) 1999 31 1
(c) 2000 1 31
(d) 2000 31 31
}
}

The Dating Game
import java.util.*;
Answers
(a) 1999 31 31
(b) 1999 31 1
(c) 2000 1 31 - maybe the Y2K
}
}
problem?
(d) 2000 31 31

The Dating Game - answer
When the Java platform was first released, its only support for
calendar calculations was the Date class. This class was limited in
power, especially when it came to support for internationalization, and
it had a basic design flaw: Date instances were mutable. In release
1.1, the Calendar class was added to the platform to rectify the
shortcomings of Date; most Date methods were deprecated.
Unfortunately, this only made a bad situation worse.

The program just illustrates some of the shortcomings in the current
API. The Calendar doesn’t warn us when it overflows and the months
start the count from 0

The program just illustrates some of the shortcomings in the current
API. The Calendar doesn’t warn us when it overflows and the months
start the count from 0
Use Joda Time which will be included in java8. . .

Some comments and references
Use Sonar which will catch some of the problems from this
presentation with the findbugs plugin

Some comments and references
Use Sonar which will catch some of the problems from this
presentation with the findbugs plugin
Clickable links:
Advanced Topics in Programming Languages: Java Puzzlers
Java-Puzzlers InfoQ

Java puzzles

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