Reading through list of objects

[Rakshan]

Hi, I have a list which contains student objects as shown below:

Registry(Student=[Student(Gender=M, School=Hamburg, FirstName=RP, Value=null), Student(Gender=F, School=Berlin, FirstName=SK, Value=null), Student(Gender=M, School=Frankfurt, FirstName=TK, Value=null)])

It symbolises a XML heirarchy.

<Registry
   <Student Gender="M" School="Hamburg">
        <FirstName>RP</FirstName>
    </Student>
    <Student Gender="F" School="Berlin">
        <FirstName>SK</FirstName>
    </Student>
    <Student Gender="M" School="Frankfurt">
        <FirstName>TK</FirstName>
    </Student>
</Registry>

Now I plan to add another entry into the list/xml based on the school of individual student.
For eg: I want to search through the list mentioned above, for school = "Berlin" and then add another new student if there is a match.

For eg:

Registry(Student=[Student(Gender=M, School=Hamburg, FirstName=RP, Value=null), Student(Gender=F, School=Berlin, FirstName=SK, Value=null),Student(Gender=M, School=Berlin, FirstName=KK, Value=null), Student(Gender=M, School=Frankfurt, FirstName=TK, Value=null)])

Ofcourse, I change he FirstName and Gender .. Please suggest me how to scan through the object properties inside list of objects?

 

[kneitzel]

Ok, I see two possible ways:

a) Use JSO Serialization / Deserialization - So you have classes for the required objects and you deserialize the JSON to the the Instances required. Then you change the Instances (e.g. add a Student) and afterwards you serialize it back to JSON.

b) You simply read the JSON to the structure and then you modify the JSON instances.

For both you need a library e.g. jackson, gson, ...

If you do not plan to do anything more then a is overkill in my eyes. Just read the JSON to the JSON classes (So you get a Structure of all the JsonElement implementations in gson e.g. JsonObject) You can go through the structure and add new elements. At the end you write it back of course.

 

[Rakshan]

You mean XML libraries?

 

[Rakshan]

So to give a complete overview:

I have a list of objects as displayed

Registry(Student=[Student(Gender=M, School=Hamburg, FirstName=RP, Value=null),
                  Student(Gender=F, School=Berlin, FirstName=SK, Value=null),
                  Student(Gender=M, School=Frankfurt, FirstName=TK, Value=null)])

This represents the XML structure after unmarshalling. The original structure of the XML is shown below

<?xml version="1.0" encoding="UTF-8"?>
<Registry xmlns="http://www.registar.com"
          xmlns:ms ="http://www.registar.com/ScoreVariant">
    <Student Gender = "M" School = "Hamburg">
        <FirstName>RP</FirstName>
    </Student>
    <Student Gender = "F" School = "Berlin">
        <FirstName>SK</FirstName>
    </Student>
    <Student Gender = "M" School = "Frankfurt">
        <FirstName>TK</FirstName>
    </Student>
</Registry>

There are classes written for Registry, Student and Value with getter and setter methods (used lombok package)

Now, I want to scan through the list of objects, Look for school location, and if the location is "Berlin", I want to add another student.

  List<Registry> entries = new ArrayList<Registry>();
    
    for (Registry e: entries) {
        for (Student s : e.getStudent()) {
            if (s.getSchool().equals("Berlin")) {
                 Student obj = new Student();
                 obj.setFirstName("MP");
                obj.setGender("F");
                obj.setSchool("Berlin");    // (1)
            }                 
        }             
    }

(1) Here I can create new object, but I am not able to add it to the XML (as the registry class has the ```List<Student> student``` as a property.


Ultimately, I want to have a output like the below

Registry(Student=[Student(Gender=M, School=Hamburg, FirstName=RP, Value=null),
                  Student(Gender=F, School=Berlin, FirstName=SK, Value=null),
                  Student(Gender=F, School=Berlin, FirstName=MP, Value=null),
                  Student(Gender=M, School=Frankfurt, FirstName=TK, Value=null)])

 

[kneitzel]

Sorry, I was a little confused and didn't read good enough - I thought that your "Java Objects" was showing some JSON that you have. My fault.

But you are correct: The same is valid with XML.

- You can use Libraries zu serialize / deserialize to/from XML. (Jackson can be used here, too) And if you already have the student java objects, then this might be the best way to go.

- You can modify the XML directly ... You load the XML to a DOM Tree (Using a DOM Parser) and then you can add Xml Elements manualy. At the end you write the DOM Tree back to the file (if it is a file).


============================

You got the classes in java already, so let us concentrate on these.

- Do you have code to serialize them to XML? Using some library? (Which if you are using one already?) Is there help required in this topic?

- Your code is creating a new student. But the new student is never added. So something like e.addStudent(obj) seems to be missing. But this might need more investigation because you might not want to change a collection that you currently iterate over.


