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Simple java music database with NullPointerException

I'm first year computer science and our assignment for semester 1 is to design a simple music database in java.
I'm using 3 classes, Interface(handles all user in/out), Song(stores artist, name, duration and filesize) and Database(stores 4 song objects; a,b,c,d). I can compile and run the the program fine, but when I enter the last field(fileSize) instead of a message returning the recently entered information I receive a NullPointerException, I understand this has something to do with assigning the value of null.
The code for the database class is;
public class songDatabase
{
song sin = new song();
private song a,b,c,d;
public songDatabase()
{
a = null;
b = null;
c = null;
d = null;
}
public void addSong(String artist, String name, double duration, int fileSize)
{
if (a==null) setData(a,artist,name,duration,fileSize);
else if (b==null) setData(b,artist,name,duration,fileSize);
else if (c==null) setData(c,artist,name,duration,fileSize);
else if (d==null) setData(d,artist,name,duration,fileSize);
}
private void setData(song sin, String artist, String name, double duration, int fileSize)
{
sin.setArtist(artist);
sin.setName(name);
sin.setDuration(duration);
sin.setFileSize(fileSize);
}
public String visconfir()
{
if (a != null) return("You have imported: "+sin.getName()+"by"+sin.getArtist()+"which is"
+sin.getFileSize()+"kB and"+sin.getDuration()+"long(mm.ss)");
else return("Error - No file imported to database memory slot a");
}
}
Can anybody help me out with this?

if (a==null) setData(a,artist,name,duration,fileSize);
if a == null you call setData with a as the first parameter (which is null).
Now, in setData you do:
sin.setArtist(artist); where sin is the first parameter. Which is like writing:
null.setArtist(artist), which of course.. throws an NPE.
Additional side note: I suggest you to follow Java Naming Conventions. After you'll read this, you might want to change the class name to begin with a capital letter.

Iterating through hashmap and creating unique objects - trying to prevent duplicates

I explain what I am trying to do in comments above the parts in the method:
public int addPatron(String name) throws PatronException {
int i = 0;
//1. Iterate through a hashmap, and confirm the new name I am trying to add to the record doesn't already exist in the hashmap
for (Map.Entry<Integer, Patron> entry : patrons.entrySet()) {
Patron nameTest = entry.getValue();
//2. If the name I am trying to add already exists, we want to throw an exception saying as much.
if (nameTest.getName() == name) {
throw new PatronException ("This patron already exists");
//3. If the name is unique, we want to get the largest key value (customer number) already in the hash, an increment by one.
} else if (nameTest.getName() != name) {
Map.Entry<Integer,Patron> maxEntry = null;
for(Map.Entry<Integer, Patron> entryCheck : patrons.entrySet()) {
if (maxEntry == null || entryCheck.getKey() > maxEntry.getKey()) {
maxEntry = entryCheck;
i = maxEntry.getKey();
i++;
}
}
} else {
throw new PatronException("Something's not working!");
}
//4. If everything is ok up to this point, we want to us the name and the new customer id number, and use those to create a new Patron object, which then gets added to a hashmap for this class which contains all the patrons.
Patron newPatron = new Patron(name, i);
patrons.put(i, newPatron);
}
return i;
}
When I try and run a simple unit test that will fail if I successfully add the same name for addPatron twice in a row, the test fails.
try {
testLibrary.addPatron("Dude");
testLibrary.addPatron("Dude");
fail("This shouldn't have worked");
The test fails, telling me the addPatron method is able to use the same name twice.
#Jon Skeet:
My Patron class looks like this:
public class Patron {
//attributes
private String name = null;
private int cardNumber = 0;
//operations
public Patron (String name, int cardNumber){
this.name = name;
this.cardNumber = cardNumber;
}
public String getName(){
return name;
}
public int getCardNumber(){
return cardNumber;
}
}

