null pointer exception while using arrays


G

gaff

Not sure why this code doesn't work, keep getting null pointer
exception problem.

Array_test is a class that has variables of type array, boolean,
String. The idea is to have an array of Array_test objects from which I
can add and delete objects but where all the objects are stored at the
very start with no ull spaces.

Am I wrong in assuming that having null spaces in an array with a class
of type that you have implemented is okay?

Anywho here's the code, and at the end are some tests and results from
the main.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

// For the app only one instance of Trackeris created, it keeps track
of all segs that are created



public class Tracker {

// Tracker has just one variable that's an array of objects
of type Array_test class which is predefined
private Array_test segholder[];


// construct.
public Tracker(int size_of_array){
segholder = new Array_test[size_of_array];
}

// retrive from array at i position
public Array_test getElementFromArray(int element_location) {
return segholder[element_location];
}




// retrive from array at i position
public String getElementFromArrayName(int element_location) {

String its_name;
its_name = segholder[element_location].getIdentifier();

return its_name;
}




// adds a object type array_test to the first null position in the
array segholder
public void addToArray(Array_test new_seg) {

int i = 0;

while ( i < (segholder.length - 1) ) {

if (segholder == null){
segholder = new_seg;
i = segholder.length;
}

i++;
}
}




// delete from an array
public void deleteFromArray(String old_seg) {

int i = 0;

while (i < segholder.length) {
if (segholder.getIdentifier() == old_seg) {
segholder = null;
i = segholder.length;
}
i++;
}

// now remove the null space

String[] remove = {null};
List templist = new ArrayList(Arrays.asList(segholder));
templist.removeAll(new ArrayList(Arrays.asList(remove)));

segholder = (Array_test[]) templist.toArray(new Array_test[1000 -
templist.size()]);


}

}


**********MAIN********
Array_test o1 = new arrray_test("o1", 50);
Array_test o2 = new arrray_test("o2", 50);
Array_test o3 = new arrray_test("o3", 50);
Array_test o4 = new arrray_test("o4", 50);
Array_test o5 = new arrray_test("o5", 50);
Array_test o6 = new arrray_test("o6", 50);

track.addToArray(o1);
track.addToArray(o2);
track.addToArray(o3);
track.addToArray(o4);
track.addToArray(o5);
track.addToArray(o6);

System.out.println(track.getElementFromArrayName(0));
System.out.println(track.getElementFromArrayName(1));
System.out.println(track.getElementFromArrayName(2));
System.out.println(track.getElementFromArrayName(3));
System.out.println(track.getElementFromArrayName(4));
System.out.println(track.getElementFromArrayName(5));

Result, works as expected:
o1
o2
o3
o4
o5
o6

add this next line and get null pointer exception
System.out.println(track.getElementFromArrayName(6));

The delete function works fine... until I put it into a GUI listener,
then I get the same exception

Could SOMEONE please explain to me what is happening and why.

Cheers,

Gaff
 
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O

Oliver Wong

gaff said:
Not sure why this code doesn't work, keep getting null pointer
exception problem.
[snip most of the code]

Note that you didn't post the code for Array_test, which sounds like
it's important here.
**********MAIN********
Array_test o1 = new arrray_test("o1", 50);
Array_test o2 = new arrray_test("o2", 50);
Array_test o3 = new arrray_test("o3", 50);
Array_test o4 = new arrray_test("o4", 50);
Array_test o5 = new arrray_test("o5", 50);
Array_test o6 = new arrray_test("o6", 50);

Here you've created 6 elements: 1, 2, 3, 4, 5, 6.
track.addToArray(o1);
track.addToArray(o2);
track.addToArray(o3);
track.addToArray(o4);
track.addToArray(o5);
track.addToArray(o6);

Here, you're adding 6 elements: 1, 2, 3, 4, 5, 6.
System.out.println(track.getElementFromArrayName(0));
System.out.println(track.getElementFromArrayName(1));
System.out.println(track.getElementFromArrayName(2));
System.out.println(track.getElementFromArrayName(3));
System.out.println(track.getElementFromArrayName(4));
System.out.println(track.getElementFromArrayName(5));

Here, you're accessing 6 elements: 0, 1, 2, 3, 4, 5 (count them, there
are 6 elements there).
Result, works as expected:
o1
o2
o3
o4
o5
o6

add this next line and get null pointer exception
System.out.println(track.getElementFromArrayName(6));

You're trying to access a 7th element, but you only have 6 elements.
That's probably why you're getting the NPE.

