Trouble reading from Storage

Question

Ive had a deeper look at the storage system now, but I just can't get it to work properly. The data seems to be saved, the saved data is definitely not empty and I am still getting a java.lang.NullPointerException.

So here is my Class i want to saved and later read to the Storage.

Band is a trivial Class with name and genre String.

public class Band implements Externalizable{
    String name;
    String genre;

    public Band(String bnd, String gen){
        this.name = bnd;
        this.genre = gen;
    }
    public Band() {

    }

    public String getName(){
        return this.name;
    }
    public String getGenre() { return this.genre; }
    public void setName(String name) {
        this.name = name;
    }
    public void setGenre(String genre) {
        this.genre = genre;
    }

    @Override
    public String toString(){
        String s;
        s = this.name;
        return s;
    }
    public int getVersion(){
        return 1;
    }

    @Override
    public void externalize(DataOutputStream out) throws IOException {
        Util.writeUTF(name, out);
        Util.writeUTF(genre, out);
    }

    @Override
    public void internalize(int version, DataInputStream in) throws IOException {
        name = Util.readUTF(in);
        genre = Util.readUTF(in);
    }

    public String getObjectId(){
        return "Band";
    }

}

BandList is just a Class with an ArrayList the Bands are added to.

public class BandList implements Externalizable{
    List <Band> bandList;

    public List getBandList(){
        return this.bandList;
    }

    public BandList(){
        bandList = new ArrayList<Band>();
    }

    public void setBandList(List<Band> bandList) {
        this.bandList = bandList;
    }

    public int getVersion(){
        return 1;
    }

    @Override
    public void externalize(DataOutputStream out) throws IOException {
        Util.writeObject(bandList, out);
    }

    @Override
    public void internalize(int version, DataInputStream in) throws IOException {
        bandList = (ArrayList<Band>) Util.readObject(in);
    }

    public String getObjectId(){
        return "BandList";
    }
}

So, in some other class where the user filled in some TextFields, the band gets added to the BandList and therefore I call saveListsToStorage();

public void saveListsToStorage(){
        Storage.getInstance().clearStorage();
        Storage.getInstance().writeObject("Bands", bandList);
    }

So now, when starting the App, the App is looking for Data in the Storage, if found, it will return the BandList Object stored, otherwise it will return a new BandList Object.

Before that, I regist the Classes with

    Util.register("Band", Band.class);
    Util.register("BandList", BandList.class);

and finally call this method in the main at the beginning:

public BandList loadSavedBandList() {
        String[] temp = Storage.getInstance().listEntries();
        for (String s : temp) {
            if (s.equals("Bands") == true) {
                BandList tempBand = (BandList) Storage.getInstance().readObject("Bands");
                return tempBand;
            }
        }
        return new BandList();
    }

After saving and then loading it throws the NullPointer exception and I have no clue why. I now used ArrayList instead of LinkedList as Shai told me and I implemented the Externalizable Interface as told in the guide.

Maybe some of you can tell me whats wrong here.

Edit:

Here is the Exception:

java.lang.NullPointerException
    at com.rosscode.gehma.Menu_Form.updateBandContainer(Menu_Form.java:95)
    at com.rosscode.gehma.Menu_Form.<init>(Menu_Form.java:73)
    at com.rosscode.gehma.main.start(main.java:61)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.codename1.impl.javase.Executor$1$1.run(Executor.java:106)
    at com.codename1.ui.Display.processSerialCalls(Display.java:1152)
    at com.codename1.ui.Display.mainEDTLoop(Display.java:969)
    at com.codename1.ui.RunnableWrapper.run(RunnableWrapper.java:120)
    at com.codename1.impl.CodenameOneThread.run(CodenameOneThread.java:176)

Edit 2, Line 95:

if(bandList.getBandList().isEmpty() == false) {
            for (int i = 0; i < bandList.getBandList().size(); i++) {
                Button temp = new Button(bandList.getBandList().get(i).toString());
                temp.addActionListener((e) ->
                        nextStep()
                );
                listCont.add(temp);
            }
            menuForm.revalidate();
        }

Answer 1

NullPointerExceptions are pretty amazing things, because not only are they easy to figure out, they even show you the exact line where they happen. All you have to do is look at anything left of a . and check if it's null. As soon as you find it, you find the problem.

So let's look at your code. Sadly I don't know which of those lines is line 95 but I'm guessing it's the one starting with if.

That means either bandList is null or bandList.getBandList() returns null. A quick look into getBandList() shows that it won't be null, as you initialize the list in the constructor of your BandList class. Which only leaves bandList.

Sadly you didn't post where you got that from, but I'm gonna assume you just did something like:

BandList bandList = loadSavedBandList();

So let's look into that method. (btw: I say a bit more about where you have to look if you didn't do this at the end of this answer).

If you couldn't find your key, it would return a new BandList, which couldn't be null. So it has to find something, meaning the key exist but the value is null.

So let's take another step and look at how you save things in the first place.

public void saveListsToStorage(){
    Storage.getInstance().clearStorage();
    Storage.getInstance().writeObject("Bands", bandList);
}

Now "in some other class" doesn't really explain much. But since you read a null, it means you are writing a null here. Which would mean in this "other class", at least in this method, bandList = null.

With the code you gave, it's impossible for me to look deeper into this issue, but it seems like in "that other class", you got a class attribute named bandList but fail to put anything inside it.

---

Or maybe you saved everything right, but didn't call

BandList bandList = loadSavedBandList();

in some form or another before your if, which would also pretty much explain the problem. I mean... if you never load the bandList, it will obviously be null...