Class instatiation via new is null?

5 Message(s) by 3 Author(s) originally posted in java language

From: Jim Michaels

Date: Monday, June 11, 2007

1. <programming> The prototype for an {object} in an
	{object-oriented language}; analogous to a {derived type} in a
	{procedural language}.  A class may also be considered to be a
	set of objects which share a common structure and behaviour.
	The structure of a class is determined by the {class
	variables} which represent the {state} of an object of that
	class and the behaviour is given by a set of {methods}
	associated with the class.

	Classes are related in a {class hierarchy}.  One class may be
	a specialisation (a "{subclass}") of another (one of its
	"{superclasses}") or it may be composed of other classes or it
	may use other classes in a {client-server} relationship.  A
	class may be an {abstract class} or a {concrete class}.

	See also {signature}.

	2. <programming> See {type class}.

	3. <networking> One of three types of {Internet addresses}
	distinguished by their most significant bits.

	3. <language> A language developed by the {Andrew Project}.
	It was one of the first attempts to add {object-oriented}
	features to {C}.

	(1995-05-01)

Vertex4D {
public double x, y, z, w;
Vertex4D() {
x = 0; y = 0; z = 0; w = 0;
}
Vertex4D(double newX, double newY, double newZ, double newW) {
x = newX; y = newY; z = newZ; w = newW;
}
public double getX() {return x;}
public double getY() {return y;}
public double getZ() {return z;}
public double getW() {return w;}
public void setX(double newX) {x = newX;}
public void setY(double newY) {y = newY;}
public void setZ(double newZ) {z = newZ;}
public void setW(double newW) {w = newW;}
}

class text

1. Executable code, especially a "pure code" portion shared
	between multiple instances of a program running in a
	{multitasking} {operating system}.

	Compare {English}.

	2. Textual material in the mainstream sense; data in ordinary
	{ASCII} or {EBCDIC} representation (see {flat ASCII}).  "Those
	are text files; you can review them using the editor."

	These two contradictory senses confuse hackers too.

	[{Jargon File}]

	(1995-03-16)

{
Vertex4D[] v;
public boolean done, rotate = false, printMode, objectLoaded=false;
Thread th;

text() {
System.out.println("text constructor");
v = new Vertex4D[20];
if (v == null

<programming> A special value used in several languages to
	represent the thing referred to by an uninitialised pointer.

	<database> A special value that may be stored in some database
	columns to represent an unknown, missing, not applicable, or
	undefined value.  Nulls are treated completely differently
	from ordinary values when evaluating SQL expressions and there
	are several SQL constructs for dealing with nulls.

	(2003-06-17)

) {System.out.println("v=null!"); return;}
v[0].setX(0);
}

public static void main(String args[]) {
System.out.println("main");
//complains about static context

That which surrounds, and gives meaning to, something else.

	<grammar> In a {grammar} it refers to the symbols before and
	after the symbol under consideration.  If the syntax of a
	symbol is independent of its context, the grammar is said to
	be {context-free}.

of v if I put the above code

<software> Instructions for a computer in some programming
	language, often {machine language}.  The word "code" is often
	used to distinguish instructions from {data} (e.g. "The code
	is marked 'read-only'") whereas "{software}" is used in
	contrast with "{hardware}" and may consist of more than just
	code.

	(2000-04-08)

in here & get rid of line

1. <hardware> An electrical conductor.  For distances larger
	than a breadbox, a single line may consist of two electrical
	conductors in twisted, parallel, or concentric arrangement
	used to transport one logical signal.

	By extension, a (usually physical) medium such as an {optical
	fibre} which carries a signal.

	(1995-09-29)

below.
new text();
}
}
C:\prj\hyprcube3\src>JAVA text
main
text constructor
Exception in thread

1. See {multithreading}.

	2. See {threaded code}.

	3. {topic thread}.

	[{Jargon File}]

"main" JAVA.lang.NullPointerException
at text.<init>(text.JAVA:41)
at text.main(text.JAVA:47)

Exception at the v[0].setX(0); line basically. this should not be happening.
I cannot see what I'm doing wrong here. I do not want to set the size of
the array

1. <programming> A collection of identically typed data items
	distinguished by their indices (or "subscripts").  The number
	of dimensions an array can have depends on the language but is
	usually unlimited.

	An array is a kind of {aggregate} data type.  A single
	ordinary variable (a "{scalar}") could be considered as a
	zero-dimensional array.  A one-dimensional array is also known
	as a "{vector}".

