LuaJ is easy to embed in Java.  I did have to change a few lines of their source to get it to work how I expected (it didn&#39;t require the IO library automatically).

http://sourceforge.net/projects/luaj/

Try also [kahlua][1] and [Mochalua][2].


  [1]: http://code.google.com/p/kahlua/
  [2]: http://code.google.com/p/mochalua/

There is http://www.keplerproject.org/luajava/manual.html, but essentially lua is more suitable for integration with C. There are a bunch of other scripting languages with good java integration around though. Consider groovy, jruby or jython for starters.

Lua is a C library, you can embed it in Java but you&#39;ll have to interface the java virtual machine and Lua with some C code. 

The LuaJava authors have already done that work - you&#39;re better off using that than writing your own.

I&#39;m working on a project that lets users track different data types over time. Part of the base idea is that a user should be able to enter data using any units that they need to. I&#39;ve been looking at both units:

http://pypi.python.org/pypi/units/

and quantities:

http://pypi.python.org/pypi/quantities/

However I&#39;m not sure the best way to go. From what I can tell, quantities is more complex, but includes a better initial list of units.

Unit Conversion in Python

From my understanding you don&#39;t gain much by setting an index in a column that will hold few distinct values. 

I have a column that holds a boolean value (actually it&#39;s a small int, but I&#39;m using it as a flag), and this column is used in the `WHERE` clauses of most of the queries I have. 
In a theoretical &quot;average&quot; case, half of the records&#39; values will be 1 and the other half, 0. 

So, in this scenario, the database engine could avoid a full table scan, but will have to read a lot of rows anyway (total rows/2). 

So, should I make this column an index?

*I&#39;m using Mysql 5, but I&#39;m more interested in a general rationale on why it does / does not make sense indexing a column that I know that will have a low cardinality.*

Does it make sense to use an index that will have a low cardinality?

Is LuaJava a must for this? Or can I embed Lua into Java without it?

How can I embed Lua in Java?

<p>Is LuaJava a must for this? Or can I embed Lua into Java without it?</p>


I have an array of numbers from 1 to 100 (both inclusive). The size of the array is 100. The numbers are randomly added to the array, but there is one random empty slot in the array. 
What is the quickest way to find that slot as well as the number that should be put in the slot? A Java solution is preferable.

Quickest way to find missing number in an array of numbers

I was going through [Questions every good .Net developer should be able to answer][1] and was highly impressed with the content and approach of this question, and so in the same spirit, I am asking this question for Java/Java EE Developer.


**What questions** do you think should a good **Java/Java EE programmer be able to answer?**

I am marking this question as community wiki as it is not user specific and it aims to serve programming community at large. 

Looking forward for some amazing responses. 

**EDIT**: Please answer questions too, as suggested in the comments, so that people could learn something new regarding the language, too. 


  [1]: https://stackoverflow.com/questions/365489/questions-every-good-net-developer-should-be-able-to-answer

Questions every good Java/Java EE Developer should be able to answer?

&gt; **Possible Duplicate:**  
&gt; [Java: Rationale of the Object class not being declared abstract](https://stackoverflow.com/questions/710362/java-rationale-of-the-object-class-not-being-declared-abstract)  

&lt;!-- End of automatically inserted text --&gt;

Why is the Object class, which is base class of &#39;em all in Java, not abstract?

I&#39;ve had this question for a really really long time and it is asked here purely out of curiosity, that&#39;s all. Nothing in my code or anybody&#39;s code is breaking because it is not abstract, but I was wondering why they made it concrete?

Why would anyone want an &quot;instance&quot; (and not its presence a.k.a. Reference) of this Object class? One case is a poor synchronization code which uses the instance of an Object for locking (at least I used it this way once.. my bad).

Is there any practical use of an &quot;instance&quot; of an Object class? And how does its instantiation fit in OOP? What would have happened if they had marked it abstract (of course after providing implementations to its methods)?

Why java.lang.Object is not abstract?

I know that using the `synchronize` keyword before a method brings synchronization to that object. That is, 2 threads running the same instance of the object will be synchronized.   

However, since the synchronization is at the object level, 2 threads running different instances of the object will not be synchronized. If we have a static variable in a Java class that is called by the method, we would like it to be synchronized across instances of the class. The two instances are running in 2 different threads.  

Can we achieve synchronization in the following way?  

    public class Test  
    {  
       private static int count = 0;  
       private static final Object lock= new Object();    
       public synchronized void foo() 
      {  
          synchronized(lock)
         {  
             count++;  
         }  
      }  
    }

Is it true that since we have defined an object `lock` that is static and we are using the keyword `synchronized` for that lock, the static variable `count` is now synchronized across instances of class `Test`?

How to synchronize a static variable among threads running different instances of a class in Java?

