Java object array

Question

Hi all,
first of all let's see some lines of source code
Foo.java
public class Foo
public Socket pSocket = null;
public nPosition = 0;
NewBie.java
public class NewBie{
public Foo[] m_Foo = null;
private int m_FooCount ;
public Init(int nFooCount)
m_Foo = new Foo[nFooCount];
m_FooCount = nFooCount;
public Operate()
int i = 0;
for( i= 0; i< m_FooCount; i++)
//follwing line will throw a exception
m_Foo.pSocket..... //Here is some operations with m_Foo[i].pSocket
Funny.java
public class Funny
public static NewBie m_pNewBie = new NewBie();
public static void main(String argv[])
m_pNewBie.Init(10);
m_pNewBie.Operate();
%>java Funny
Failed I/O:java.lang.NullPointerException
when I debug the program , seems m_Foo[i].pSocket in method Operate of NewBie object is null. I am puzzled, because I have created a NewBie object array with "new", why it still be null?
Any one can give me a detailed explaination? Thanks a lot.

Answer 1

Arrays contain references, not objects. You must initialize the references within the array to point to constructed objects. Otherwise, the references will be null:
m_Foo = new Foo[nFooCount];
// m_Foo[0] is currently null -- the reference does not point to any object
m_Foo[0] = new Foo();
// now m_Foo[0] points to an object so is not null