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  • codingBat repeatEnd using regex

    - by polygenelubricants
    I'm trying to understand regex as much as I can, so I came up with this regex-based solution to codingbat.com repeatEnd: Given a string and an int N, return a string made of N repetitions of the last N characters of the string. You may assume that N is between 0 and the length of the string, inclusive. public String repeatEnd(String str, int N) { return str.replaceAll( ".(?!.{N})(?=.*(?<=(.{N})))|." .replace("N", Integer.toString(N)), "$1" ); } Explanation on its parts: .(?!.{N}): asserts that the matched character is one of the last N characters, by making sure that there aren't N characters following it. (?=.*(?<=(.{N}))): in which case, use lookforward to first go all the way to the end of the string, then a nested lookbehind to capture the last N characters into \1. Note that this assertion will always be true. |.: if the first assertion failed (i.e. there are at least N characters ahead) then match the character anyway; \1 would be empty. In either case, a character is always matched; replace it with \1. My questions are: Is this technique of nested assertions valid? (i.e. looking behind during a lookahead?) Is there a simpler regex-based solution?

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  • codingbat wordEnds using regex

    - by polygenelubricants
    I'm trying to solve wordEnds from codingbat.com using regex. This is the simplest as I can make it with my current knowledge of regex: public String wordEnds(String str, String word) { return str.replaceAll( String.format( ".*?(?=%s)(?<=(.|^))%1$s(?=(.|$))|.+", java.util.regex.Pattern.quote(word) ), "$1$2" ); } String.format is used to inject word into the pattern for both readability and convenience (it's injected twice). Pattern.quote isn't necessary to pass their tests, but I think it's required for a proper regex-based solution. The regex has two major parts: If after matching as few characters as possible ".*?", word can still be found "(?=%s)", then lookbehind to capture any character immediately preceding it "(?<=(.|^))", match word "%1$s" and lookforward to capture any character following it "(?=(.|$))". The initial "if" test ensures that the atomic lookbehind captures only if there's a word Using lookahead to capture the following character doesn't consume it, so it can be used as part of further matching Otherwise match what's left "|.+" Groups 1 and 2 would capture empty strings I think this works in all cases, but it's obviously quite complex. I'm just wondering if others can suggest a simpler regex to do this. Note: I'm not looking for a solution using indexOf and a loop. I want a regex-based replaceAll solution. I also need a working solution that I can just copy-paste into codingbat and passes.

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  • codingBat plusOut using regex

    - by polygenelubricants
    This is similar to my previous efforts (wordEnds and repeatEnd): as a mental exercise, I want to solve this toy problem using regex only. Description from codingbat.com: Given a string and a non-empty word string, return a version of the original string where all chars have been replaced by pluses ("+"), except for appearances of the word string which are preserved unchanged. plusOut("12xy34", "xy") ? "++xy++" plusOut("12xy34", "1") ? "1+++++" plusOut("12xy34xyabcxy", "xy") ? "++xy++xy+++xy" There is no mention whether or not to allow overlap (e.g. what is plusOut("+xAxAx+", "xAx")?), but my non-regex solution doesn't handle overlap and it passes, so I guess we can assume non-overlapping occurrences of word if it makes it simpler. In any case, I'd like to solve this using regex (of the same style that I did before with the other two problems), but I'm absolutely stumped. I don't even have anything to show, because I have nothing that works. So let's see what the stackoverflow community comes up with.

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  • Al Zimmermann's Son of Darts

    - by polygenelubricants
    There's about 2 months left in Al Zimmermann's Son of Darts programming contest, and I'd like to improve my standing (currently in the 60s) to something more respectable. I'd like to get some ideas from the great community of stackoverflow on how best to approach this problem. The contest problem is known as the Global Postage Stamp Problem in literatures. I don't have much experience with optimization algorithms (I know of hillclimbing and simulated annealing in concept only from college), and in fact the program that I have right now is basically sheer brute force, which of course isn't feasible for the larger search spaces. Here are some papers on the subject: A Postage Stamp Problem (Alter & Barnett, 1980) Algorithms for Computing the h-Range of the Postage Stamp Problem (Mossige, 1981) A Postage Stamp Problem (Lunnon, 1986) Two New Techniques for Computing Extremal h-bases Ak (Challis, 1992) Any hints and suggestions are welcome. Also, feel free to direct me to the proper site if stackoverflow isn't it.