As others have said, the use of == for comparing strings is almost certainly inappropriate. However, it shouldn't actually have caused a problem in your test case, as you're using the same constant string twice, so == should have worked. Of course, you should still fix the code to use equals.
It's also not clear what the Patron constructor or getName methods do - either of those could cause a problem (e.g. if they create a new copy of the string - that would cause your test to fail, but would also be unnecessary usually).
What's slightly more worrying to me is this comment:
// 3. If the name is unique, we want to get the largest key value (customer number)
// already in the hash, an increment by one.
This comment is within the main loop. So by that point we don't know that the name is unique - we only know that it doesn't match the name of the patron in this iteration.
Even more worrying - and I've only just noticed this - you perform the add within the iteration block too. It seems to me that you should have something more like this:
public int addPatron(String name) throws PatronException {
int maxKey = -1;
for (Map.Entry<Integer, Patron> entry : patrons.entrySet()) {
if (entry.getValue().getName().equals(name)) {
// TODO: Consider using IllegalArgumentException
throw new PatronException("This patron already exists");
}
maxKey = Math.max(maxKey, entry.getKey());
}
int newKey = maxKey + 1;
Patron newPatron = new Patron(name, newKey);
patrons.put(newKey, newPatron);
return newKey;
}
Additionally, it sounds like really you want a map from name to patron, possibly as well as the id to patron map.

You need to use equals to compare String objects in java, not ==. So replace:
if (nameTest.getName() == name) {
with:
if (nameTest.getName().equals(name)) {

Try to use
nameTest.getName().equals(name)
instead of
nameTest.getName() == name
because now you're comparing references and not the value of the String.
it's explained here
Took another look on your code
Well i took another look on your code and the problem is, that your HashMap is empty at the start of the Test. So the loop will never be runned ==> there will never bee a Patron added or an Exception thrown.

The cause of the problem is how you have used the compare operator ==.
When you use this operator against two objects, what you test is that variable point to the same reference.
To test two objects for value equality, you should use equals() method or compareTo if available.
For String class, invoke of equals is sufficient the check that the store same characters more.
What is equals method ?
To compare the values of Object
The problem is how you compare names.

null pointer exception in array

I am getting a null exception error from this segment of code and I am not sure what causing it. The array itemcatalog has being populate for i =0 to 8. I am new to java so any assistance will be greatly appreciated. The error message points to the line of the while statement. Thanks
public class ItemCatalog {
private static ItemCatalog instance = new ItemCatalog();
private Item itemCatalog[] = new Item[9];
private ItemCatalog(){
};
public static synchronized ItemCatalog getInstance() {
return instance;
}
public void populateCatalog()
{
itemCatalog[0] = new Item("bb","Baked Beans",new BigDecimal("0.35"));
itemCatalog[1] = new Item("cf","Cornflakes",new BigDecimal("1.00"));
itemCatalog[2] = new Item("s0","Sugar",new BigDecimal("0.50"));
itemCatalog[3] = new Item("tb","Tea Bags",new BigDecimal("1.15"));
itemCatalog[4] = new Item("ic","Instant Coffee",new BigDecimal("2.50"));
itemCatalog[5] = new Item("b0","Bread",new BigDecimal("0.50"));
itemCatalog[6] = new Item("s0","Sausages",new BigDecimal("1.30"));
itemCatalog[7] = new Item("e0","Eggs",new BigDecimal("0.75"));
itemCatalog[8] = new Item("m0","Milk",new BigDecimal("0.65"));
}
public BigDecimal getPrice(String itemCode)
{
int i = 0;
while (!itemCode.equals(itemCatalog[i].getItemCode()))
{
i++;
}
BigDecimal itemPrice = itemCatalog[i].getItemprice();
return itemPrice;
}
}
I solved the issue. I was populating the catalog in the main class which was giving the null exception error. I instantiate it in the jframe instead and it works. The follow code solved the issue, but is this the best place to populate the catalog?
private void saleButtonActionPerformed(java.awt.event.ActionEvent evt) {
String itemCode = this.itemCodeinput.getText();
int itemQuantity =Integer.parseInt(this.itemQuantityinput.getText());
ItemCatalog catalog = ItemCatalog.getInstance();
catalog.populateCatalog();
BigDecimal price = catalog.getPrice(itemCode);
itemCostoutput.setText(price.toString());
}

