Saving/loading multiple ArrayLists into one file in Java

Question

Currently I have a program that can save and load ArrayLists of type "Variable" from a file. However, I would like to be able to save and load different objects(instances of different classes such as "Variables", "Functions"..etc) from just one file. Here is the code I have now:

 public SaveState(){
    ObjectOutputStream out = null;
    try {
        out = new ObjectOutputStream(new FileOutputStream("vars.stt"));
    } catch (IOException e) {
        e.printStackTrace();
    }
    ArrayList<Variables> vars = new ArrayList<>();
    vars.add(new Variables("g", 5));
    vars.add(new Variables("f", -3.12));
    try {
        out.writeObject(vars);
        out.flush();
        out.close();
    } catch (IOException e) {
        e.printStackTrace();
    }

}

public LoadState(){
    ObjectInputStream in = null;
    try {
        in = new ObjectInputStream(new FileInputStream("vars.stt"));
    } catch (IOException e) {
        e.printStackTrace();
    }
    try {
        //final Object o = in.readObject();
        final ArrayList<Variables> vars = (ArrayList<Variables>) in.readObject();
        System.out.println(vars.toString());
    } catch (IOException | ClassNotFoundException e) {
        e.printStackTrace();
    }
}

This code only allows me to load one object type from a file. Is there any way to save/load multiple object types in this manner?

didn't you try making a base class for Functions and Variables and using the array list of type Base Class? — Imesha Sudasingha
– Imesha Sudasingha, Commented Apr 13, 2016 at 3:23
@ImeshaSudasingha Honestly I'm not sure if I would be able to do that, it would require me to refactor a lot of my code (hundreds of lines). Is there another solution. — Arman
– Arman, Commented Apr 13, 2016 at 3:25
No, you just can create some base class(say "ProgramElement"), them just extent the Variables and Functions class from it. No need to refactor the code. — Imesha Sudasingha
– Imesha Sudasingha, Commented Apr 13, 2016 at 3:26
@ImeshaSudasingha So if I had classes "Variables", "Functions", and "Lists" how would I extend all 3 from one class? — Arman
– Arman, Commented Apr 13, 2016 at 3:31
Why don't you use Object class as that 'base class'. You can use polymorphism. — Lasitha Yapa
– Lasitha Yapa, Commented Apr 13, 2016 at 3:31

KayV · Accepted Answer · 2016-04-13 06:39:16Z

2

You can simply use a map conataining the arraylists of varaible or functions. Map is good in terms of performance also.

    Map<String, List> map = new HashMap<String, List>();
    map.put("variable", arralist1);
    map.put("functions", arralist2);
    map.put("list", arralist3);

answered Apr 13, 2016 at 6:39

KayV

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3 Comments

Arman Over a year ago

So this would work even if the ArrayLists are of different types?

KayV Over a year ago

Yes. Or you can use generics to make it parameterized.

Arman Over a year ago

Thank you very much for this solution. It worked perfectly

devonlazarus · Accepted Answer · 2016-04-13 06:03:37Z

2

Create a class hierarchy like this:

//base class. No need to write any content
class ProgramElement {

}

//your variables class
class Variables extends ProgramElement {
    //your code
}

//your functions class
class Functions extends ProgramElement {
    //your code
}

//your lists class
class Lists extends ProgramElement {
    //your code
}

Then use an ArrayList of type ProgramElement. Remember List here is not your Lists class. It's java's List interface.

List<ProgramElement> elements = new ArrayList<>();
elements.add(aFunction); //add function object
elements.add(aList);  //lists object
elements.add(aVariable); //variables object

Note: You can achieve the same by using a List of type Object. But it is not recommended since it is not a good practice.

edited Apr 13, 2016 at 6:03

devonlazarus

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answered Apr 13, 2016 at 5:40

Imesha Sudasingha

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1 Comment

Arman Over a year ago

Thank you very much, at first I thought you were telling me to do something along the lines of "class ProgramElement extends Variables, Functions, Lists { }" That is where I got confused. Thank you again.

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