- Oliver
 
G

gaff

When the array is created (sorry didn't include this line of code):
final Tracker track = new Tracker(1000);

the object track is created and the 1000 indicated the size of the
array that it has as it's variable.

therefore shouldn't it be an array of 1000 null elements?

when I look for the 7th element, after occuping the first six, it
should return 1-6 of the objects and then null?
 
O

Oliver Wong

gaff said:
When the array is created (sorry didn't include this line of code):
final Tracker track = new Tracker(1000);

the object track is created and the 1000 indicated the size of the
array that it has as it's variable.

therefore shouldn't it be an array of 1000 null elements?

when I look for the 7th element, after occuping the first six, it
should return 1-6 of the objects and then null?

If you post an SSCCE, it'll probably a lot easier for me to tell you
exactly why you're getting a NullPointerException:
http://mindprod.com/jgloss/sscce.html

- Oliver
 
P

Patricia Shanahan

gaff said:
// adds a object type array_test to the first null position in the
array segholder
public void addToArray(Array_test new_seg) {

int i = 0;

while ( i < (segholder.length - 1) ) {

if (segholder == null){
segholder = new_seg;
i = segholder.length;
}

i++;
}
}


As far as I can tell, given the loop structure, there is a bug in
addToArray. If the first length-1 entries are non-null, the reference
will not be added to the array.

I have a web page on an approach to debugging at
http://home.earthlink.net/~patricia_shanahan/debug/

Patricia
 
G

gaff

The addToArray method seems to be working fine, it finds the first null
space in the array and adds the object in this space.

Haven't tried to fill the array yet but see what you mean with the -1
part.
 
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G

gaff

The addToArray method seems to be working fine, it finds the first null
space in the array and adds the object in this space.

Haven't tried to fill the array yet but see what you mean with the -1
part.
 
G

gaff

Compliable code uses two two classes and one main, you'll notice the
classes are similar, the first having an array of Strings as variable
and the second having an array of the first class as objects. They have
similar functions but never got any grief from the first class, does
this problem have something to do with nulls combined with Strings in
an array working fine and objects created with a class that I define
not working with nulls in an array, does that happen? :

//********Class one********// relaise this should be Array_test, typo I
never fixed, but code compiles

import java.util.*;

public class arrray_test {

private String outro_text[];
private String identifier;

public arrray_test(String new_name, int array_size) {

outro_text = new String[array_size];
identifier = new_name;
}

// Assume that the array always has the entires at the start with the
first null to be the end.
// Code should keep account of this


// retrieve name of object
public String getIdentifier() {
return identifier;
}



// adds a string to the first null position in the array
public void addToArray(String new_string) {

int i = 0;

while ( i < (outro_text.length - 1) ) {

if (outro_text == null){
outro_text = new_string;
i = outro_text.length;
}

i++;
}
}



//method to retrieve single element from array in this object
public String getElementFromArray(int element_location) {
return outro_text[element_location];
}


// delete from an array only first occurence
public void deleteFromArray(String old_string) {

int i = 0;

while (i < outro_text.length) {
if (outro_text == old_string) {
outro_text = null;
i = outro_text.length;
}
i++;
}

// now remove the null space

String[] remove = {null};
List templist = new ArrayList(Arrays.asList(outro_text));
templist.removeAll(new ArrayList(Arrays.asList(remove)));

outro_text = (String[]) templist.toArray(new String[50 -
templist.size()]);


}










//remove all nulls form a string array... then add them back in at the
end so more strings can be added if necessarey
public void removeNulls() {
String[] remove = {null};
List templist = new ArrayList(Arrays.asList(outro_text));
templist.removeAll(new ArrayList(Arrays.asList(remove)));

outro_text = (String[]) templist.toArray(new String[50 -
templist.size()]);

}





//list all strings in the array in the object as a string
public String listAll() {

int i = 0;
String all_strings = null;

while ((i < outro_text.length) &&(outro_text != null)) {



all_strings = all_strings + " " + outro_text;

i++;
}

return all_strings;
}


// return the size of the array, i.e. the places that are not null
public int sizeOfArray() {
int i = 0;
int array_size = 0;

while (i < outro_text.length) {
if (outro_text != null) {
array_size = array_size + 1;
}
else i = outro_text.length;
i++;
}

return array_size;
}


}





//public void addString(String new_String) {
// new_String = outro_text[0];
//}


/*
private Vector v;


public arrray_test(String new_name, int vector_size) {
v = new Vector(vector_size);
namey = new_name;
}




public void addToVector(String new_string) {

v.add(new_string);
}

public String getFirst() {
String firsty;

Iterator iter = v.iterator();
//iter.hasNext();

firsty = (String)iter.next();

return firsty;
}*/


//********Class two********//
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

