Uniqueness of map keys of user-defined types is determined by the equals and hashCode methods,

public class PairNumbers {

    Integer x,y;

    public PairNumbers(Integer a, Integer b) {

        x=a;

        y=b;

    }

    public Boolean equals(Object obj) {

        if (obj instanceof PairNumbers) {

            PairNumbers p = (PairNumbers)obj;

            return ((x==p.x) && (y==p.y));

        }

        return false;

    }

    public Integer hashCode() {

        return (31 * x) * y;

    }

}

Use case:

Map<PairNumbers, String> m = new Map<PairNumbers, String>();

PairNumbers p1 = new PairNumbers(1,2);

PairNumbers p2 = new PairNumbers(3,4);

// Duplicate key

PairNumbers p3 = new PairNumbers(1,2);

PairNumbers p4 = new PairNumbers(2,1);

m.put(p1, 'first');

m.put(p2, 'second');

m.put(p3, 'third');

m.put(p4, 'third');

// Map size is 2 because the entry with 

// the duplicate key overwrote the first entry.

System.assertEquals(2, m.size());

// Use the == operator

if (p1 == p3) {

    System.debug('p1 and p3 are equal.');

}

// Perform some other operations

System.assertEquals(true, m.containsKey(p1));

System.assertEquals(true, m.containsKey(p2));

System.assertEquals(false, m.containsKey(new PairNumbers(5,6)));

for(PairNumbers pn : m.keySet()) {

    System.debug('Key: ' + pn);

}

List<String> mValues = m.values();

System.debug('m.values: ' + mValues);

// Create a set

Set<PairNumbers> s1 = new Set<PairNumbers>();

s1.add(p1);

s1.add(p2);

s1.add(p3);

// Verify that we have only two elements

// since the p3 is equal to p1.

System.assertEquals(2, s1.size());