Statement.execute(sql) vs executeUpdate(sql) and executeQuery(sql)

What do they mean by "one or more ResultSet objects"?

The javadoc for the execute method says this:

"Executes the given SQL statement, which may return multiple results. In some (uncommon) situations, a single SQL statement may return multiple result sets and/or update counts. Normally you can ignore this unless you are (1) executing a stored procedure that you know may return multiple results or (2) you are dynamically executing an unknown SQL string."

That pretty much explains it. Sometimes a query can deliver more than one ResultSet.

if so how is it possible to manage them once got an array of ResultSet?

I'm not sure what you mean but:

• you can't get them as an array: you must get them one at a time, and
• you could put the ResultSets into an array ...

It's not (at least to me) the aim of st.execute(sql) which can also return an int as if it was updated a table.

One use of execute is to execute an SQL statement if you don't know if it is a query, an update (of some kind) ... or something else that potentially delivers multiple result sets. It is a generalization of executeQuery() and executeUpdate() ...

boolean execute(): Executes the SQL statement in this Prepared Statement object, which may be any kind of SQL statement.

ResultSet executeQuery(): Executes the SQL query in this Prepared Statement object and returns the ResultSet object generated by the query.

int executeUpdate(): Executes the SQL statement in this Prepared Statement object, which must be an SQL INSERT, UPDATE or DELETE statement; or an SQL statement that returns nothing, such as a DDL statement.

The best place to find answers to questions like this is the Javadocs: Here

execute() : The method used for all types of SQL statements, and that is, returns a boolean value of TRUE or FALSE. If the method return TRUE, return the ResultSet object and FALSE returns the int value.

executeUpdate() : This method is used for execution of DML statement(INSERT, UPDATE and DELETE) which is return int value, count of the affected rows.

executeQuery() : This method is used to retrieve data from database using SELECT query. This method returns the ResultSet object that returns the data according to the query.