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  • Regex for circular replacement

    - by polygenelubricants
    How would you use regex to write functions to do the following: Replace lowercase 'a' with uppercase and vice versa Where words are separated by whitespaces and > and < are special markers, replace >word with word< and vice versa Replace postincrement (i++;) with preincrement (++i;) and vice versa. Variable names are [a-z]+. Input is just a bunch of these statements. Bonus: also do decrement. Also interested in solutions in other flavors. Note: this is NOT a homework question. See also my previous explorations of regex: Regex split into overlapping strings (Alan Moore's answer is especially instructive) Can you use zero-width matching regex in String split? (my solution exploits a known Java regex bug with regards to non-obvious length lookbehind!)

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  • Regex split into overlapping strings

    - by polygenelubricants
    I'm exploring the power of regular expressions, so I'm just wondering if something like this is possible: public class StringSplit { public static void main(String args[]) { System.out.println( java.util.Arrays.deepToString( "12345".split(INSERT_REGEX_HERE) ) ); // prints "[12, 23, 34, 45]" } } If possible, then simply provide the regex (and preemptively some explanation on how it works). If it's only possible in some regex flavors other than Java, then feel free to provide those as well. If it's not possible, then please explain why.

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  • Regex one-to-one mapping pattern replace

    - by polygenelubricants
    How would you use regex to write a function that replaces all lowercase letters with uppercase and vice versa? Note: this is NOT a homework question. See also my previous explorations of regex: Regex split into overlapping strings (Alan Moore's answer is especially instructive) Can you use zero-width matching regex in String split? (my solution exploits a known Java regex bug with regards to non-obvious length lookbehind!)

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  • codingBat last2 using regex

    - by polygenelubricants
    Okay guys, this is similar to my repeatEnd and wordEnds efforts; I want to solve this codingBat Warmup-2 question using only regex-based techniques as a "brain gymnastics" exercise. This solution works for codingBat tests: public int last2(String str) { return str.isEmpty() ? 0 : str.split( str.replaceAll( ".*(.)(.)", "$1(?=$2)" //.replaceAll("(\\$.)", "\\\\\\\\Q$1\\\\\\\\E") ), -1 ).length - 1 - 1; } The monstrous octo-slashes aren't needed to pass codingBat, but is needed for a proper regex-based solution. That is, if I want this (and I do!): assert last2("..+++...++") == 2; I'd have to uncomment the second .replaceAll. I'm just wondering if others can come up with a simpler, more elegant regex solution for this problem. Preferably one that doesn't contain octo-slashes.

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  • Is it a bad idea if equals(null) throws NullPointerException instead?

    - by polygenelubricants
    The contract of equals with regards to null, is as follows: For any non-null reference value x, x.equals(null) should return false. This is rather peculiar, because if o1 != null and o2 == null, then we have: o1.equals(o2) // returns false o2.equals(o1) // throws NullPointerException The fact that o2.equals(o1) throws NullPointerException is a good thing, because it alerts us of programmer error. And yet, that error would not be catched if for various reasons we just switched it around to o1.equals(o2), which would just "silently fail" instead. So the questions are: Why is it a good idea that o1.equals(o2) should return false instead of throwing NullPointerException? Would it be a bad idea if wherever possible we rewrite the contract so that anyObject.equals(null) always throw NullPointerException instead?

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  • codingBat separateThousands using regex (and unit testing how-to)

