Implement Java version of the stable marriage problem. (#345)

Author: bsamseth

contents/stable_marriage_problem/code/java/stable-marriage.java

@@ -0,0 +1,140 @@
+import java.util.List;
+import java.util.ArrayList;
+import java.util.Collections;
+
+class StableMarriage {
+
+    /*
+     * Use the stable marriage algorithm to find stable pairs from the
+     * lists of men and women.
+     */
+    public static void findStableMarriages(List<Woman> women, List<Man> men) {
+        // We might have more men/women than women/men. In this case, not everybody can
+        // get a mate. We should aim to give every member of the less numerous gender a mate,
+        // as this is always possible.
+        List<? extends Person> leastCommonGender = women.size() <= men.size() ? women : men;
+        do {
+            // Every single man proposes to a woman.
+            for (Man man : men)
+                if (man.isLonely())
+                    man.propose();
+
+            // The women pick their favorite suitor.
+            for (Woman woman : women)
+                woman.chooseMate();
+
+            // End the process if everybody has a mate.
+            if (!leastCommonGender.stream().anyMatch(Person::isLonely))
+                break;
+
+        } while (true);
+
+        women.forEach(w -> System.out.println(w + " married to " + w.getMate()));
+    }
+
+    public static void main(String[] args) {
+        int nPairs = 5;
+        List<Woman> women = new ArrayList<>();
+        List<Man> men = new ArrayList<>();
+        for (char i = 'A'; i < 'A' + nPairs; ++i) {
+            women.add(new Woman("" + i));
+            men.add(new Man("" + i));
+        }
+        // Make the genders unbalanced:
+        women.add(new Woman("X"));
+
+        women.forEach(w -> {
+            w.receiveOptions(men);
+            System.out.println(w + " prefers " + w.getPreferredMates());
+        });
+        men.forEach(m -> {
+            m.receiveOptions(women);
+            System.out.println(m + " prefers " + m.getPreferredMates());
+        });
+
+        findStableMarriages(women, men);
+    }
+
+}
+
+class Person {
+    private final String name;
+    protected Person mate;
+    protected List<Person> preferredMates;
+
+    public Person(String name) {
+        this.name = name;
+    }
+
+    public boolean isLonely() {
+        return mate == null;
+    }
+
+    public void setMate(Person mate) {
+        // Only set mates if there is a change.
+        if (this.mate != mate) {
+            // Remove old mates mate.
+            if (this.mate != null)
+                this.mate.mate = null;
+
+            // Set the new mate.
+            this.mate = mate;
+
+            // If new mate is someone, update their mate.
+            if (mate != null)
+                mate.mate = this;
+        }
+    }
+
+    public Person getMate() {
+        return mate;
+    }
+
+    public void receiveOptions(List<? extends Person> mates) {
+        // Preferences are subjective.
+        preferredMates = new ArrayList<>(mates);
+        Collections.shuffle(preferredMates);
+    }
+
+    public List<Person> getPreferredMates() {
+        return preferredMates;
+    }
+
+    public String toString() {
+        return getClass().getName() + "(" + name + ")";
+    }
+}
+
+class Woman extends Person {
+    private List<Man> suitors = new ArrayList<>();
+
+    public Woman(String name) {
+        super(name);
+    }
+
+    public void recieveProposal(Man suitor) {
+        suitors.add(suitor);
+    }
+
+    public void chooseMate() {
+        for (Person mostDesired : preferredMates) {
+            if (mostDesired == mate || suitors.contains(mostDesired)) {
+                setMate(mostDesired);
+                break;
+            }
+        }
+    }
+}
+
+class Man extends Person {
+    public Man(String name) {
+        super(name);
+    }
+
+    public void propose() {
+        if (!preferredMates.isEmpty()) {
+            Woman fiance = (Woman) preferredMates.remove(0);
+            fiance.recieveProposal(this);
+        }
+    }
+}

contents/stable_marriage_problem/stable_marriage_problem.md

@@ -42,6 +42,8 @@ I am incredibly interested to see what you guys do and how you implement the alg
 [import, lang:"csharp"](code/csharp/Program.cs)
 ##### ListExtensions.cs
 [import, lang:"csharp"](code/csharp/ListExtensions.cs)
+{% sample lang="java" %}
+[import, lang:"java"](code/java/stable-marriage.java)
 {% endmethod %}
 
 <script>