Class Composition with Array's - New to Java

Question

Greetings.
I have the basic foundtion to my program laid and it seems to be doing what i need it to. However in my transaction method I have an "If" statement that is supposed to process a transaction differently based on the type of member. Based on what I have done the program operates fine if I make member type static. I will include the areas of code that refer to the problem of the member type and the error i recieve. Hopefully it will be enough to demonstrate what I am doing wrong.
Outline of the Problem
Create an Array of Users (user) - Working
Create and Array of Accounts containing Beginning balance and Account Number (accountNum) - Working
Set Array owner using last name - Working
Process Transactions - Returning the returning the following error:
Exception in thread "main" java.lang.NullPointerException
     at account.transaction(account.java:90)
     at accountTest.main(accountTest.java:64)
If i set memberType to static It worked earlier. I hope this is enough code and description to help identify the problem. I can include more if you need.
Thank you in advance!
Lava
Class Customer
     private String firstName;
     private String lastName ;
     private String memberType;
     public customer(String first, String last, String member)
          firstName = first;
          lastName = last;
          memberType = member;
     public void setMemberType( String m )
           memberType = m;
     public String getMemberType()
          return memberType;
     }Class Account
     private customer owner;
public void setOwner(customer user)
          owner = user;
     public customer getOwner()
          return owner;
public void transaction(account[] accountNum)
if ( owner.getMemberType().equals("Premier" ));

Answer 1

>
Based on what I have done the program operates fine if I make member type static. >
It has to do with how static fields and instance fields are or are not given default values at runtime. Making the customer member static means that for all instances of the Account class, the customer variable will refer to the same object. For different objects of the Account class, will your customer variable refer to the same object? If so, it should be static and thus a class member. If not, it should be an instance member.