How to write and read java serialized objects into a file

Question

I am going to write multiple objects to a file and then retrieve them in another part of my code  My code has no error  but it is not working properly  Could you please help me find what is wrong about my code  I read different codes from different websites  but none of them worked for me   Here is my code to write my objects to a file  MyClassList is an arraylist which includes objects of my class  which must be written to a file    for  int cnt   0  cnt  lt  MyClassList size    cnt          FileOutputStream fout   new FileOutputStream  G   address ser   true       ObjectOutputStream oos   new ObjectOutputStream fout       oos writeObject MyClassList get cnt        I added  true  to the constructor of the outputstream  because I want to add each object to end of the file  Is that correct   And here is my code to read the objects from the file    try        streamIn   new FileInputStream  G   address ser         ObjectInputStream objectinputstream   new ObjectInputStream streamIn        MyClass readCase    MyClass  objectinputstream readObject         recordList add readCase        System out println recordList get i       catch  Exception e         e printStackTrace         It finally prints out just one object  Now  I don t know if I am not writing correctly or reading correctly

User · Accepted Answer

Why not serialize the whole list at once?

FileOutputStream fout = new FileOutputStream("G:\\address.ser");
ObjectOutputStream oos = new ObjectOutputStream(fout);
oos.writeObject(MyClassList);

Assuming, of course, that MyClassList is an ArrayList or LinkedList, or another Serializable collection.

In the case of reading it back, in your code you ready only one item, there is no loop to gather all the item written.

User · Answer

As others suggested  you can serialize and deserialize the whole list at once  which is simpler and seems to comply perfectly with what you intend to do   In that case the serialization code becomes  ObjectOutputStream oos   null  FileOutputStream fout   null  try      fout   new FileOutputStream  G   address ser   true       oos   new ObjectOutputStream fout       oos writeObject myClassList     catch  Exception ex        ex printStackTrace      finally       if oos    null           oos close               And deserialization becomes  assuming that myClassList is a list and hoping you will use generics    ObjectInputStream objectinputstream   null  try       FileInputStream streamIn   new FileInputStream  G   address ser        objectinputstream   new ObjectInputStream streamIn       List lt MyClass gt  readCase    List lt MyClass gt   objectinputstream readObject        recordList add readCase       System out println recordList get i      catch  Exception e        e printStackTrace      finally       if objectinputstream    null           objectinputstream  close               You can also deserialize several objects from a file  as you intended to   ObjectInputStream objectinputstream   null  try       streamIn   new FileInputStream  G   address ser        objectinputstream   new ObjectInputStream streamIn       MyClass readCase   null      do           readCase    MyClass  objectinputstream readObject            if readCase    null               recordList add readCase                    while  readCase    null              System out println recordList get i      catch  Exception e        e printStackTrace      finally       if objectinputstream    null           objectinputstream  close               Please do not forget to close stream objects in a finally clause  note  it can throw exception    EDIT  As suggested in the comments  it should be preferable to use try with resources and the code should get quite simpler    Here is the list serialization    try      FileOutputStream fout   new FileOutputStream  G   address ser   true       ObjectOutputStream oos   new ObjectOutputStream fout          oos writeObject myClassList     catch  Exception ex        ex printStackTrace

User · Answer

Simple program to write objects to file and read objects from file    x000D   x000D  package program  x000D   x000D  import java io File  x000D  import java io FileInputStream  x000D  import java io FileOutputStream  x000D  import java io ObjectInputStream  x000D  import java io ObjectOutputStream  x000D  import java io Serializable  x000D   x000D  public class TempList   x000D   x000D    public static void main String   args  throws Exception   x000D      Counter counter   new Counter 10   x000D   x000D      File f   new File  MyFile txt    x000D      FileOutputStream fos   new FileOutputStream f   x000D      ObjectOutputStream oos   new ObjectOutputStream fos   x000D      oos writeObject counter   x000D      oos close    x000D   x000D      FileInputStream fis   new FileInputStream f   x000D      ObjectInputStream ois   new ObjectInputStream fis   x000D      Counter newCounter    Counter  ois readObject    x000D      System out println newCounter count   x000D      ois close    x000D      x000D   x000D    x000D   x000D  class Counter implements Serializable   x000D   x000D    private static final long serialVersionUID   -628789568975888036 L  x000D   x000D    int count  x000D   x000D    Counter int count    x000D      this count   count  x000D      x000D    x000D   x000D   x000D    After running the program the output in your console window will be 10 and you can find the file inside Test folder by clicking on the icon show in below image

User · Answer

if you serialize the whole list you also have to de-serialize the file into a list when you read it back  This means that you will inevitably load in memory a big file  It can be expensive  If you have a big file  and need to chunk it line by line  -  object by object  just proceed with your initial idea   Serialization   LinkedList lt YourObject gt  listOfObjects    lt something gt   try       FileOutputStream file   new FileOutputStream  lt filePath gt        ObjectOutputStream writer   new ObjectOutputStream file       for  YourObject obj   listOfObjects            writer writeObject obj             writer close        file close      catch  Exception ex        System err println  failed to write     filePath         ex       De-serialization   try       FileInputStream file   new FileInputStream  lt filePath gt        ObjectInputStream reader   new ObjectInputStream file       while  true            try                YourObject obj    YourObject reader readObject                System out println obj            catch  Exception ex                System err println  end of reader file                 break                    catch  Exception ex        System err println  failed to read     filePath         ex

User · Answer

I think you have to write each object to an own File or you have to split the one when reading it  You may also try to serialize your list and retrieve that when deserializing

[java] How to write and read java serialized objects into a file

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