Question

I creating a program that will calculate something and store the information into a hashTable. What I want it to do is the next time it runs to restore the previous values stored in the hashTable and resume. So I ask how to save an object to a file?

In my code, I am using FileInputStream and FileOutputStream. Here is a snip-bit of my code, so far:

@SuppressWarnings("unchecked")
private void obtainSaveFile(){
    String file = new String("./saveFile.txt");
    try{
        //Create the file if it doesn't exist
        if( !( (new File(file)).exists() ) ){

            //Create new empty file
            (new File(file)).createNewFile();

            //Creates a fresh new hashTable
            hashTable = new QuadraticProbingHashTable<TicTacToeBoard>();
        }
        else{
            //Obtain the hashTables saved config.txt file
            FileInputStream fis = new FileInputStream(file);
            ObjectInputStream ois = new ObjectInputStream(fis);

            //Obtain the previously saved hashTable
            //-----------------------------------
            //This line of code gives me an error
            //-----------------------------------
            hashTable = (QuadraticProbingHashTable<TicTacToeBoard>)ois.readObject();
            ois.close();
        }

        //Create save file configs.txt
        FileOutputStream fos = new FileOutputStream(file);
        saveFile = new ObjectOutputStream(fos);     

    }catch(Exception e){
        e.printStackTrace();

        //Creates a fresh new hashTable
        //This line of code gives me an error
        hashTable = new QuadraticProbingHashTable<TicTacToeBoard>();
    }
}

Initially, when there is no file, it runs fine. But after that, it keeps giving me errors. It doesn’t like when I take an object from the file and cast it as a QuadraticProbingHashTable and set that equal to hashTable.

Also, here is what is outputed as the error:

java.io.WriteAbortedException: writing aborted; java.io.NotSerializableException: proj4.QuadraticProbingHashTable$HashEntry
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readArray(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.defaultReadFields(Unknown Source)
    at java.io.ObjectInputStream.readSerialData(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at proj4.TicTacToeSolver.obtainSaveFile(TicTacToeSolver.java:308)
    at proj4.TicTacToeSolver.solve(TicTacToeSolver.java:120)
    at proj4.Main.main(Main.java:41)
Caused by: java.io.NotSerializableException: proj4.QuadraticProbingHashTable$HashEntry
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeArray(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.io.ObjectOutputStream.writeObject(Unknown Source)
    at proj4.TicTacToeSolver.solve(TicTacToeSolver.java:246)
    ... 1 more
Exception in thread "main" java.lang.NullPointerException
    at proj4.TicTacToeSolver.solve(TicTacToeSolver.java:246)
    at proj4.Main.main(Main.java:41)

Answer 1

Well, the problem seems to be, that you are trying to serialize something which does not implement Serializable.
You can see that here:

java.io.NotSerializableException: proj4.QuadraticProbingHashTable$HashEntry

You have two choices:

1. You let HashEntry implement Serializable if thats possible
2. You mark it as transient so it will not be serialized (probably not what you want).

For more details you could post the class.