public abstract class SimpleComparisonPredicateBuilder<T, V> {

    private final V targetValue;

    /**
     * Constructor.
     *
     * @param targetValue value to check against, possibly null
     */
    protected SimpleComparisonPredicate(V targetValue) {
        this.targetValue = targetValue;
    }

    /**
     * Create a predicate that compares the target expression with this instance's target value.
     */
    @Override
    public Predicate getPredicate(CriteriaQuery<?> query, CriteriaBuilder builder, From<?, ? extends T> from) {
        final Expression<V> expression = this.getExpression(query, builder, from);
        return this.targetValue != null ? builder.equal(expression, this.targetValue) : builder.isNull(expression);
    }

    /**
     * Get criteria expression representing the value to test.
     */
    protected abstract Expression<V> getExpression(CriteriaQuery<?> query, CriteriaBuilder builder,
      From<?, ? extends T> from);
}

public class IsFoobarPrimary extends SimpleComparisonPredicateBuilder<Foobar, Boolean> {

    public IsFoobarPrimary() {
        super(true);
    }

    @Override
    protected Predicate getExpression(CriteriaQuery<?> query, CriteriaBuilder builder,
      From<?, ? extends Foobar> foobar) {
        return builder.isTrue(foobar.get(Foobar_.primary));
    }
}

public class IsFoobarPrimary extends SimpleComparisonPredicate<Foobar, Boolean> {

    public IsFoobarPrimary() {
        super(true);
    }

    @Override
    protected Expression<Boolean> getExpression(CriteriaQuery<?> query, CriteriaBuilder builder,
      From<?, ? extends Foobar> foobar) {
        return foobar.get(Foobar_.primary);
    }
}