// For the app only one instance of Trackeris created, it keeps track
of all segs that are created



public class Tracker {

// Tracker has just one variable that's an array of objects of type
Array_test class which is predefined
private arrray_test segholder[];


// construct.
public Tracker(int size_of_array){
segholder = new arrray_test[size_of_array];
}

// retrive from array at i position
public arrray_test getElementFromArray(int element_location) {
return segholder[element_location];
}




// retrive from array at i position
public String getElementFromArrayName(int element_location) {

String its_name;
its_name = segholder[element_location].getIdentifier();

return its_name;
}




// adds a object type array_test to the first null position in the
array segholder
public void addToArray(arrray_test new_seg) {

int i = 0;

while ( i < segholder.length ) {

if (segholder == null){
segholder = new_seg;
i = segholder.length;
}

i++;
}
}




// delete from an array only first occurence but impossible to have
duplicate objects
public void deleteFromArray(String old_seg) {

int i = 0;

while (i < segholder.length) {
if (segholder.getIdentifier() == old_seg) {
segholder = null;
i = segholder.length;
}
i++;
}

// now remove the null space

String[] remove = {null};
List templist = new ArrayList(Arrays.asList(segholder));
templist.removeAll(new ArrayList(Arrays.asList(remove)));

segholder = (arrray_test[]) templist.toArray(new arrray_test[1000 -
templist.size()]);


}





}


//********MAIN-1********//
public static void main(String[] args){
//ButtonExample ss = new ButtonExample();
final Tracker track = new Tracker(1000);

arrray_test o1 = new arrray_test("o1", 50);
arrray_test o2 = new arrray_test("o2", 50);
arrray_test o3 = new arrray_test("o3", 50);
arrray_test o4 = new arrray_test("o4", 50);
arrray_test o5 = new arrray_test("o5", 50);
arrray_test o6 = new arrray_test("o6", 50);

track.addToArray(o1);
track.addToArray(o2);
track.addToArray(o3);
track.addToArray(o4);
track.addToArray(o5);
track.addToArray(o6);

System.out.println(track.getElementFromArrayName(0));
System.out.println(track.getElementFromArrayName(1));
System.out.println(track.getElementFromArrayName(2));
System.out.println(track.getElementFromArrayName(3));
System.out.println(track.getElementFromArrayName(4));
System.out.println(track.getElementFromArrayName(5));
System.out.println(track.getElementFromArrayName(6));
//System.out.println("Break");

track.deleteFromArray("o1");
System.out.println(track.getElementFromArrayName(0));
System.out.println(track.getElementFromArrayName(1));
System.out.println(track.getElementFromArrayName(2));
System.out.println(track.getElementFromArrayName(3));
System.out.println(track.getElementFromArrayName(4));
}
 
G

gaff

Compliable code uses two two classes and one main, you'll notice the
classes are similar, the first having an array of Strings as variable
and the second having an array of the first class as objects. They have
similar functions but never got any grief from the first class, does
this problem have something to do with nulls combined with Strings in
an array working fine and objects created with a class that I define
not working with nulls in an array, does that happen? :

//********Class one********// relaise this should be Array_test, typo I
never fixed, but code compiles

import java.util.*;

public class arrray_test {

private String outro_text[];
private String identifier;

public arrray_test(String new_name, int array_size) {

outro_text = new String[array_size];
identifier = new_name;
}

// Assume that the array always has the entires at the start with the
first null to be the end.
// Code should keep account of this


// retrieve name of object
public String getIdentifier() {
return identifier;
}



// adds a string to the first null position in the array
public void addToArray(String new_string) {

int i = 0;

while ( i < (outro_text.length - 1) ) {

if (outro_text == null){
outro_text = new_string;
i = outro_text.length;
}

i++;
}
}



//method to retrieve single element from array in this object
public String getElementFromArray(int element_location) {
return outro_text[element_location];
}


// delete from an array only first occurence
public void deleteFromArray(String old_string) {

int i = 0;

while (i < outro_text.length) {
if (outro_text == old_string) {
outro_text = null;
i = outro_text.length;
}
i++;
}

// now remove the null space

String[] remove = {null};
List templist = new ArrayList(Arrays.asList(outro_text));
templist.removeAll(new ArrayList(Arrays.asList(remove)));

outro_text = (String[]) templist.toArray(new String[50 -
templist.size()]);


}










//remove all nulls form a string array... then add them back in at the
end so more strings can be added if necessarey
public void removeNulls() {
String[] remove = {null};
List templist = new ArrayList(Arrays.asList(outro_text));
templist.removeAll(new ArrayList(Arrays.asList(remove)));

outro_text = (String[]) templist.toArray(new String[50 -
templist.size()]);

