How to Conditionally Filter the data in text file in Java

If you perform conditional filtering on text files in Java alone, you may meet the following problems:

1. The text file is not a database,soit cannot be accessed by SQL. The code needs to be modified if filtering conditions are changed. Besides, if you want a flexible conditional filtering as that in SQL, you have to self-program the dynamic expression parsing and evaluating, resulting in a great amount of programming work.
2. Stepwise loading is required for the big files that cannot be loaded into the memory all at once. If the performance must be taken into account, you have to deal with some complicated programming like the management of file buffer and line-splitting computing.

But if esProc is used to help with Java programming, these problems can be solved without self-programmed code. The following example will teach you how to do this in detail.

The text file employee.txt has the employee information. You are required to fetch the data and select from them the female employees who were born on and after January 1, 1981.

The text fileemployee.txtis in a format as follows:

Implementation approach: call esProc script with Java, import and compute the data, then return the result in the form of ResultSet to Java. Because esProc supports dynamic expression parsing and evaluating, it enables Java to perform the conditional filtering as flexibly as SQL does.

For example, it is required to query the information of female employees who were born on and after January 1, 1981. In this case, esProc can use an input parameter “where”as the condition, which is shown below:

“where” is a string, its values is

BIRTHDAY>=date(1981,1,1) && GENDER=="F"

The code written in esProc is as follows:

A1:Define a file object and import the data, with the first row being the title. tab is used as the field separator by default. esProc’s IDE can display the imported data visually, as shown in the right part of the above figure.

A2:Perform the conditional filtering, using macro to realize parsing the expression dynamically. The “where” in this process is an input parameter. In executing, esProc will first compute the expression enclosed by ${…}, then replace ${…} with the computed result acting as the macro string value and interpret and execute the code. The final code to be executed in this example is

=A1.select(BIRTHDAY>=date(1981,1,1) && GENDER=="F")

A3：Return the eligible result set to the external program.

In esProc, when the filtering condition is changed, you just need to modify “where”– the parameter. For example, it is required to query the information of female employees who were born on and after January 1, 1981, or employeeswhose NAME+SURNAME is equivalent to “RebeccaMoore”. The value of “where” can be written as

BIRTHDAY>=date(1981,1,1) && GENDER=="F" || NAME+SURNAME=="RebeccaMoore"

After the code is executed, the result set in A2 is as follows:

Call this piece of code in Java with esProc JDBC and get the result. Detailed code is as follows (save the above program in esProc as test.dfx):

//create a connection withesProc JDBC

Class.forName("com.esproc.jdbc.InternalDriver");

con= DriverManager.getConnection("jdbc:esproc:local://");

//call the program in esProc (the stored procedure); test is the name of file dfx

st =(com.esproc.jdbc.InternalCStatement)con.prepareCall("call test(?)");

//set the parameters 

st.setObject(1," BIRTHDAY>=date(1981,1,1) && GENDER==\"F\" ||NAME+SURNAME==\"RebeccaMoore\"");//the parameters are the dynamic filtering conditions

//execute the esProcstored procedure

st.execute();

//get the result set, which is the eligible set of employees 

ResultSet set = st.getResultSet();

If the script is simple, the code can be written directly into the program in Java that calls the esProc JDBC. It won’t benecessary to write a special script file (test.dfx):

st=(com. esproc.jdbc.InternalCStatement)con.createStatement();

ResultSet set=st.executeQuery("=file(\"D:/employee.txt\").import@t().select(BIRTHDAY>=date(1981,1,1)&&GENDER==\"F\" || NAME+SURNAME==\"RebeccaMoore\")");

This piece of code in Java calls a line of code in esProc scriptdirectly, that is, getthe data from the text file, perform conditional filteringand return the result set to set– the object of ResultSet.

It is assumed, in the above approach, that the file is small enough to be loaded to the memory all together. In reality, there may be huge files that cannot be loaded all together or the situation where it is believed that it is unnecessary to increase memory usage even if the file is not huge. In these occasions, file cursorcan be used to handle the operation, thus the program in esProc can be modified in this way:

A1：Define a file cursor object, with the first row being the title and tab being the field separator by default.

A2:Perform conditional filtering on the cursor, using macro to realize parsing the expression dynamically. The “where” in this process is an input parameter. In executing, esProc will first compute the expression enclosed by ${…}, then replace ${…} with the computed result acting as the macro string value and interpret and execute the code. The final code to be executed in this example is

=A1.select(BIRTHDAY>=date(1981,1,1) && GENDER=="F")

A3：Return the cursor.

Despite a cursor returned to Java by esProc, it is no need to modify the calling program of Java. esProc will automatically fetch the data corresponding to the cursor while Java is traversing the data with ResultSet.

If it is needed to write the filtered data to another file, instead of returning them to the main program, you just modify the expression in A3 into

=file("D:/employee_group.txt")<a href="mailto:.export@t(A2)">.export@t(A2)</a>

esProc will write out the data of the cursor to a file.