Saving Objects to File. Custom methods to add/remove Object

Question

I want to be able to save the objects created in my program to a file. I have watched a few tutorials on ObjectOutputStream, but the problem is, they only show how to save a specific object created in the main method. What I want, I that the program automatically saves every created object. Taking the Group Object in my program as an example. This is the add method:

public void addGroup(int gid, String groupname) {
    Group newgroup = new Group(gid, groupname);
    if (!Groups.contains(newgroup)) {
        Groups.add(newgroup);
        return;
    }else
        JOptionPane.showMessageDialog(null, "Group with ID " + gid
                + " already exists!", "Error",
                JOptionPane.WARNING_MESSAGE);
}

It is part of my database class. I wan to automatically save every created group to the file. How would this be done? where do I declare the new file, in the database class? in the main method?

My second question is, if I delete a group, using the remove method:

public void removeGroup(int gid) {

    if (!Groups.remove(new Group(gid, null))) {
        JOptionPane.showMessageDialog(null, "Group with ID[" + gid
                + "] not present. System unchanged.");
    }

}

How do I delete it from the file? I know, that I cant really delete an Object from the file, but how will I blank out the space?

Thanks in advance for all the help :)

Answer 1

If you have access to 3rd party libraries, just use XStream to serialize to XML. If not, you can serialize and save.

Follow a serialization tutorial like this: http://www.tutorialspoint.com/java/java_serialization.htm

I don't think you want to try to remove anything from the file.. just rewrite it when you make a change. Make a couple methods to read in the file and also to serialize objects and save to file. Here is an example

Group.java

import java.io.Serializable;

public class Group implements Serializable
{
/**
 * 
 */
private static final long serialVersionUID = 1L;

private String guid;
private String groupName;

public Group(String guid, String groupName) {
    super();
    this.guid = guid;
    this.groupName = groupName;
}

public String getGuid() {
    return guid;
}
public void setGuid(String guid) {
    this.guid = guid;
}
public String getGroupName() {
    return groupName;
}
public void setGroupName(String groupName) {
    this.groupName = groupName;
}
}

GroupData.java

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.ArrayList;

public class GroupData extends ArrayList<Group>
{
private static final long serialVersionUID = 1L;

public GroupData(){}

public void addGroup(Group group)
{
    this.add(group);
    saveGroupData();
}

public void removeGroup(Group group)
{
    this.remove(group);
    saveGroupData();
}

public void saveGroupData()
{
      try
      {
         FileOutputStream fileOut = new FileOutputStream("C:\\Users\\tester\\group-data.ser");
         ObjectOutputStream out = new ObjectOutputStream(fileOut);

         out.writeObject(this);

         out.close();
         fileOut.close();
      }
      catch(IOException i)
      {
          i.printStackTrace();
      }
}

public void loadGroupData()
{
      try
      {
         FileInputStream fileIn = new FileInputStream("C:\\Users\\tester\\group-data.ser");
         ObjectInputStream in = new ObjectInputStream(fileIn);
         GroupData tmp = (GroupData) in.readObject();

         this.clear();
         this.addAll(tmp);

         in.close();
         fileIn.close();
      }
      catch(IOException i)
      {
         i.printStackTrace();
         return;
      }
      catch(ClassNotFoundException c)
      {
         c.printStackTrace();
         return;
      }
}
}

Here is a test

TestGroup.java

public class TestGroup {

/**
 * @param args
 */
public static void main(String[] args) 
{

    Group group1 = new Group("123", "testers");
    Group group2 = new Group("456", "programmers");
    Group group3 = new Group("687", "students");

    GroupData groupData = new GroupData();

    groupData.add(group1);
    groupData.add(group2);
    groupData.add(group3);

    groupData.remove(group3);
}

}