	A reference to an array element is written something like
	A[i,j,k] where A is the array name and i, j and k are the
	indices.  The {C} language is peculiar in that each index is
	written in separate brackets, e.g. A[i][j][k].  This expresses
	the fact that, in C, an N-dimensional array is actually a
	vector, each of whose elements is an N-1 dimensional array.

	Elements of an array are usually stored contiguously.
	Languages differ as to whether the leftmost or rightmost index
	varies most rapidly, i.e. whether each row is stored
	contiguously or each column (for a 2D array).

	Arrays are appropriate for storing data which must be accessed
	in an unpredictable order, in contrast to {lists} which are
	best when accessed sequentially.

	See also {associative array}.

	2. <architecture> A {processor array}, not to be confused with
	an {array processor}.

	(1995-01-25)

until I know how big it should be.
This should work even if the Vertex4D class doesn't have a constructor.
the class is instantiated the same way I see examples of String arrays
being instantiated. (http://JAVA.about.com/od/beginningJAVA/l/aa_array.htm)

------------------------------------
Jim Michaels
for email, edit the address

1. <networking> {e-mail address}.

	2. <networking> {Internet address}.

	3. <networking> {MAC address}.

	4. <storage, programming> An unsigned integer used to select
	one fundamental element of storage, usually known as a {word}
	from a computer's {main memory} or other storage device.  The
	{CPU} outputs addresses on its {address bus} which may be
	connected to an {address decoder}, {cache controller}, {memory
	management unit}, and other devices.

	While from a hardware point of view an address is indeed an
	integer most {strongly typed} programming languages disallow
	mixing integers and addresses, and indeed addresses of
	different data types.  This is a fine example for {syntactic
	salt}: the compiler could work without it but makes writing
	bad programs more difficult.

	(1997-07-01)

From: Jack Marsh

Date: Tuesday, June 12, 2007

wrote in message:
[snip]

class text {
Vertex4D[] v;
public boolean done, rotate = false, printMode, objectLoaded=false;
Thread th;
text() {
System.out.println("text constructor");

here you allocate pointer

1. <programming> An {address}, from the point of view of a
	programming language.  A pointer may be typed, with its {type}
	indicating the type of data to which it points.

	The terms "pointer" and "reference" are generally
	interchangable although particular programming languages often
	differentiate these two in subtle ways.  For example, {Perl}
	always calls them references, never pointers.  Conversely, in
	C, "pointer" is used, although "a reference" is often used to
	denote the concept that a pointer implements.

	{Anthony Hoare} once said:

	Pointers are like jumps, leading wildly from one part of the
	data structure to another.  Their introduction into high-level
	languages has been a step backward from which we may never
	recover.

	[C.A.R.Hoare "Hints on Programming Language Design", 1973,
	Prentice-Hall collection of essays and papers by Tony Hoare].

	2. <operating system> (Or "mouse pointer") An {icon}, usually
	a small arrow, that moves on the screen in response to
	movement of a {pointing device}, typically a {mouse}.  The
	pointer shows the user which object on the screen will be
	selected etc. when a mouse button is clicked.

	(1999-07-07)

s to reference

1. <programming> An {address}, from the point of view of a
	programming language.  A pointer may be typed, with its {type}
	indicating the type of data to which it points.

	The terms "pointer" and "reference" are generally
	interchangable although particular programming languages often
	differentiate these two in subtle ways.  For example, {Perl}
	always calls them references, never pointers.  Conversely, in
	C, "pointer" is used, although "a reference" is often used to
	denote the concept that a pointer implements.

	{Anthony Hoare} once said:

	Pointers are like jumps, leading wildly from one part of the
	data structure to another.  Their introduction into high-level
	languages has been a step backward from which we may never
	recover.

	[C.A.R.Hoare "Hints on Programming Language Design", 1973,
	Prentice-Hall collection of essays and papers by Tony Hoare].

	2. <operating system> (Or "mouse pointer") An {icon}, usually
	a small arrow, that moves on the screen in response to
	movement of a {pointing device}, typically a {mouse}.  The
	pointer shows the user which object on the screen will be
	selected etc. when a mouse button is clicked.

	(1999-07-07)

20 Vertex4D objects. Note that
these pointers are all null at this point.

v = new Vertex4D[20];

here you test that these pointers were allocated

if (v == null) {System.out.println("v=null!"); return;}

here you try to use the first pointer without creating a Vertex4D
object. NullPointerException results...
try adding
v[0] = new Vertex4D();

v[0].setX(0);
}

[snip]
Your contention that you've seen code like this used for Strings is
quite wrong. Find that code and look again.