    - by polygenelubricants
    This question is a combination of regex practice and unit testing practice. Regex part I authored this problem separateThousands for personal practice: Given a number as a string, introduce commas to separate thousands. The number may contain an optional minus sign, and an optional decimal part. There will not be any superfluous leading zeroes. Here's my solution: String separateThousands(String s) { return s.replaceAll( String.format("(?:%s)|(?:%s)", "(?<=\\G\\d{3})(?=\\d)", "(?<=^-?\\d{1,3})(?=(?:\\d{3})+(?!\\d))" ), "," ); } The way it works is that it classifies two types of commas, the first, and the rest. In the above regex, the rest subpattern actually appears before the first. A match will always be zero-length, which will be replaceAll with ",". The rest basically looks behind to see if there was a match followed by 3 digits, and looks ahead to see if there's a digit. It's some sort of a chain reaction mechanism triggered by the previous match. The first basically looks behind for ^ anchor, followed by an optional minus sign, and between 1 to 3 digits. The rest of the string from that point must match triplets of digits, followed by a nondigit (which could either be $ or \.). My question for this part is: Can this regex be simplified? Can it be optimized further? Ordering rest before first is deliberate, since first is only needed once No capturing group Unit testing part As I've mentioned, I'm the author of this problem, so I'm also the one responsible for coming up with testcases for them. Here they are: INPUT, OUTPUT "1000", "1,000" "-12345", "-12,345" "-1234567890.1234567890", "-1,234,567,890.1234567890" "123.456", "123.456" ".666666", ".666666" "0", "0" "123456789", "123,456,789" "1234.5678", "1,234.5678" "-55555.55555", "-55,555.55555" "0.123456789", "0.123456789" "123456.789", "123,456.789" I haven't had much experience with industrial-strength unit testing, so I'm wondering if others can comment whether this is a good coverage, whether I've missed anything important, etc (I can always add more tests if there's a scenario I've missed).

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  • Managing highly repetitive code and documentation in Java

    - by polygenelubricants
    Highly repetitive code is generally a bad thing, and there are design patterns that can help minimize this. However, sometimes it's simply inevitable due to the constraints of the language itself. Take the following example from java.util.Arrays: /** * Assigns the specified long value to each element of the specified * range of the specified array of longs. The range to be filled * extends from index <tt>fromIndex</tt>, inclusive, to index * <tt>toIndex</tt>, exclusive. (If <tt>fromIndex==toIndex</tt>, the * range to be filled is empty.) * * @param a the array to be filled * @param fromIndex the index of the first element (inclusive) to be * filled with the specified value * @param toIndex the index of the last element (exclusive) to be * filled with the specified value * @param val the value to be stored in all elements of the array * @throws IllegalArgumentException if <tt>fromIndex &gt; toIndex</tt> * @throws ArrayIndexOutOfBoundsException if <tt>fromIndex &lt; 0</tt> or * <tt>toIndex &gt; a.length</tt> */ public static void fill(long[] a, int fromIndex, int toIndex, long val) { rangeCheck(a.length, fromIndex, toIndex); for (int i=fromIndex; i<toIndex; i++) a[i] = val; } The above snippet appears in the source code 8 times, with very little variation in the documentation/method signature but exactly the same method body, one for each of the root array types int[], short[], char[], byte[], boolean[], double[], float[], and Object[]. I believe that unless one resorts to reflection (which is an entirely different subject in itself), this repetition is inevitable. I understand that as a utility class, such high concentration of repetitive Java code is highly atypical, but even with the best practice, repetition does happen! Refactoring doesn't always work because it's not always possible (the obvious case is when the repetition is in the documentation). Obviously maintaining this source code is a nightmare. A slight typo in the documentation, or a minor bug in the implementation, is multiplied by however many repetitions was made. In fact, the best example happens to involve this exact class: Google Research Blog - Extra, Extra - Read All About It: Nearly All Binary Searches and Mergesorts are Broken (by Joshua Bloch, Software Engineer) The bug is a surprisingly subtle one, occurring in what many thought to be just a simple and straightforward algorithm. // int mid =(low + high) / 2; // the bug int mid = (low + high) >>> 1; // the fix The above line appears 11 times in the source code! So my questions are: How are these kinds of repetitive Java code/documentation handled in practice? How are they developed, maintained, and tested? Do you start with "the original", and make it as mature as possible, and then copy and paste as necessary and hope you didn't make a mistake? And if you did make a mistake in the original, then just fix it everywhere, unless you're comfortable with deleting the copies and repeating the whole replication process? And you apply this same process for the testing code as well? Would Java benefit from some sort of limited-use source code preprocessing for this kind of thing? Perhaps Sun has their own preprocessor to help write, maintain, document and test these kind of repetitive library code? A comment requested another example, so I pulled this one from Google Collections: com.google.common.base.Predicates lines 276-310 (AndPredicate) vs lines 312-346 (OrPredicate). The source for these two classes are identical, except for: AndPredicate vs OrPredicate (each appears 5 times in its class) "And(" vs Or(" (in the respective toString() methods) #and vs #or (in the @see Javadoc comments) true vs false (in apply; ! can be rewritten out of the expression) -1 /* all bits on */ vs 0 /* all bits off */ in hashCode() &= vs |= in hashCode()

