new to java - NEED HELP!

[Sandro]

Hello i am new to java and in my java class we just started using classes. I am getting an error in one of my programs saying:
"java.lang.NullPointerException
at LogicOfProgramming.Assignment4.MonthlyCalendar.cal endar(MonthlyCalendar.java:30)
at LogicOfProgramming.Assignment4.MonthlyCalendarAppl ication.main(MonthlyCalendarApplication.java:20)"

Here are my classes:

[KevinWorkman]

Did you google the cause of that Exception?

If you want help, you'll have to provide an SSCCE that demonstrates the problem.

[Sandro]

I have googled the problem but I couldn't find exactly what was causeing it... something about me goofing up on a class.... also there are only 3 class files.. i believe thats pretty short?

[KevinWorkman]

I have googled the problem but I couldn't find exactly what was causeing it... something about me goofing up on a class.... also there are only 3 class files.. i believe thats pretty short?

I'd be pretty surprised if you couldn't figure out what causes an NPE by a google search, or a search of these forums. And three classes is actually quite huge for this type of problem- you have a line number, right? So you already know the exact line causing the problem. Isolate that line in a standalone program, and copy and paste it here. Don't forget the code tags.

Chances are, in the course of creating your standalone program, you'll figure the problem out on your own. But if not, then many people hesitate to click on .zip attachments, and we don't have time to wade through too much code. Make it easier for people to help you, and you'll get help faster.

[Sandro]

NPE? I dont know what that is.
I don't know which parts of the code i need and which parts I wouldn't even with the lines.

I haven't done much codeing in general and i hardly know how to fix bugs. You need to remember ive been doing classes in general about 2days now and my teacher didn't even trully explain it.

[Norm]

java.lang.NullPointerException
at LogicOfProgramming.Assignment4.MonthlyCalendar.cal endar(MonthlyCalendar.java:30)

Look at line 30 in the MonthlyCalendar class. There is a variable on that line that has a null value.
Determine what variable is null and then backtrack in your code and find out why the variable does not have a valid value.
If you can not determine which variable is null, add a println just before line 30 that prints the values of all the variables used on line 30

[al_Marshy_1981]

Date was never initialized in MonthlyCalendar

[Sandro]

I thought i just had to put "Date theDate;" in my variable declaration and then i could call on it? Am i wrong?

[al_Marshy_1981]

If an object is not initialized it will be assigned to null.

[Norm]

There are two steps:
define the variable and
give it a value.

[Sandro]

im sorry guys... but i still have no clue what i did wrong T_T im trying to understand what you guys are telling me but sadly half of it is going right over my head... (partially because i don't know the terminology of most of what im actually doing.)

[al_Marshy_1981]

You are passing an initialized Date object to MonthlyCalendars constructor. You have an uninitialized Date object inside MonthlyCalander. Perhaps some kind of assignment can be done inside the the constructor?

[Norm]

Do you know what an assignment statment is? It is used to give a value to a variable:
x = 123; // assign the value 123 to the variable x

Do you know how to define a variable:
int x; // define an int variable named x

These can be done in one statement:
int x = 123; // define a variable and give it a value.

[Sandro]

i don't even call of the MonthlyCalendars constructor though.

[al_Marshy_1981]

hmmmm Lesson: Classes and Objects (The Java™ Tutorials > Learning the Java Language)

MonthlyCalendar newMonthlyCalendar = new MonthlyCalendar (newDate);

[Sandro]

Ohhh your right (feels stupid). So what would i put into this constructor? The Date class?

[Sandro]

Did this:
public MonthlyCalendar (Date myDate)
{
theDate = myDate;
}

Worked. Love you al_Marshy_1981 :P