Re: Newbie Q. Accessing a variable in one method from another.
Mike Barnard wrote:
"G:\java\Projects>javac starnumbers.java
starnumbers.java:28: non-static variable numberStore cannot be
referenced from a
static context
numberStore = new int[number] ;
public class starnumbers {
int[] numberStore; // A place to store all of the numbers.
static int[] numberStore;
//This was originally in main with 'number'.
public static void main (String[] args)
{
markspace wrote:
This is an easy one. Just make the change that I added above and you're
good to go.
To try to explain this a bit more, static on a method means that the
method isn't attached to any one instance of the class, it's attached to
the class itself. Same for variables. Static means they're attached to
the class (sort like a "global" variable) and not static means the
variable is attached to an instance.
The problem with what you had was a static method was trying to access a
non-static (instance) variable. And you don't have an instance in that
case. So, it fails.
The other thing you could have done, is not make the change I gave you,
then do something like this:
public static void main( String... args ) {
starnumbers sn = new starnumbers();
int number = 5;
sn.numberStore = new int[number];
// ...
See? Now I have an instance, referred to as "sn", so I can use the
instance variable called numberStore with it.
Hope that made sense.
(P.S., also, please use proper capitalization of your class names, e.g.
"StarNumbers".)
There are three basic kinds of variables - static, instance and local.
Static variables belong to the class itself.
Instance variables belong to an instance, or object, of the class.
Local variables are restricted to a block (code between curly braces).
Static methods cannot refer to instance variables.
Code outside a block cannot refer to variables local to that block.
Then there's the matter of 'public', 'protected', package-private and 'private'.
public class FooBar
{
static String belongaType = "This is attached to the class itself";
String belongaInstance = "This is attached to a particular instance";
public static void doSomething()
{
String belongaMethod = "This method cannot use 'belongaInstance'";
System.out.println( belongaType );
// System.out.println( belongaInstance ); // would cause compiler error
System.out.println( belongaMethod );
if ( belongaBlock != null )
{
String belongaBlock = "This is local to the 'if' block";
System.out.println( belongaBlock );
}
// System.out.println( belongaBlock ); // would cause compiler error
}
public void instanceLevel()
{
String belongaLocal = "This method cannot use 'belongaInstance'";
System.out.println( belongaType );
System.out.println( belongaInstance ); // no compiler error
System.out.println( belongaLocal );
if ( belongaBlock != null )
{
String belongaBlock = "This is not the same 'belongaBlock'";
System.out.println( belongaBlock );
}
// System.out.println( belongaBlock ); // would cause compiler error
}
public static void main( String [] args )
{
doSomething();
new FooBar().instanceLevel();
}
}
For a fast introduction to all this, since you have limited time:
<http://java.sun.com/docs/books/tutorial/java/nutsandbolts/index.html>
--
Lew