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  • How to do inclusive range queries when only half-open range is supported (ala SortedMap.subMap)

    - by polygenelubricants
    On SortedMap.subMap This is the API for SortedMap<K,V>.subMap: SortedMap<K,V> subMap(K fromKey, K toKey) : Returns a view of the portion of this map whose keys range from fromKey, inclusive, to toKey, exclusive. This inclusive lower bound, exclusive upper bound combo ("half-open range") is something that is prevalent in Java, and while it does have its benefits, it also has its quirks, as we shall soon see. The following snippet illustrates a simple usage of subMap: static <K,V> SortedMap<K,V> someSortOfSortedMap() { return Collections.synchronizedSortedMap(new TreeMap<K,V>()); } //... SortedMap<Integer,String> map = someSortOfSortedMap(); map.put(1, "One"); map.put(3, "Three"); map.put(5, "Five"); map.put(7, "Seven"); map.put(9, "Nine"); System.out.println(map.subMap(0, 4)); // prints "{1=One, 3=Three}" System.out.println(map.subMap(3, 7)); // prints "{3=Three, 5=Five}" The last line is important: 7=Seven is excluded, due to the exclusive upper bound nature of subMap. Now suppose that we actually need an inclusive upper bound, then we could try to write a utility method like this: static <V> SortedMap<Integer,V> subMapInclusive(SortedMap<Integer,V> map, int from, int to) { return (to == Integer.MAX_VALUE) ? map.tailMap(from) : map.subMap(from, to + 1); } Then, continuing on with the above snippet, we get: System.out.println(subMapInclusive(map, 3, 7)); // prints "{3=Three, 5=Five, 7=Seven}" map.put(Integer.MAX_VALUE, "Infinity"); System.out.println(subMapInclusive(map, 5, Integer.MAX_VALUE)); // {5=Five, 7=Seven, 9=Nine, 2147483647=Infinity} A couple of key observations need to be made: The good news is that we don't care about the type of the values, but... subMapInclusive assumes Integer keys for to + 1 to work. A generic version that also takes e.g. Long keys is not possible (see related questions) Not to mention that for Long, we need to compare against Long.MAX_VALUE instead Overloads for the numeric primitive boxed types Byte, Character, etc, as keys, must all be written individually A special check need to be made for toInclusive == Integer.MAX_VALUE, because +1 would overflow, and subMap would throw IllegalArgumentException: fromKey > toKey This, generally speaking, is an overly ugly and overly specific solution What about String keys? Or some unknown type that may not even be Comparable<?>? So the question is: is it possible to write a general subMapInclusive method that takes a SortedMap<K,V>, and K fromKey, K toKey, and perform an inclusive-range subMap queries? Related questions Are upper bounds of indexed ranges always assumed to be exclusive? Is it possible to write a generic +1 method for numeric box types in Java? On NavigableMap It should be mentioned that there's a NavigableMap.subMap overload that takes two additional boolean variables to signify whether the bounds are inclusive or exclusive. Had this been made available in SortedMap, then none of the above would've even been asked. So working with a NavigableMap<K,V> for inclusive range queries would've been ideal, but while Collections provides utility methods for SortedMap (among other things), we aren't afforded the same luxury with NavigableMap. Related questions Writing a synchronized thread-safety wrapper for NavigableMap On API providing only exclusive upper bound range queries Does this highlight a problem with exclusive upper bound range queries? How were inclusive range queries done in the past when exclusive upper bound is the only available functionality?

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  • Is regex too slow? Real life examples where simple non-regex alternative is better

    - by polygenelubricants
    I've seen people here made comments like "regex is too slow!", or "why would you do something so simple using regex!" (and then present a 10+ lines alternative instead), etc. I haven't really used regex in industrial setting, so I'm curious if there are applications where regex is demonstratably just too slow, AND where a simple non-regex alternative exists that performs significantly (maybe even asymptotically!) better. Obviously many highly-specialized string manipulations with sophisticated string algorithms will outperform regex easily, but I'm talking about cases where a simple solution exists and significantly outperforms regex. What counts as simple is subjective, of course, but I think a reasonable standard is that if it uses only String, StringBuilder, etc, then it's probably simple.

