Java puzzles

Introduction - what this is about

Puzzles in java are those situations where the code is broken andyou are tricked by some API or the language itself.

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

Nikola Petrov<[email protected]> Java puzzlers - Traps, Pitfalls


Puzzles in java are those situations where the code is broken andyou are tricked by some API or the language itself.Those were started mostly by Joch Bloch and there even abook(some of the examples are taken from there)

Want it to be interactive



Puzzles in java are those situations where the code is broken andyou are tricked by some API or the language itself.Those were started mostly by Joch Bloch and there even abook(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) 1|2(b) 1|0(c) 0|2(d) Throws an exception

(e) None of the above

SplittingSizes




}


Answers

(a) 1|2(b) 1|0 - Explanation follows. . .

(c) 0|2(d) Throws an exception


If you navigate through the source for String.split, you’ll discoverthat 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 inputthen the resulting array has just one element, namely theinput sequence in string form.

If n is zero then the pattern will be applied as many times aspossible, the array can have any length, and trailing emptystrings 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://javapuzzlers.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






}}


Answers(a) 4

(b) 5

(c) Throws an exception

(d) None of the above

Joy of sets






}}


Answers(a) 4 (assuming you are connected

to the net)

(b) 5

(c) Throws an exception

(d) None of the above - it variesfrom 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 thesame file and fragment of the file.

Two hosts are considered equivalent if both host names can beresolved into the same IP addresses; else if either host name can’t be

resolved, the host names must be equal without regard to case; orboth host names equal to null.

Since hosts comparison requires name resolution, this operation is ablocking operation.


Another look

public class Histogram {private static final String[] words = { "I", "recommend",

"polygene", "lubricants" };

public static void main(String[] args) {int[] histogram = new int[5];for (String word1 : words) {

for (String word2 : 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

Another look








}}


Answers(a) 83

(b) C16

(c) S

(d) None of the above - throwsArrayOutOfBoundsException

Math.abs(int) can return a negative number, and so can the %operator

The hashcode of “polygenelubricants” is actuallyInteger.MIN_VALUE i.e.

"polygenelubricants".hashCode() == Integer.MIN_VALUE


Long division

public class LongDivision {public static void main(String[] args) {

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;System.out.println(MICROS_PER_DAY / MILLIS_PER_DAY);

}}

Answers(a) 0

(b) 5

(c) 1000

(d) Throws an exception



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;System.out.println(MICROS_PER_DAY / MILLIS_PER_DAY);

}}

Answers(a) 0

(b) 5 - not actually long division

(c) 1000

(d) Throws an exception



The Dating Game

import java.util.*;

public class DatingGame {public static void main(String[] args) {

Calendar cal = Calendar.getInstance();cal.set(1999, 12, 31); // Year, Month, DaySystem.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 Y2Kproblem?

(d) 2000 31 31


The Dating Game - answer

When the Java platform was first released, its only support forcalendar calculations was the Date class. This class was limited inpower, especially when it came to support for internationalization, andit had a basic design flaw: Date instances were mutable. In release1.1, the Calendar class was added to the platform to rectify theshortcomings 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 currentAPI. The Calendar doesn’t warn us when it overflows and the monthsstart 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 thispresentation with the findbugs plugin

Clickable links:Advanced Topics in Programming Languages: Java Puzzlers

Java-Puzzlers InfoQ


http://www.youtube.com/watch?v=wDN_EYUvUq0

http://www.infoq.com/presentations/Java-Puzzlers

Java puzzles

Software

Transcript of Java puzzles