如何读取/写入在文件中具有链接列表的对象

Question

I have these classes: "MyClass1", "MyClass2", "MyClass3" and "MyMainClass",

public class MyMainClass implements Serializable {
     private String att1, att2, att3;
     private int att4, att5, att6;
     private LinkedList <MyClass1> myClass1List = new LinkedList<MyClass1>();
     private LinkedList <MyClass2> myClass2List = new LinkedList<MyClass2>();
     private LinkedList <MyClass3> myClass3List = new LinkedList<MyClass3>();
}

My program create registers (Objects) of "MyMainClass" and deposit it in a LinkedList. I want to save the LinkedList of the objects in a file to get them after i reopen my program. What's the way to do it? I have tried with ObjectOutputStream, but doesn't work. Thanks.

Edit: My code to add an object(I just read an example and tried):

public static void addObject (MyMainClass p) {
    try {
        outputStream = new ObjectOutputStream(new FileOutputStream("myfile.dat"));
        outputStream.writeObject(p);
    } catch (FileNotFoundException ex) {
        ex.printStackTrace();
        System.exit(1);
    } catch (IOException ex) {
        ex.printStackTrace();
        System.exit(1);
    } finally {
        try {
            if (outputStream != null) {
                outputStream.flush();
                outputStream.close();
            }
        } catch (IOException ex) {
            ex.printStackTrace();
            System.exit(1);
        }
    }
}

Note: "MyClass1", "MyClass2", "MyClass3" are Serializable.

Answer 1

I used following for my highschool project long time ago. Due to my poor English skills I do not really understand what class you wish to save and load (LinkedList or myMainClass), but I used this solution to successfully store and load any of my custom objects. I hope you find it handy.

Usage:

myMainClass object;
//
// ... your code fillin up the content of object
//
MyIO io = new MyIO();
io.save("", "myfile.dat", object); // "" as first argument will make java use current working directory

// to load the object:

myMainObject object = (myMainObject) io.load("", "myfile.dat"); 

Source:

import java.util.zip.GZIPInputStream;
import java.util.zip.GZIPOutputStream;
import java.io.FileOutputStream;
import java.io.FileInputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.IOException;

public class MyIO {

    // String path - path to the directory where the file is supposed to be saved.
    // String filename - the name of the file
    // Object data - object that you wish to save in the file. In your case "myMainClass"

    public void save(String path, String filename, Object data) {
        try {
            FileOutputStream fos = new FileOutputStream(path + filename, false);
            GZIPOutputStream gzos = new GZIPOutputStream(fos);
            ObjectOutputStream out = new ObjectOutputStream(gzos);
            out.writeObject(data);
            out.flush();
            out.close();
        } catch (IOException e) {
            System.out.println(e);
        }
    }

    // String path - path to the directory where the file is stored
    // String filename - the name of the file
    // The function returns java object which can be cast to myMainClass.

    public Object load(String path, String filename) {
        try {
            FileInputStream fis = new FileInputStream(path + filename);
            GZIPInputStream gzis = new GZIPInputStream(fis);
            ObjectInputStream in = new ObjectInputStream(gzis);
            Object data = in.readObject();
            in.close();
            return data;
        } catch (Exception e) {
            System.out.println(e);
        }
        return null;
    }
}

Answer 2

I would make "myClass1", "myClass2", and "myClass3" Serializable , then wrap myClass1List , myClass2List , and myClass3List (and any other data you want to save) in another serializable class so you can use serialization/deserialization to save and restore all of the program state at once.

Unless myMainClass is that wrapper, in which case you need to declare that they all implement Serializable.

Answer 3

myMainClass isn't marked Serializable . Also, are myClass1 , myClass2 , and myClass3 serializable as well? If not, they should be.

On another note, please follow Java naming conventions; class name should start with an uppercase letter.

UPDATE

Are you sure that it's not writing to the file, or is it that the code is throwing exceptions that you cannot see?

In all your catch blocks, you have System.exit(1) , which gives you absolutely no information about any exceptions that are happening; you're essentially swallowing them. You should at least print out the stacktrace ( ex.printStackTrace() ) so you can see what is going wrong.