PostgreSQL function array_agg in JPA

Question

In PostgreSQL, array_agg (column) allows to aggregate. I try to use it with JPA.

ParameterExpression<Integer> param = criteriaBuilder.parameter(Integer.class, "empId");
Expression<Integer> groupId = criteriaBuilder.function("array_agg", Integer.class, param);
criteriaQuery.select(criteriaBuilder.construct(Integer.class, groupId));

I am setting parameter here

//Other where and group by clauses.

TypedQuery<Integer> query = entityManager.createQuery(criteriaQuery);
query.setParameter("empId", empId);
return query.getResultList();

Even I have registered the function in PostgresSqlDialect

public class PGDialect extends PostgreSQL94Dialect {

    public PGDialect() {
        super();
        this.registerFunction("array_agg", new SQLFunctionTemplate(StandardBasicTypes.INTEGER, "array_agg(?1)"));
    }   
}

I am getting this Exception

java.lang.NullPointerException
    at org.hibernate.hql.internal.ast.tree.ConstructorNode.formatMissingContructorExceptionMessage(ConstructorNode.java:192)
    at org.hibernate.hql.internal.ast.tree.ConstructorNode.resolveConstructor(ConstructorNode.java:182)
    at org.hibernate.hql.internal.ast.tree.ConstructorNode.prepare(ConstructorNode.java:144)
    at org.hibernate.hql.internal.ast.HqlSqlWalker.processConstructor(HqlSqlWalker.java:1092)
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectExpr(HqlSqlBaseWalker.java:2359)
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectExprList(HqlSqlBaseWalker.java:2225)
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectClause(HqlSqlBaseWalker.java:1496)
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.query(HqlSqlBaseWalker.java:581)
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectStatement(HqlSqlBaseWalker.java:309)
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.statement(HqlSqlBaseWalker.java:257)
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.analyze(QueryTranslatorImpl.java:262)
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:190)
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.compile(QueryTranslatorImpl.java:142)
    at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:115)
    at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:76)
    at org.hibernate.engine.query.spi.QueryPlanCache.getHQLQueryPlan(QueryPlanCache.java:150)
    at org.hibernate.internal.AbstractSessionImpl.getHQLQueryPlan(AbstractSessionImpl.java:302)
    at org.hibernate.internal.AbstractSessionImpl.createQuery(AbstractSessionImpl.java:240)
    at org.hibernate.internal.SessionImpl.createQuery(SessionImpl.java:1894)
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.createQuery(AbstractEntityManagerImpl.java:531)
    at org.hibernate.jpa.criteria.CriteriaQueryImpl$1.buildCompiledQuery(CriteriaQueryImpl.java:316)
    at org.hibernate.jpa.criteria.compile.CriteriaCompiler.compile(CriteriaCompiler.java:130)
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.createQuery(AbstractEntityManagerImpl.java:699)
    at sun.reflect.GeneratedMethodAccessor152.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)

All I want is to get all IDS falling in one group.

I am trying it to do by HQL.