}





//list all strings in the array in the object as a string
public String listAll() {

int i = 0;
String all_strings = null;

while ((i < outro_text.length) &&(outro_text != null)) {



all_strings = all_strings + " " + outro_text;

i++;
}

return all_strings;
}


// return the size of the array, i.e. the places that are not null
public int sizeOfArray() {
int i = 0;
int array_size = 0;

while (i < outro_text.length) {
if (outro_text != null) {
array_size = array_size + 1;
}
else i = outro_text.length;
i++;
}

return array_size;
}


}





//public void addString(String new_String) {
// new_String = outro_text[0];
//}


/*
private Vector v;


public arrray_test(String new_name, int vector_size) {
v = new Vector(vector_size);
namey = new_name;
}




public void addToVector(String new_string) {

v.add(new_string);
}

public String getFirst() {
String firsty;

Iterator iter = v.iterator();
//iter.hasNext();

firsty = (String)iter.next();

return firsty;
}*/


//********Class two********//
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

// For the app only one instance of Trackeris created, it keeps track
of all segs that are created



public class Tracker {

// Tracker has just one variable that's an array of objects of type
Array_test class which is predefined
private arrray_test segholder[];


// construct.
public Tracker(int size_of_array){
segholder = new arrray_test[size_of_array];
}

// retrive from array at i position
public arrray_test getElementFromArray(int element_location) {
return segholder[element_location];
}




// retrive from array at i position
public String getElementFromArrayName(int element_location) {

String its_name;
its_name = segholder[element_location].getIdentifier();

return its_name;
}




// adds a object type array_test to the first null position in the
array segholder
public void addToArray(arrray_test new_seg) {

int i = 0;

while ( i < segholder.length ) {

if (segholder == null){
segholder = new_seg;
i = segholder.length;
}

i++;
}
}




// delete from an array only first occurence but impossible to have
duplicate objects
public void deleteFromArray(String old_seg) {

int i = 0;

while (i < segholder.length) {
if (segholder.getIdentifier() == old_seg) {
segholder = null;
i = segholder.length;
}
i++;
}

// now remove the null space

String[] remove = {null};
List templist = new ArrayList(Arrays.asList(segholder));
templist.removeAll(new ArrayList(Arrays.asList(remove)));

segholder = (arrray_test[]) templist.toArray(new arrray_test[1000 -
templist.size()]);


}





}


//********MAIN-1********//
public static void main(String[] args){
//ButtonExample ss = new ButtonExample();
final Tracker track = new Tracker(1000);

arrray_test o1 = new arrray_test("o1", 50);
arrray_test o2 = new arrray_test("o2", 50);
arrray_test o3 = new arrray_test("o3", 50);
arrray_test o4 = new arrray_test("o4", 50);
arrray_test o5 = new arrray_test("o5", 50);
arrray_test o6 = new arrray_test("o6", 50);

track.addToArray(o1);
track.addToArray(o2);
track.addToArray(o3);
track.addToArray(o4);
track.addToArray(o5);
track.addToArray(o6);

System.out.println(track.getElementFromArrayName(0));
System.out.println(track.getElementFromArrayName(1));
System.out.println(track.getElementFromArrayName(2));
System.out.println(track.getElementFromArrayName(3));
System.out.println(track.getElementFromArrayName(4));
System.out.println(track.getElementFromArrayName(5));
System.out.println(track.getElementFromArrayName(6));
//System.out.println("Break");

track.deleteFromArray("o1");
System.out.println(track.getElementFromArrayName(0));
System.out.println(track.getElementFromArrayName(1));
System.out.println(track.getElementFromArrayName(2));
System.out.println(track.getElementFromArrayName(3));
System.out.println(track.getElementFromArrayName(4));
}
 
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O

Oliver Wong

gaff said:
Compliable code uses two two classes and one main, you'll notice the
classes are similar, the first having an array of Strings as variable
and the second having an array of the first class as objects. They have
similar functions but never got any grief from the first class, does
this problem have something to do with nulls combined with Strings in
an array working fine and objects created with a class that I define
not working with nulls in an array, does that happen? :

[code snipped]

The code is too long for me to take a close look at it. Again, I suspect
the NPE is due to you add 6 objects, and trying to extract 7 objects. If you
want me to take another look, try to get your SSCCE down to 50 lines or
less. I don't think the array_test class is relevant to the problem. You
could just use Tracker, and add instances of Strings instead of instances of
array_test.

- Oliver
 

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