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  • Reordering arguments using recursion (pro, cons, alternatives)

    - by polygenelubricants
    I find that I often make a recursive call just to reorder arguments. For example, here's my solution for endOther from codingbat.com: Given two strings, return true if either of the strings appears at the very end of the other string, ignoring upper/lower case differences (in other words, the computation should not be "case sensitive"). Note: str.toLowerCase() returns the lowercase version of a string. public boolean endOther(String a, String b) { return a.length() < b.length() ? endOther(b, a) : a.toLowerCase().endsWith(b.toLowerCase()); } I'm very comfortable with recursions, but I can certainly understand why some perhaps would object to it. There are two obvious alternatives to this recursion technique: Swap a and b traditionally public boolean endOther(String a, String b) { if (a.length() < b.length()) { String t = a; a = b; b = t; } return a.toLowerCase().endsWith(b.toLowerCase()); } Not convenient in a language like Java that doesn't pass by reference Lots of code just to do a simple operation An extra if statement breaks the "flow" Repeat code public boolean endOther(String a, String b) { return (a.length() < b.length()) ? b.toLowerCase().endsWith(a.toLowerCase()) : a.toLowerCase().endsWith(b.toLowerCase()); } Explicit symmetry may be a nice thing (or not?) Bad idea unless the repeated code is very simple ...though in this case you can get rid of the ternary and just || the two expressions So my questions are: Is there a name for these 3 techniques? (Are there more?) Is there a name for what they achieve? (e.g. "parameter normalization", perhaps?) Are there official recommendations on which technique to use (when)? What are other pros/cons that I may have missed?

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  • Backreferences in lookbehind

    - by polygenelubricants
    Can you use backreferences in a lookbehind? Let's say I want to split wherever behind me a character is repeated twice. String REGEX1 = "(?<=(.)\\1)"; // DOESN'T WORK! String REGEX2 = "(?<=(?=(.)\\1)..)"; // WORKS! System.out.println(java.util.Arrays.toString( "Bazooka killed the poor aardvark (yummy!)" .split(REGEX2) )); // prints "[Bazoo, ka kill, ed the poo, r aa, rdvark (yumm, y!)]" Using REGEX2 (where the backreference is in a lookahead nested inside a lookbehind) works, but REGEX1 gives this error at run-time: Look-behind group does not have an obvious maximum length near index 8 (?<=(.)\1) ^ This sort of make sense, I suppose, because in general the backreference can capture a string of any length (if the regex compiler is a bit smarter, though, it could determine that \1 is (.) in this case, and therefore has a finite length). So is there a way to use a backreference in a lookbehind? And if there isn't, can you always work around it using this nested lookahead? Are there other commonly-used techniques?

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  • Naming convention when casually referring to methods in Java

    - by polygenelubricants
    Is there a Java convention to refer to methods, static and otherwise, any specific one or the whole overload, etc? e.g. String.valueOf - referring to all overloads of static valueOf String.valueOf(char) - specific overload, formal parameter name omittable? String.split - looks like a static method, but actually an instance method Maybe aString.split is the convention? String#split - I've seen this HTML anchor form too, which I guess is javadoc-influenced Is there an authoritative recommendation on how to clearly refer to these things?

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  • "The left hand side of an assignment must be a variable" due to extra parentheses

    - by polygenelubricants
    I know why the following code doesn't compile: public class Main { public static void main(String args[]) { main((null)); // this is fine! (main(null)); // this is NOT! } } What I'm wondering is why my compiler (javac 1.6.0_17, Windows version) is complaining "The left hand side of an assignment must be a variable". I'd expect something like "Don't put parentheses around a method invokation, dummy!", instead. So why is the compiler making a totally unhelpful complaint about something that is blatantly irrelevant? Is this the result of an ambiguity in the grammar? A bug in the compiler? If it's the former, could you design a language such that a compiler would never be so off-base about a syntax error like this?