If your itemCode doesn't match any entries in your itemCatalog, then eventually
while (!itemCode.equals(itemCatalog[i].getItemCode()))
{
i++;
}
will increment i to 11, in which case itemCatalog[11] is either empty or out of bounds.
If addition, you should use a for loop to iterate through the itemCatalog:
for (int i = 0; i < itemCatalog.length; i++) {
if (itemCode.equals(itemCatalog[i].getItemCode()) {
return (BigDecimal) itemCatalog[i].getItemprice();
}
}
return null // you can change this from null to a flag
// value for not finding the item.

From the comments, it's clear the design isn't sound.
Here's a possible solution :
public BigDecimal getPrice(String itemCode) {
for (int i=0; i<itemCatalog.length; i++) { // not going outside the array
if (itemCatalog[i].getItemCode().equals(itemCode)) { // inversing the test to avoid npe if itemCode is null
return itemCatalog[i].getItemprice();
}
}
return null; // default value
}
This supposes your array is correctly filled with itemCatalogs having an itemCode.

How do you end your loop?
Seems that the loop will keep going until i is 10. Then your will have exceeded the limit.
Unless this is a uni assignment where you have to use arrays, I'd also suggest using a map, rather than an array. This way your lookup will be the same time, whether your collection has 100,000 entries or 10.
You will also reduce risk of NPE or ArrayOutOfBounds exception
See http://docs.oracle.com/javase/1.4.2/docs/api/java/util/HashMap.html
When adding the object use the item code as the key. Then lookup by the key.
The cost of using a map is increased memory usage.

Java: Copy Constructor not going as planned

I have a bit of a problem. I'm making a Finite Automata checker.
Given an input, and the DFA, does it end on a accepting state.
My problem is creating a new DFA_State from another's target.
DFA_State state0, state1, curr_state, init_state, temp; //fine, I think
state0 = new DFA_State();
state1 = new DFA_State();
state0 = new DFA_State("State 0",true, state0, state1); //fine, I think
init_state = new DFA_State(state0); //fine, I think
but, this bit is throwing up problems.
temp = new DFA_State(curr_state.nextState(arr1[i]));
*
*
curr_state = new DFA_State(temp);
Thanks for any help,
Dave
Edit:
God I was retarded when I did this, AFAIK, I just wasn't thinking straight, added methods to set the values to the DFA_State object.
//in DFA_State class
public void set(DFA_State on_0, DFA_State on_1, Boolean is_accepting, String name){
this.on_0 = on_0;
this.on_1 = on_1;
this.is_accepting = is_accepting;
this.name = name;
}
//in main
DFA_State state0, state1, curr_state;
state0 = new DFA_State();
state1 = new DFA_State();
state0.set(state0, state1, false, "State 0");
state1.set(state1, state0, true, "State 1");
curr_state = state0;//initial state
//iterate across string input changing curr_state depending on char c
curr_state = getNextState(c);
//at end
if(curr_state.isAccepting())
System.out.println("Valid, " + curr_state.getName() + " is accepting);
else
System.out.println("Invalid, " + curr_state.getName() + " is not accepting);