============================

And last but not least: please allow me some advice regarding your code:

- Please try to choose more reasonable names. "e" is a registry? "s" student? "obj" a new student? I would recommend to use the longer versions:
e -> registry
s -> student
obj -> newStudent
And getStudent() is giving multiple Students? So it should be getStudents (plural) and not getStudent (singular).
And I am not sure if the Attribute names were given to you. But I would like to even do changes there e.g. School seems to be a schoolLocation

- Something I always try: Keep methods more simple. for each / for each / if -> That is already something where I would think about splitting into multiple methods. You are inside a class that holds registries. And the logic to add a student is something that happens inside a registry. Maybe the code to add another student even belongs more in the registry class? (So 2 points: 1st is the depth of Blocks, 2nd is the question: Whre does some logic belong to?) But feel free to ignore this point! This is just something I got into my mind without knowing anything about the real structure of you project and the responsibilities.

 

[Mart]

if you parse your xml file

when you dug into the students you need to get their Attribute School and compare it if its Berlin if so
you can continue digging their with your xml parser

 

[Rakshan]

Ofcourse, I would gladly take your advice. Infact, I would really appreciate because I am just starting and would be guided in the right direction. I will start naming them currently in a manner that is understandable.

First and foremost, yes so there is an input xml file that has been parsed using JAXB standard.

Registry.java

import lombok.Data;
@XmlRootElement(name="Registry")//,namespace = "http://www.registar.com")
@XmlAccessorType(XmlAccessType.FIELD)
@Data
public class Registry {  
    @XmlElement(name="Student")//,namespace = "http://www.registar.com")
    private List<Student> students;
}

Student.java

@Data

@XmlAccessorType(XmlAccessType.FIELD)
public class Student {   
    @XmlAttribute(name = "Gender")
    private String Gender;   
    @XmlAttribute(name = "School")
    private String School;   
    @XmlElement(name = "FirstName")//namespace = "http://www.registar.com")
    private String FirstName;   
    @XmlElement(name = "Value")
    private Value Value;
}

So in my main parser now, I search for the student having school at Berlin, and I add one student there( practically it doesnt make sense, but just for me to understand and get familiar)

public class Mainparser {
    public static void main(String[] args) {
        try {
            File xmlFile = new File("MultipleNS.xml");
            JAXBContext jaxbContext;
            jaxbContext = JAXBContext.newInstance(Registry.class);
            Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
            Registry xmlentries = (Registry) jaxbUnmarshaller.unmarshal(xmlFile);           
            Student newStudent = null;               
            List<Student> students = xmlentries.getStudents();
            for (Student s : students) {
                if (s.getSchool().equals("Berlin")) {
                    newStudent=new Student();
                    newStudent.setFirstName("MP");
                    newStudent.setGender("F");
                    newStudent.setSchool("Berlin");
                    break;
                }
            }
            if(newStudent!=null) {
                students.add(newStudent);
            }       
            Marshaller marshaller = jaxbContext.createMarshaller();
            marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
            marshaller.marshal(xmlentries, System.out);
        }catch (JAXBException e) {
            e.printStackTrace();
        } catch (FactoryConfigurationError e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

I could achieve the result as below

<Registry xmlns:ms="http://www.registar.com/ScoreVariant" xmlns="http://www.registar.com">
    <Student Gender="M" School="Hamburg">
        <FirstName>RP</FirstName>
    </Student>
    <Student Gender="F" School="Berlin">
        <FirstName>SK</FirstName>
    </Student>
    <Student Gender="M" School="Frankfurt">
        <FirstName>TK</FirstName>
    </Student>
    <Student Gender="F" School="Berlin">
        <FirstName>MP</FirstName>
    </Student>
</Registry>

 

[kneitzel]

So if I understood you correctly, then you have all major parts already. The only problem is, that the order is not as wanted?

So you want to insert an item at a specific position. List<T> has a method add(int index, E element) which you could use. But you need to get the index where you want to insert the student.

One possible way could be to not use a for each loop. Instead you use a normal loop:
for (Student s : students) {
is replaced with:

for (int index=0; index < students.length(); index++) {
    Student s = students.get(index);
    // ...
}

That way you have the index when you found a matching element. You can store that inside a variable outside of the loop and then you can use something like

if(newStudent!=null) {
    students.add(storedIndex+1, newStudent); // +1 to add behind the found element.
}

 

[Mart]

            List<Student> students = xmlentries.getStudents();
            for (Student s : students) {
                if (s.getSchool().equals("Berlin")) {
                }

this is basically what i meant ...

you dug until you reached all Students and you dug through them

my digging was always getElementsByTagName but ive used another xml parser where you have to dig from the top level down to the bottom