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  • Beginner SQL question: querying gold and silver tag badges in Stack Exchange Data Explorer

    - by polygenelubricants
    I'm using the Stack Exchange Data Explorer to learn SQL, but I think the fundamentals of the question is applicable to other databases. I'm trying to query the Badges table, which according to Stexdex (that's what I'm going to call it from now on) has the following schema: Badges Id UserId Name Date This works well for badges like [Epic] and [Legendary] which have unique names, but the silver and gold tag-specific badges seems to be mixed in together by having the same exact name. Here's an example query I wrote for [mysql] tag: SELECT UserId as [User Link], Date FROM Badges Where Name = 'mysql' Order By Date ASC The (slightly annotated) output is: as seen on stexdex: User Link Date --------------- ------------------- // all for silver except where noted Bill Karwin 2009-02-20 11:00:25 Quassnoi 2009-06-01 10:00:16 Greg 2009-10-22 10:00:25 Quassnoi 2009-10-31 10:00:24 // for gold Bill Karwin 2009-11-23 11:00:30 // for gold cletus 2010-01-01 11:00:23 OMG Ponies 2010-01-03 11:00:48 Pascal MARTIN 2010-02-17 11:00:29 Mark Byers 2010-04-07 10:00:35 Daniel Vassallo 2010-05-14 10:00:38 This is consistent with the current list of silver and gold earners at the moment of this writing, but to speak in more timeless terms, as of the end of May 2010 only 2 users have earned the gold [mysql] tag: Quassnoi and Bill Karwin, as evidenced in the above result by their names being the only ones that appear twice. So this is the way I understand it: The first time an Id appears (in chronological order) is for the silver badge The second time is for the gold Now, the above result mixes the silver and gold entries together. My questions are: Is this a typical design, or are there much friendlier schema/normalization/whatever you call it? In the current design, how would you query the silver and gold badges separately? GROUP BY Id and picking the min/max or first/second by the Date somehow? How can you write a query that lists all the silver badges first then all the gold badges next? Imagine also that the "real" query may be more complicated, i.e. not just listing by date. How would you write it so that it doesn't have too many repetition between the silver and gold subqueries? Is it perhaps more typical to do two totally separate queries instead? What is this idiom called? A row "partitioning" query to put them into "buckets" or something?

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  • Iterator performance contract (and use on non-collections)

    - by polygenelubricants
    If all that you're doing is a simple one-pass iteration (i.e. only hasNext() and next(), no remove()), are you guaranteed linear time performance and/or amortized constant cost per operation? Is this specified in the Iterator contract anywhere? Are there data structures/Java Collection which cannot be iterated in linear time? java.util.Scanner implements Iterator<String>. A Scanner is hardly a data structure (e.g. remove() makes absolutely no sense). Is this considered a design blunder? Is something like PrimeGenerator implements Iterator<Integer> considered bad design, or is this exactly what Iterator is for? (hasNext() always returns true, next() computes the next number on demand, remove() makes no sense). Similarly, would it have made sense for java.util.Random implements Iterator<Double>? Should a type really implement Iterator if it's effectively only using one-third of its API? (i.e. no remove(), always hasNext())

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  • Beginner SQL question: arithmetic with multiple COUNT(*) results

    - by polygenelubricants
    Continuing with the spirit of using the Stack Exchange Data Explorer to learn SQL, (see: Can we become our own “Northwind” for teaching SQL / databases?), I've decided to try to write a query to answer a simple question (on meta): What % of stackoverflow users have over 10,000 rep?. Here's what I've done: Query#1 SELECT COUNT(*) FROM Users WHERE Users.Reputation >= 10000 Result: 556 Query#2 SELECT COUNT(*) FROM USERS Result: 227691 Now, how do I put them together into one query? What is this query idiom called? What do I need to write so I can get, say, a one-row three-column result like this: 556 227691 0,00244190592

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  • Comparable and Comparator contract with regards to null