In that first line, you declare the variables state0, state1, curr_state, init_state and temp as being variables of type DFA_State. However, that only declares them, they are not yet initialized. The next few lines are all okay. Second line creates a state without anything in it and assigns it to state0, so does the third line for state1. Fourth line overwrites your previous state0 assignment with a new DFA_State that has actual contents. Fifth line creates a DFA_State as a copy of state0 and assigns it to init_state.
Assuming there's nothing in between this and the first line of your second code block, now you'll get a problem. You're assigning temp with a new DFA_State that uses a copy-constructor with an argument relying on curr_state. But at that point, that variable hasn't been initialized yet. Just because it was declared doesn't mean it has somehow already been structured in memory. When you call nextState on it, there's simply no variable to resolve this to. Don't expect to get something like a pointer that will eventually point to a part of what you put in curr_state.
I'm just guessing, but from your code style I'd say you have a background in C or C++. Look into the differences between those languages and Java. If possible, I'd also advise you to make your DFA_State class immutable, since this is more reliable and will avoid mistakes. That means getting rid of the no-args constructor. Here's a reworking of it (not actually compiled, might contain errors):
package foundations.of.computing;
/**
*
* #author Kayotic
*/
class DFA_State {
private final String state;
private final DFA_State on_0;
private final DFA_State on_1;
private final boolean isAccepting;
//private DFA_State dummy;
public DFA_State(DFA_State arg) {
//this(arg.is_accepting(), arg.on0(), arg.on1());
state = arg.get_name();
isAccepting = arg.is_accepting();
on_0 = arg.on0();
on_1 = arg.on1();
}
public DFA_State(String name, Boolean accepting, DFA_State on0, DFA_State on1) {
state = name;
isAccepting = accepting;
on_0 = on0;
on_1 = on1;
}
public String get_name(){
return state;
}
public Boolean is_accepting() {
return isAccepting;
}
public DFA_State on0() {
return on_0;
}
public DFA_State on1() {
return on_1;
}
public DFA_State nextState(char i) {
if (i == '0') {
return on0();
} else if (i == '1') {
return on1();
} else {
System.out.println("Error with input");
return null;
}
}
}
Even if you can't make the instance variables final, it's best to at least make them private, since you already have methods for getting them.

There are better memory representations of DFAs than the object-oriented.
You should use a simple lookuptable:
int[] table = new int[vocabularyCount][stateCount];
Every State and every word gets a number, starting with 0.
Fill the table with the state transitions, or -1, if there is no transition. Now you just need the translation methods for the states and the words.
Heres a generic DFA algorithm:
public boolean checkSentence(String s, int[] finishes) {
// fill table
int state = 0; // assuming S0 is the start state
for (int i = 0; i < s.length(); i++) {
state = table[translate(s.charAt(i))][s];
}
for (int i = 0; i < finishes.length; i++) {
if (finishes[i] == state) {
return true;
}
}
return false;
}

The program is quite poorly written. Look at this in your FoundationsOfComputing.java:
state0 = new DFA_State();
state1 = new DFA_State();
state0 = new DFA_State("State 0",true, state0, state1);
You essentially created 3 instances of state - two instances which are not initialized (first two lines in your code) - all their instance variables are null.
Then you create the third instance, which you point to the first two uninitialized ones, and assign it to state0 variable. Please note, at this point, it is only the value of the variable that changes, not the values you passed in the DFA-State constructor!!! So, what you now have in state0 is a state that points to two uninitialized states.
Now let's look at the code further down in the FoundationsOfComputing.java:
while (i < arr1.length) {//loops through array
System.out.println(i + ". scan shows " + arr1[i]);
temp = new DFA_State(curr_state.nextState(arr1[i]));
System.out.println(" "+curr_state.get_name()+ " moves onto " + temp.get_name());
curr_state = new DFA_State(temp);
i++;
}
I am guessing this throws NullPointerException - that code moves to the on_0 state of state0 - which is a state that has not been initialized (all it's instance variables are null), so in the following pass of the loop, when it calls curr_state.nextState(whatever), it would return null and you are trying to pass that to the copy-constructor which would result in NPE.

