Oracle's RETURNING INTO usage in Java (JDBC, Prepared Statement)

Because parameters specified in returning clauses are handled in a different way compared to normal output parameters(getReturnResultSet vs getResultSet vs returning parameters in a callablestatement).
They need to be handled with OraclePreparedStatement. In the second case when you wrap the insert statement in begin..end the insert is handled by the database itself and al jdbc sees is an anonymous plsql block.
http://docs.oracle.com/cd/E11882_01/java.112/e16548/oraint.htm#BABJJDDA

To get auto generated key we have method getGeneratedKeys method in preparestatement which return resultset that contain key value all we need is pass key column name to preparestatement

pstm = con.prepareStatement("insert query",new String[]{primarykeycolumnname});
int i = pstm.executeUpdate();
if (i > 0) 
{
    ResultSet rs = pstm.getGeneratedKeys();
    while(rs.next())
    {
        System.out.println(rs.getString(1)); 
    }
}