Class cast Exception when reading an saved array list (object) from a file

Question

I want to actually read data from a file into an array list of a Flight_registrie class. I am able to save an array list in a file, but when I try to read from the file, it's not working and giving a class cast exception. I also tried to get values in the array list directly, but it didn't work. Can you tell me what should I write to read data from a file into an array list?

try {
    ObjectInputStream ois = new ObjectInputStream(new FileInputStream(" Flight Registrie.bin"));

    Flight_registrie b =(Flight_registrie) ois.readObject();
    alr.add(b);
    ois.close();
} catch (FileNotFoundException e) {
    System.out.println("\t\t\tFile not Found.");    
} catch (IOException e) {
    System.out.println("An I/O error occurs");
} catch (ClassNotFoundException e) {
    System.out.println("\t\tClass Flight_registrie not found ");
}

and below code is how i took input .

try {al.add(a);
    ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(" Flight Registrie.bin"));
    oos.writeObject(al);
    System.out.println("Data Saved");
    al.clear();
oos.close();
} catch (IOException e) {
System.out.println("An I/O error occurs");
}

Error:

 Exception in thread "main" java.lang.ClassCastException: class
java.util.ArrayList cannot be cast to class
source__module.Flight_registrie (java.util.ArrayList is in module
java.base of loader 'bootstrap'; source__module.Flight_registrie is in
unnamed module of loader 'app')
    at source__module.Functions.get_data(Functions.java:64)
    at file.Launcher.main(Launcher.java:41)

It looks like your code is trying to read a Flight_registrie object, not an ArrayList. — khelwood
– khelwood, Commented Oct 20, 2019 at 8:29
It seems you write into file ArrayList but read list item. You need to update question with serialization code. — Maxim Tulupov
– Maxim Tulupov, Commented Oct 20, 2019 at 8:34
i edited the other code which shows how i am taking input . so what should i read from file. — Aman Verma
– Aman Verma, Commented Oct 20, 2019 at 8:42
If you write a banana, you must read a banana. Not an apple. Not a box of bananas. If you want to read a registry, you must write a registry. If you write an ArrayList, you must read an ArrayList. — JB Nizet
– JB Nizet, Commented Oct 20, 2019 at 8:43

javaGroup456 · Accepted Answer · 2019-10-20 10:19:52Z

1

Seems like you are saving a List in your file. But while reading you are trying to read a List into Flight_registrie.

That id why the error is coming as you are trying to cast an ArrayList object into and object of Flight_registrie class. The faulty line is :

 Flight_registrie b =(Flight_registrie) ois.readObject();

Try changing it to this :

List<Flight_registrie> b =(List<Flight_registrie>) ois.readObject();

And this should fix the problem.

answered Oct 20, 2019 at 10:19

javaGroup456

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1 Comment

javaGroup456 Over a year ago

One should read the object of a class of which the object was written in the file in the very first place. List can be written and read from files as they implement Serializable by default.

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