searching for specific keys in multiple independen

Question

I have a slightly theoretical question that I want to clear up before I start programing my solution any further...

Background. I need to perform comparisons between 2 (or possible more) MS Access files.

Each file should contain data that is found in one of the others.

Due to the 'limitation' I have encountered with JDBC and connecting to Access result sets (they are only forward scrollable!) I have created 'java objects' (classes) that models the structure of a result set.

in essence I have an object that models a single record line in the result set (lets call it a rowSet) A resSet object then has an 'array' of rowSets.

However to 'speed things up' I catch the values in the key and index columns and create a hashMap of 'key_Index' this to the relevant rowSet object.

Me comparison then does the following.

take the first resSet object (to use as the master), from this collect the individual the Key_Index hashmaps (lets call it 'aKey'....

Now use this 'aKey' object to search into the other avalailable resSet objects and see if any contain a value of a key_Index that matches the values in 'aKey'.

However I have just had a rather annoying thought.

If I use the code resSet.get(aKey) on one of my other resSet objects will I have problems as the 'aKey' object is obviously not the same object - although it's contents should be the same (ie comparable).

I have to say that as I read my question I think it is not well worded... so I think I will include a copy of the class that I have created...

The important parts:

members: challenge - an arrayList of 'result set' type objects.

method: the runChallenge()

package KWhite;

/**
 * the RScomparator is designed for the instance of comparing a double entry database system, as
 * often occurs in localy run medical trials data.
 * 
 * The double entry is to ensure that there are no errors made during input, the entry is performed
 * in separate independant instances. The RScolmparator object is specifically able to take any
 * number of result sets as its input (ie queries from multiple independantly created databases)
 * 
 * It should be recognised that this object should probably be called as part of a DBcomparator
 * object.
 * 
 */
//imports here

//import of logger class and required dependencies...
import java.util.ArrayList;
import java.util.HashMap;

import org.apache.log4j.PropertyConfigurator;

import MrBlue.DB_Table;
import TawuaiLogger.tawuaiLogger;

public class RScomparator {

//Static variables

//the logger instance
private static final TawuaiLogger.tawuaiLogger errLog = new tawuaiLogger("RScomparator");

//class member variables

/** this is a selection of ResSet objects that are going to have thier data challenged */
private ArrayList<ResSet_KW> challenge;
/**the name of the current table being challenged*/
private String tableName;
/**a 'table' object for meta data reference purposes, this can be used for getting column types etc */
private DB_Table table;

//These are our report objects
/**a report array for challenge failures */
private ArrayList<report_KW> fail;
/**a report array for errors, ie no challenger able to be made, no equivalent value found */
private ArrayList<report_KW> errors;
/**a report array for the good values */
private ArrayList<report_KW> success;

/** this is either the main class or the constructor
 * 
 * If it is a constructor rename to reflect the name of the class
 * @param args
 */
public RScomparator(DB_Table t) //TODO add arguments as required
{
    PropertyConfigurator.configure("Log4j.properties");
    // TODO Auto-generated method stub
    challenge = new ArrayList<ResSet_KW>();
    //initialise our report objects for this challenge scenario
    fail =  new ArrayList<report_KW>();
    errors = new ArrayList<report_KW>();
    success = new ArrayList<report_KW>();
    table = t;
    tableName = t.getTblName();
}

//class methods go here

/**
 * add a result set object into this comparator
 * 
 * @param r the result set object being inserted into this challenge.
 * 
 */
public void addChallenger(ResSet_KW r)
{
this.challenge.add(r);  
}


/**
 * this runs the comparison process...
 * Although no details in of itself are returned it calls other methods that do return a value
 *
 */

public void runChallenge()
{
//TODO finish this method, creating a report object on the way

    //these are the 2 result set objects that will be compared
    ResSet_KW gold = new ResSet_KW  ();
    ResSet_KW silver = new ResSet_KW ();



    //ensure the challenger list has objects in it.
    if(challenge.size() < 2)
    {
        //it must have 2 objects..
        errLog.add(3, "there are no results available for comparison of table " + this.tableName);
        //either way we should create report object.
        this.errors.add( new report_KW(tableName));
        //break out of the method.
        return;
    }

    //get the first row of data

    gold = challenge.get(0);//the first result set.

    //for each column in the result set, perform a search for the same key in the others..
    for(HashMap<String, String> c : gold.getRS().keySet())
    {//c is the key value in the map
        //cycle over the challenge object
        for (int i=1; i<challenge.size(); i++)//we don't want to use the first element, so start from 1 not zero
        {
            silver = challenge.get(i);
            if (silver.hasKey(c))
            {
                //a temp object for meta data referencing

                //only get the actual result values if there is a match
                Column_KW a = gold.getRS().get(c);
                Column_KW b = silver.getRS().get(c);
                //make the comparison
                a.compareTo(b, this.table);
                //get the reports from the comparison
                for(report_KW k :a.getFailure())
                    {
                    this.fail.add(k);
                    }
                for(report_KW k :a.getPassed())
                {
                    this.success.add(k);
                }
                for(report_KW k :a.getPassed())
                {
                this.errors.add(k);
                }
            }
            else
            {
                break;//return to the next item in the for loop
            }
        }
    }









}


/**
 *a helper method to create the error message creator
 *@ param m the extra message if any, 
 *@return s the full message
 */
private String getErrMessage(String m) {
    //the third element in the current stact trace should be the calling method
    StackTraceElement caller = Thread.currentThread().getStackTrace()[3];
    String s = m + "\ncalled from line " + caller.getLineNumber()
            + "\nmethod: " + this.getClass() + "." + caller.getMethodName();
        return s;
    }
}//end class

ps. any coments on my code or more that welcome

Thanks in advance

David

edit: I just found this question, Nested Maps or combined keys in java is this going to be my solution, create a'custom' key_Index object and then define a hashcode() and equals object for it. However I am allready using a 'hashMap' for my key_index object, so does this perform this automatically somewhere that I haven' noticed?

Answer 1

You said :

If I use the code resSet.get(aKey) on one of my other resSet objects will I have problems as the 'aKey' object is obviously not the same object - although it's contents should be the same (ie comparable).

You have to implement equals() and hashcode() respectively. It is not enough to implement equals(), hashcode() must be the same if the objects are equal.

Example:

import java.util.HashMap;
import java.util.Map;

class A {
    String name;
    Integer number;

    public A(String name, Integer number) {
    super();
    this.name = name;
    this.number = number;
    }

}

class B {
    String name;
    Integer number;

    public B(String name, Integer number) {
    super();
    this.name = name;
    this.number = number;
    }

    @Override
    public boolean equals(Object obj) {
    if (obj instanceof B) {
        return obj == this || (name.equals(((B) obj).name) && number.equals(((B) obj).number));
    }
    return false;
    }

    @Override
    public int hashCode() {
    return name.hashCode() + number.hashCode();
    }
}

public class TestHashMap {

    public static void main(String... args) {
        A a1 = new A("a", 1);
        A anotherA1 = new A("a", 1);

        Map<A, String> as = new HashMap<A, String>();

        as.put(a1, "a1");

        System.out.println(as.get(anotherA1)); // prints null

        B b1 = new B("b", 1);
        B anotherB1 = new B("b", 1);

        Map<B, String> bs = new HashMap<B, String>();

        bs.put(b1, "b1");

        System.out.println(bs.get(anotherB1)); // prints b1

    }

}