I&#39;m having trouble getting data from a [`ResultSet`][1] object. Here is my code:

		String sql = &quot;SELECT type FROM node WHERE nid = ?&quot;;
		PreparedStatement prep = conn.prepareStatement(sql);
		int meetNID = Integer.parseInt(node.get(BoutField.field_meet_nid));
		prep.setInt(1, meetNID);

		ResultSet result = prep.executeQuery();
		result.beforeFirst();
		String foundType = result.getString(1);

		if (! foundType.equals(&quot;meet&quot;)) {
			throw new IllegalArgumentException(String.format(&quot;Node %d must be of type &#39;meet&#39;, but was %s&quot;, meetNID, foundType));
		}

The error trace:

    Exception in thread &quot;main&quot; java.sql.SQLException: Before start of result set
    	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1072)
    	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:986)
    	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:981)
    	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:926)
    	at com.mysql.jdbc.ResultSetImpl.checkRowPos(ResultSetImpl.java:841)
    	at com.mysql.jdbc.ResultSetImpl.getStringInternal(ResultSetImpl.java:5656)
    	at com.mysql.jdbc.ResultSetImpl.getString(ResultSetImpl.java:5576)
    	at nth.cumf3.nodeImport.Validator.validate(Validator.java:43)
    	at nth.cumf3.nodeImport.Main.main(Main.java:38)

What am I doing wrong here?


  [1]: https://docs.oracle.com/en/java/javase/11/docs/api/java.sql/java/sql/ResultSet.html

ResultSet exception - before start of result set

Is there a method for checking if a table contains a value ? I have my own (naive) function, but I was wondering if something &quot;official&quot; exists for that ? Or something more efficient...

    function table.contains(table, element)
      for _, value in pairs(table) do
        if value == element then
          return true
        end
      end
      return false
    end

By the way, the main reason I&#39;m using this functions is to use tables as sets, ie with no duplicate elements. Is there something else I could use ?

How to check if a table contains an element in Lua?

According to the computer language benchmark game, the LuaJIT implementation [seems to beat every other JIT-ed dynamic language][1] (V8, Tracemonkey, PLT Scheme, Erlang HIPE) by **an order of magnitude**. 

I know that these benchmarks are not representative (as they say: &quot;Which programming language implementations have the fastest benchmark programs?&quot;), but this is still really impressive.

In practice, is it really the case? Someone have tested that Lua implementation?


  [1]: http://shootout.alioth.debian.org/u32/which-programming-languages-are-fastest.php?gpp=on&amp;luajit=on&amp;sbcl=on&amp;mzscheme=on&amp;hipe=on&amp;v8=on&amp;vw=on&amp;lua=on&amp;calc=chart

Is LuaJIT really faster than every other JIT-ed dynamic languages?

I&#39;m currently learning lua. regarding pattern-matching in lua I found the following sentence in the lua documentation on lua.org:

&gt; Nevertheless, pattern matching in Lua is a powerful tool and includes some features that are difficult to match with standard POSIX implementations.

As I&#39;m familiar with posix regular expressions I would like to know if there are any common samples where lua pattern matching is &quot;better&quot; compared to regular expression -- or did I misinterpret the sentence? and if there are any common examples: why is any of pattern-matching vs. regular expressions better suited?


Lua pattern matching vs. regular expressions

Sounds like a &quot;let me google it for you&quot; question, but somehow I can&#39;t find an answer. The Lua `#` operator only counts entries with integer keys, and so does `table.getn`:

    tbl = {}
    tbl[&quot;test&quot;] = 47
    tbl[1] = 48
    print(#tbl, table.getn(tbl))   -- prints &quot;1     1&quot;

    count = 0
    for _ in pairs(tbl) do count = count + 1 end
    print(count)            -- prints &quot;2&quot;

How do I get the number of _all_ entries without counting them?

How to get number of entries in a Lua table?

I don&#39;t understand the rationale behind the decision of this part of Lua. Why does indexing start at 1? I have read (as many others did) [this great paper][1]. It seems to me a strange corner of a language that is very pleasant to learn and program. Don&#39;t get me wrong, Lua is just great but there has to be an explanation somewhere. Most of what I found (on the web) is just saying the index starts at 1. Full stop.

It would be very interesting to read what its designers said about the subject.

Note that I am &quot;very&quot; beginner in Lua, I hope I am not missing something obvious about tables.

  [1]: http://www.cs.utexas.edu/~EWD/ewd08xx/EWD831.PDF

Content Type	Original Author	Original Content on Stackoverflow
Question	anon	View Question on Stackoverflow
Solution 1 - Java	Ben Ziegler	View Answer on Stackoverflow
Solution 2 - Java	lhf	View Answer on Stackoverflow
Solution 3 - Java	Manfred Moser	View Answer on Stackoverflow
Solution 4 - Java	nos	View Answer on Stackoverflow

How can I embed Lua in Java?

Java Problem Overview

Java Solutions

Solution 1 - Java

Solution 2 - Java

Solution 3 - Java

Solution 4 - Java

Does it make sense to use an index that will have a low cardinality?

Unit Conversion in Python

Attributions