    - by polygenelubricants
    Comparable contract specifies that e.compareTo(null) must throw NullPointerException. From the API: Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false. On the other hand, Comparator API mentions nothing about what needs to happen when comparing null. Consider the following attempt of a generic method that takes a Comparable, and return a Comparator for it that puts null as the minimum element. static <T extends Comparable<? super T>> Comparator<T> nullComparableComparator() { return new Comparator<T>() { @Override public int compare(T el1, T el2) { return el1 == null ? -1 : el2 == null ? +1 : el1.compareTo(el2); } }; } This allows us to do the following: List<Integer> numbers = new ArrayList<Integer>( Arrays.asList(3, 2, 1, null, null, 0) ); Comparator<Integer> numbersComp = nullComparableComparator(); Collections.sort(numbers, numbersComp); System.out.println(numbers); // "[null, null, 0, 1, 2, 3]" List<String> names = new ArrayList<String>( Arrays.asList("Bob", null, "Alice", "Carol") ); Comparator<String> namesComp = nullComparableComparator(); Collections.sort(names, namesComp); System.out.println(names); // "[null, Alice, Bob, Carol]" So the questions are: Is this an acceptable use of a Comparator, or is it violating an unwritten rule regarding comparing null and throwing NullPointerException? Is it ever a good idea to even have to sort a List containing null elements, or is that a sure sign of a design error?

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  • Is 1/0 a legal Java expression?

    - by polygenelubricants
    The following compiles fine in my Eclipse: final int j = 1/0; // compiles fine!!! // throws ArithmeticException: / by zero at run-time Java prevents many "dumb code" from even compiling in the first place (e.g. "Five" instanceof Number doesn't compile!), so the fact this didn't even generate as much as a warning was very surprising to me. The intrigue deepens when you consider the fact that constant expressions are allowed to be optimized at compile time: public class Div0 { public static void main(String[] args) { final int i = 2+3; final int j = 1/0; final int k = 9/2; } } Compiled in Eclipse, the above snippet generates the following bytecode (javap -c Div0) Compiled from "Div0.java" public class Div0 extends java.lang.Object{ public Div0(); Code: 0: aload_0 1: invokespecial #8; //Method java/lang/Object."<init>":()V 4: return public static void main(java.lang.String[]); Code: 0: iconst_5 1: istore_1 // "i = 5;" 2: iconst_1 3: iconst_0 4: idiv 5: istore_2 // "j = 1/0;" 6: iconst_4 7: istore_3 // "k = 4;" 8: return } As you can see, the i and k assignments are optimized as compile-time constants, but the division by 0 (which must've been detectable at compile-time) is simply compiled as is. javac 1.6.0_17 behaves even more strangely, compiling silently but excising the assignments to i and k completely out of the bytecode (probably because it determined that they're not used anywhere) but leaving the 1/0 intact (since removing it would cause an entirely different program semantics). So the questions are: Is 1/0 actually a legal Java expression that should compile anytime anywhere? What does JLS say about it? If this is legal, is there a good reason for it? What good could this possibly serve?

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  • Java for each vs regular for -- are they equivalent?

    - by polygenelubricants
    Are these two constructs equivalent? char[] arr = new char[5]; for (char x : arr) { // code goes here } Compared to: char[] arr = new char[5]; for (int i = 0; i < arr.length; i++) { char x = arr[i]; // code goes here } That is, if I put exactly the same code in the body of both loops (and they compile), will they behave exactly the same??? Full disclaimer: this was inspired by another question (Java: are these 2 codes the same). My answer there turned out not to be the answer, but I feel that the exact semantics of Java for-each has some nuances that needs pointing out.

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  • Fun things you can do by mutating Java strings

    - by polygenelubricants
    So I've come around since I asked how to limit setAccessible to only “legitimate” uses and have come to embrace its power for fun. Enabled by its power, of course, is string mutation. import java.lang.reflect.Field; public class Mutator { static void mutate(Object obj, String field, Object newValue) { try { Field f = obj.getClass().getDeclaredField(field); f.setAccessible(true); f.set(obj, newValue); } catch (Exception e) { } } public static void mutate(String from, String to) { mutate(from, "value", to.toCharArray()); mutate(from, "count", to.length()); } public static void main(String args[]) { Mutator.mutate(System.getProperty("line.separator"), "<br/>\n"); System.out.println("Hello world!"); Mutator.mutate(Integer.toString(Integer.MIN_VALUE), "OMG!"); System.out.println(-2147483648); Mutator.mutate(String.valueOf((Object) null), "LOL!"); System.out.println(Arrays.toString(new int[3][])); Mutator.mutate(Arrays.toString(new int[0]), ":("); System.out.println(Arrays.toString(new byte[0])); } } Output (if no exception is thrown): Hello world!<br/> OMG!<br/> [LOL!, LOL!, LOL!]<br/> :(<br/> Let's see what other fun things we can come up with.

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