如何在Java中使用ObjectInputStream从文件中读取对象列表？

Question

I have a List of list of some objects defined in a data structure: List > someObject1=new ArrayList();

And I have written few someObject items to a fileXY using function writeData :

void writeData(List <List<Integer>> someObject1, File XY) throws IOException {
    try(ObjectOutputStream outFile = new ObjectOutputStream(new FileOutputStream(XY,true)))
    outFile.writeObject(someObject1);
    }
    catch( IOException ex) { ex.printStackTrace(); } 
}

Now I tried to read it using function readData:

void readData(File XY) throws IOException {
    try (ObjectInputStream inFile=new ObjectInputStream(new FileInputStream(XY))) {
    List <List <MyClass>> someObject2=(List <List <MyClass>>)inFile.readObject();
    }
catch( IOException ex) { ex.printStackTrace(); } 
}

But it is giving me error :

java.io.OptionalDataException
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1363)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:371)

Can anyone help me with it. Is there something wrong with the casting while reading this data structure?

Had it been just a list of objects, I could have read it by casting the readObject() with

(List<MyClass>).

Why the same thing doesn't work for the list-of-list of objects?

This is outline of MyClass:

public class MyClass implements Serializable {
private long aa;    
private List <Integer> bb;
public MyClass(long a,List <Integer> b) {
    aa=a;
    bb=b;         
}
public long getA() {    
    return aa;
}         

public List<Integer> getB () {
   return bb; 
} 

}

Answer 1

This definitely works for me:

public static final String FILE = "file.b";

public static class MyClass implements Serializable {
    private long aa;
    private List<Integer> bb;

    public MyClass(long a, List<Integer> b) {
        aa = a;
        bb = b;
    }
}

public static void main(String[] args) throws FileNotFoundException, IOException,
        ClassNotFoundException {
    List<List<MyClass>> list = new ArrayList<>();
    list.add(new ArrayList<>());
    list.get(0).add(new MyClass(5, new ArrayList<>()));

    ObjectOutputStream outFile = new ObjectOutputStream(new FileOutputStream(new File(FILE)));
    outFile.writeObject(list);
    outFile.close();

    ObjectInputStream inFile = new ObjectInputStream(new FileInputStream(new File(FILE)));
    List<List<MyClass>> list2 = (List<List<MyClass>>) inFile.readObject();
    System.out.println(list2);
}

Note: this works even if I don't add any MyClass to list.get(0) , and even if I don't add a list to list .

Several possible causes for your problem come to my mind:

• you forgot to close the output stream after writing to it
• you don't read the object at the right position in the file. If you have several objects written to the file, they must be read in the same order as they were written.
• your problem is with your custom class

---

UPDATE:

Maybe you use a non serializable attribute in your serializable class MyClass . If so, you should override writeObject() and readObject() to serialize/deserialize such an attribute of your class properly.

Take a look at this: http://marxsoftware.blogspot.fr/2014/02/serializing-java-objects-with-non.html