JAVA isn't a particularly easy language

1. <language, programming> {programming language}.

	2. <human language> {natural language}.

	(1998-09-07)

to learn by example, but do not
give up, you are making good progress.

From: Jim Michaels

Date: Tuesday, June 12, 2007

wrote in message:
wrote in message:
[snip]
class text {
Vertex4D[] v;
public boolean done, rotate = false, printMode, objectLoaded=false;
Thread th;

text() {
System.out.println("text constructor");
here you allocate pointers to reference 20 Vertex4D objects. Note that
these pointers are all null at this point.
v = new Vertex4D[20];
here you test that these pointers were allocated
if (v == null) {System.out.println("v=null!"); return;}
here you try to use the first pointer without creating a Vertex4D
object. NullPointerException results...
try adding
v[0] = new Vertex4D();
v[0].setX(0);
}
[snip]
Your contention that you've seen code like this used for Strings is
quite wrong. Find that code and look again.
JAVA isn't a particularly easy language to learn by example, but do not
give up, you are making good progress.

for (int index

(Plural "indices" or "indexes")

	1. <programming> A number used to select an element of a list,
	vector, {array} or other sequence.  Such indices are nearly
	always non-negative integers but see {associative array}.

	2. <database> See {inverted index}.  [Other kinds?]

	3. <World-Wide Web> A {search engine}.

	4. <World-Wide Web> A {subject index}.

	4. <jargon> See {coefficient of X}.

	[{Jargon File}]

	(1997-04-09)

=0; index < v.length; index++) {
v[0] = new Vertex4D();
}

this caused it to work without NPE's. and now I think I understand how
JAVA arrays work. something no example or tutorial has yet showed me.
thanks.
There ought to be a thick book called Real-world JAVA.
--

------------------------------------
Jim Michaels
for email, edit the address

RAM

1. <storage> {Random Access Memory}.

	2. <humour> Rarely Adequate Memory.

	A humorous reference to the fact that programs and data
	expand to fill the memory available.

	(1995-04-22)

Disk is *not* an installation method.

From: Jack Marsh

Date: Tuesday, June 12, 2007

wrote in message:
wrote in message:
wrote in message:
[snip]
for (int index=0; index < v.length; index++) {
v[0] = new Vertex4D();

^--- I'm sure you meant to put v[index] here.

}
this caused it to work without NPE's. and now I think I understand how
JAVA arrays work. something no example or tutorial has yet showed me.
thanks.
There ought to be a thick book called Real-world JAVA.

There are plenty of thick JAVA books.

From: Lew

Date: Tuesday, June 12, 2007

wrote in message:
wrote in message:
wrote in message:
wrote in message:
[snip]
for (int index=0; index < v.length; index++) {
v[0] = new Vertex4D();
^--- I'm sure you meant to put v[index] here.
}

this caused it to work without NPE's. and now I think I understand
how JAVA arrays work. something no example or tutorial has yet showed
me. thanks.
There ought to be a thick book called Real-world JAVA.
There are plenty of thick JAVA books.

When you create a new array, you don't create n new objects, you create n new
references, all null. You've to set an individual array element

1. <data, programming> One of the items of data in an {array}.

	2. <language, text> One kind of node in an {SGML}, {HTML}, or
	{XML} {document} {tree}.  An SGML element is typically
	represented by a start {tag} ("<p>") and an end tag ("</p>").
	In some SGML implementations, some tags are omissible, as with
	"</p>" in {HTML}.

	The start tag can contain {attributes} ("<p lang="en-UK"
	class='stuff'>"), which are an unordered set of key-value
	bindings for that element.  Both the start tag and end tag for
	an element typically contain the "tag name" (also called the
	"{GI}" or generic identifier) for that element.

	In {XML}, an element is always represented either by an
	explicit start tag and end tag, or by an empty element tag
	("<img src='thing.{png}' alt='a dodad' />").

	Other kinds of SGML node are: a section of character data
	("foo"), a comment ("<!-- bar -->"), a markup declaration
	("<!ENTITY reg CDATA '&#174'>"), or a processing instruction
	("<?xml-stylesheet href="shop-english.xsl" type="text/xsl"
	?>").

	(2001-01-30)

to a
non-null value to avoid the NPE.

--
Lew

Next Message: class or interface expected