Java Program to Implement Gale Shapley Algorithm

This is a Java Program to Implement Gale Shapley Algorithm. Gale Shapley Algorithm is used to solve the stable marriage problem (SMP). SMP is the problem of finding a stable matching between two sets of elements given a set of preferences for each element.

Algorithm is as follows:

function stableMatching {
    Initialize all m ∈ M and w ∈ W to free
    while ∃ free man m who still has a woman w to propose to {
       w = m's highest ranked such woman to whom he has not yet proposed
       if w is free
         (m, w) become engaged
       else some pair (m', w) already exists
         if w prefers m to m'
           (m, w) become engaged
           m' becomes free
         else
           (m', w) remain engaged
    }
}

Here is the source code of the Java Program to Implement Gale Shapley Algorithm. The Java program is successfully compiled and run on a Windows system. The program output is also shown below.

/**
 *    Java Program to Implement Gale Shapley Algorithm 
 **/
 
/** Class GaleShapley **/
public class GaleShapley
{
    private int N, engagedCount;
    private String[][] menPref;
    private String[][] womenPref;
    private String[] men;
    private String[] women;
    private String[] womenPartner;
    private boolean[] menEngaged;
 
    /** Constructor **/
    public GaleShapley(String[] m, String[] w, String[][] mp, String[][] wp)
    {
        N = mp.length;
        engagedCount = 0;
        men = m;
        women = w;
        menPref = mp;
        womenPref = wp;
        menEngaged = new boolean[N];
        womenPartner = new String[N];
        calcMatches();
    }
    /** function to calculate all matches **/
    private void calcMatches()
    {
        while (engagedCount < N)
        {
            int free;
            for (free = 0; free < N; free++)
                if (!menEngaged[free])
                    break;
 
            for (int i = 0; i < N && !menEngaged[free]; i++)
            {
                int index = womenIndexOf(menPref[free][i]);
                if (womenPartner[index] == null)
                {
                    womenPartner[index] = men[free];
                    menEngaged[free] = true;
                    engagedCount++;
                }
                else
                {
                    String currentPartner = womenPartner[index];
                    if (morePreference(currentPartner, men[free], index))
                    {
                        womenPartner[index] = men[free];
                        menEngaged[free] = true;
                        menEngaged[menIndexOf(currentPartner)] = false;
                    }
                }
            }            
        }
        printCouples();
    }
    /** function to check if women prefers new partner over old assigned partner **/
    private boolean morePreference(String curPartner, String newPartner, int index)
    {
        for (int i = 0; i < N; i++)
        {
            if (womenPref[index][i].equals(newPartner))
                return true;
            if (womenPref[index][i].equals(curPartner))
                return false;
        }
        return false;
    }
    /** get men index **/
    private int menIndexOf(String str)
    {
        for (int i = 0; i < N; i++)
            if (men[i].equals(str))
                return i;
        return -1;
    }
    /** get women index **/
    private int womenIndexOf(String str)
    {
        for (int i = 0; i < N; i++)
            if (women[i].equals(str))
                return i;
        return -1;
    }
    /** print couples **/
    public void printCouples()
    {
        System.out.println("Couples are : ");
        for (int i = 0; i < N; i++)
        {
            System.out.println(womenPartner[i] +" "+ women[i]);
        }
    }
    /** main function **/
    public static void main(String[] args) 
    {
        System.out.println("Gale Shapley Marriage Algorithm\n");
        /** list of men **/
        String[] m = {"M1", "M2", "M3", "M4", "M5"};
        /** list of women **/
        String[] w = {"W1", "W2", "W3", "W4", "W5"};
 
        /** men preference **/
        String[][] mp = {{"W5", "W2", "W3", "W4", "W1"}, 
                         {"W2", "W5", "W1", "W3", "W4"}, 
                         {"W4", "W3", "W2", "W1", "W5"}, 
                         {"W1", "W2", "W3", "W4", "W5"},
                         {"W5", "W2", "W3", "W4", "W1"}};
        /** women preference **/                      
        String[][] wp = {{"M5", "M3", "M4", "M1", "M2"}, 
                         {"M1", "M2", "M3", "M5", "M4"}, 
                         {"M4", "M5", "M3", "M2", "M1"},
                         {"M5", "M2", "M1", "M4", "M3"}, 
                         {"M2", "M1", "M4", "M3", "M5"}};
 
        GaleShapley gs = new GaleShapley(m, w, mp, wp);                        
    }
}

Gale Shapley Marriage Algorithm
 
Couples are :
M4 W1
M2 W2
M5 W3
M3 W4
M1 W5