GADT.java

abstract class Term<T> {
    abstract T eval();
}

class IntegerTerm extends Term<Integer> {
    Integer value;
    IntegerTerm(Integer value) { this.value = value; }
    Integer eval() { return value; }
}

class StringTerm extends Term<String> {
    String value;
    StringTerm(String value) { this.value = value; }
    String eval() { return value; }
}

class IsZeroTerm extends Term<Boolean> {
    Term<Integer> subTerm;
    IsZeroTerm(Term<Integer> subTerm) { this.subTerm = subTerm; }
    Boolean eval() {
        return subTerm.eval() == 0 ? true : false;
    }
}

class IfTerm<T> extends Term<T> {
    Term<Boolean> testTerm;
    Term<T> thenTerm, elseTerm;
    IfTerm(Term<Boolean> testTerm, Term<T> thenTerm, Term<T> elseTerm) {
        this.testTerm = testTerm; this.thenTerm = thenTerm; this.elseTerm = elseTerm;
    }
    T eval() {
        if (testTerm.eval())
            return thenTerm.eval();
        else
            return elseTerm.eval();
    }
}

public class GADT {
    public static void main(String[] args) {
        String result =
            new IfTerm<String>(new IsZeroTerm(new IntegerTerm(12)),
                               new StringTerm("It's zero"),
                               new StringTerm("It's not zero")).eval();
        System.out.println(result);
    }
}

alternative implementation that dissociate eval() from Term:
https://gist.github.com/jbgi/208a1733f15cdcf78eb5