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Setting up an array of objects

Joe Lank
Greenhorn

Joined: Jun 08, 2004
Posts: 9
I'm having trouble accessing arrays of objects, probably something wrong with my syntax declaring the array.



This gives a java.lang.NullPointerException error. Any help is greatly appreciated.
[ June 08, 2004: Message edited by: Joe Lank ]
Jason Fox
Ranch Hand

Joined: Jan 22, 2004
Posts: 114
When you initiate the Student1 array, you are creating a new Student1[] (array) object, you are not creating any Student1 objects, and since all array objects (or primitives) are assigned there default value, each Student1 object in the Student1[] array object is set to null. Try:


I hope this makes sense.
Mark Vedder
Ranch Hand

Joined: Dec 17, 2003
Posts: 624

Hi Joe,
Welcome to JavaRanch� A quick administrative note, it is generally a lot easier if you place any sample code in UBB [code] [/code] tags. It makes reading the code much easier. Take a look hereto learn how to use UBB�

In addition to Jason�s comment, there is another "problem" in your code that should be pointed out so you do not confuse yourself about something. Looking at your code with line numbers for easy referencing:


The code on lines 3-6 is not actually a constructor, as you indicate in your System out comment; based on the way you have it defined, it is a method. Constructors must be named the same as the class and do not have a return type, not even void. I think you just made a typo since you have similarly named classes, Student and Student1. To be a constructor, you need to change line so the name matches the class, and remove the return type:



This will now output the comment when you construct an instance of Student1 using the syntax Jason gave you.

Previously, a Student1 object could still be constructed since by default, all objects have a no argument constructor such as this:


If we change lines 3-6 to be a constructor instead of a method named "Student", it will make line 14 syntactically incorrect if your intent was to call the constructor since you do not call constructors with dot-notation. As Jason indicates, you need to use the keyword new.

If you then wanted to call a method on the object, you use the dot-notation on any methods you have defined. Since having a method named Student inside the class Student1 which is used in the Class Student is rather confusing, let�s instead add a method named sayHello to the Student1 class so you can see how to call a method. So if we take all the above along with Jason�s comments, we would have this:



This should run and give you the output:

Call to constructor complete.
sayHello method called

As you continue to read about Array�s, you should learn how to iterate through the entire array and place an object in each position.

Hope that helps
Joe Lank
Greenhorn

Joined: Jun 08, 2004
Posts: 9
Wow! I can't thank you both enough! And thanks for clearing up about methods and constructors.
 
 
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