Ok so we know this is homework. Let's do this instead of telling you the answer let's try and work through it on your own. If you are seeing a NullPointerException (NPE). Grab the second line of the exception:
java.lang.NullPointerException: null
at com.blah.blah.SomeObject.someMethod(SomeArgumentType):1234 <<< here
....
That 1234 is the line number in the file that contains SomeObject. If you goto that line number you can see exactly where the NPE is being generated from. For example if line 1234 was:
this.foo = bar.indexOf("caramel");
You can easily deduce what was null. No clue? Well this can never be null so this.foo isn't the problem. If this could be null you couldn't be inside that method because this points to the instance you are currently within. Therefore, the only other statement where a variable is being dereferenced is bar so bar must be null. Let's look at your code:
temp = new DFA_State(curr_state.nextState(arr1[i]));
Say you find out the line above is tossing an exception. Well there could be several things that could be null. curr_state could be null, or arr1 could be null in which case this line would blow up. However, if arr1[i] is null or curr_state.nextState() is returning null then you won't see the NPE pointing at this line, but would be coming out of the constructor should someone try to call methods on that method parameter.
Hopefully, this will give you the tools you need to track down problems in your application by understanding exception stack traces.

BST intersection, NullPointerException

I am trying to create a new BST from the intersection of 2 known BSTs. I am getting a NullPointerException in the intersect2 method int he second case, at the line "cur3.item.set_account_id(cur1.item.get_accountid()+ cur2.item.get_accountid());". I know you get the error when you try to dereference the variable without initializing it but i think i am initializing it? I'm not really sure. I would appreciate the help.
public static Bst<Customer> intersect(Bst<Customer> a, Bst<Customer> b){
return( intersect2(a.root, b.root));
}
public static Bst<Customer> intersect2(BTNode<Customer> cur1, BTNode<Customer> cur2){
Bst<Customer> result = new Bst<Customer>();
// 1. both empty -> true
if (cur1==null && cur2==null){
result=null;
}
// 2. both non-empty -> compare them
else if (cur1!=null && cur2!=null) {
BTNode<Customer> cur3 = new BTNode<Customer>();
cur3.item.set_account_id(cur1.item.get_accountid()+ cur2.item.get_accountid());
result.insert(cur3.item);
intersect2(cur1.left, cur2.left);
intersect2(cur1.right, cur2.right);
}
// 3. one empty, one not -> false
else if (cur1==null ||cur2==null){
BTNode<Customer> cur3 = new BTNode<Customer>();
cur3.item=null;
intersect2(cur1.left, cur2.left);
intersect2(cur1.right, cur2.right);
}
return result;
}
Here is the image of the problem:

A NullPointerException can be caused by a number of things. In your given example, cur1 and cur2 are not null, but there is no guarantee that cur1.item, cur1.item.accountId (and similarly for cur2) are not null.
Being as you have no description for the underlying implementation, I cannot assist further.
I can suggest that you do some of a few things:
1.) check the implementation of your objects (if this happens EVERY time, there may be some sort of initialization problem.
2.) Whenever you create an instance of your item, do you make sure to specify the accountId field? Try giving a default value for this field so it cannot be null. (try some sort of illegal value [eg -1, false, etc] and test for it.
If you would post more implementation details, I (or someone) may be able to directly identify the problem.
Regards.
Edit:4/20#17:11
Here's an example of what you should be doing.
public class Customer {
private int accountId;
public Customer() {
this.accountId = 0;
}
public Customer(int account_identification) {
this.accountId = account_identification);
}
//As a side note, general practice implies fields be private
//Use a method (hence the term 'getter' and the reciprocal, 'setter')
public int getId() {
return this.accountId;
}
public void setId(int replacement_account_identification) {
this.accountId = replacement_account_identification;
}
}

It is because the item variable in Customer object is not initialized.

Does creating a BTNode automatically allocate its member item ?
You do:
cur3.item.set_account_id(.. )
For this to succeed, both cur3 and cur3.item need to be not null.
Same applies to cur1 and cur2 as well, that you reference later in that line.
And the example of the 3rd case shows that BTNode.item can be null in some scenarios:
cur3.item=null;