How to return multiple objects from a Java method?

Question

I want to return two objects from a Java method and was wondering what could be a good way of doing so?

The possible ways I can think of are: return a HashMap<

Answer 1

If you know you are going to return two objects, you can also use a generic pair:

public class Pair {
    public final A a;
    public final B b;

    public Pair(A a, B b) {
        this.a = a;
        this.b = b;
    }
};

Edit A more fully formed implementation of the above:

package util;

public class Pair {

    public static  Pair makePair(P p, Q q) {
        return new Pair(p, q);
    }

    public final A a;
    public final B b;

    public Pair(A a, B b) {
        this.a = a;
        this.b = b;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((a == null) ? 0 : a.hashCode());
        result = prime * result + ((b == null) ? 0 : b.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        @SuppressWarnings("rawtypes")
        Pair other = (Pair) obj;
        if (a == null) {
            if (other.a != null) {
                return false;
            }
        } else if (!a.equals(other.a)) {
            return false;
        }
        if (b == null) {
            if (other.b != null) {
                return false;
            }
        } else if (!b.equals(other.b)) {
            return false;
        }
        return true;
    }

    public boolean isInstance(Class classA, Class classB) {
        return classA.isInstance(a) && classB.isInstance(b);
    }

    @SuppressWarnings("unchecked")
    public static  Pair cast(Pair pair, Class
 pClass, Class qClass) {

        if (pair.isInstance(pClass, qClass)) {
            return (Pair) pair;
        }

        throw new ClassCastException();

    }

}

Notes, mainly around rustiness with Java & generics:

• both a and b are immutable.
• makePair static method helps you with boiler plate typing, which the diamond operator in Java 7 will make less annoying. There's some work to make this really nice re: generics, but it should be ok-ish now. (c.f. PECS)
• hashcode and equals are generated by eclipse.
• the compile time casting in the cast method is ok, but doesn't seem quite right.
• I'm not sure if the wildcards in isInstance are necessary.
• I've just written this in response to comments, for illustration purposes only.