Develop Reference

Processing multidimensional array

Hey I can't get processing to run my code due to a NullPointerException on my array value in the println statement.
for (bx=0; bx<=7; bx++) {
for (by=0; by<=4; by++) {
rect(bx*BRICK_WIDTH, by*BRICK_HEIGHT, BRICK_WIDTH, BRICK_HEIGHT);
int[][] a = {{bx}, {by}};
}
println (a[bx][by]);
}

From just the code you posted, I wouldn't expect you to get a NullPointerException. I would expect you to get a The variable "a" does not exist error.
So I'm guessing that you have another a variable at the top of your sketch, like this:
int[][] a;
void draw(){
for (bx=0; bx<=7; bx++) {
for (by=0; by<=4; by++) {
rect(bx*BRICK_WIDTH, by*BRICK_HEIGHT, BRICK_WIDTH, BRICK_HEIGHT);
int[][] a = {{bx}, {by}};
}
println (a[bx][by]);
}
}
Please note that this is why it's so important for you to post a MCVE, so we don't have to guess at what your code is doing.
If this is the case, your problem is caused because the int[][] a = {{bx}, {by}}; line inside the for loop is declaring a different variable with the same name. It's not touching the skethc-level a variable. So the sketch-level a variable still has the default value of null, hence the NullPointerException when you try to use it.
Also note that it doesn't make a ton of sense to assign a to anything inside the for loop. To see why, consider this simpler example:
int x = 0;
for(int i = 0; i < 10; i++){
x = i;
}
println(x);
You'll see that the x variable only "keeps" the last value we assigned to it. The same thing is true of arrays. Maybe you meant to set a particular index of your array?
If you're still having trouble then please post a MCVE. Good luck.

Java StingIndexoutOfBounds

I keep getting an error
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String
index out of range: 10
at java.lang.String.charAt(Unknown Source)
at Field.setLocations(Field.java:175)
at Field.(Field.java:39)
at SeaBattle.onePlayer(SeaBattle.java:76)
at SeaBattle.play(SeaBattle.java:36)
at MainClass.main(MainClass.java:7)
public void setLocations() {
Random rand = new Random();
ArrayList<String> locationToSet = new ArrayList<String>();
ArrayList<String> temp = null;
int let, num, incl, incn;
String alpha = "ABCDEFGHIJ";
boolean worked;
for (int i = 0; i < ships.size(); i++) {
worked = false;
start: while (!worked) {
locationToSet.clear();
worked = true;
let = rand.nextInt(9);
num = 1 + rand.nextInt(9);
if (num % 2 == 0) {
//num even or odd
incl = 1;
incn = 0;
} else {
incl = 0;
incn = 1;
}
for (int j = 0; j < ships.get(i).getLength(); j++) {
String loc = "" + alpha.charAt(let) + num;
let += incl;
num += incn;
for (int t = 0; t < ships.size(); t++) {
if (t != i) {
temp = ships.get(t).getLocation();
if (temp.contains(loc)) {
worked = false;
continue start;
}
}
}
locationToSet.add(loc);
}
ships.get(i).setLocation(locationToSet);
}
}
}
the line 175 is
for (int j = 0; j < ships.get(i).getLength(); j++) {
I have no idea why I get this error. If I change the < in == then I don't get the error. But then it won't give my objects their locations.

There are already many resources on this in Stack Overflow which can be found with Googling, but I think the larger issue here is you need to understand how exceptions and errors are telling you something went wrong, and Java can't do what you told it to do because it's not able to.
Looking at the error you posted. That is a stacktrace. You'll see a ton of this in programming, you might hate seeing them but they're your friend, because without them, we can't tell if our program ran properly or not. You can read it from bottom up right after it tells it that an exception of java.lang.StringIndexOutOfBoundsException occurred.
Here's how to read your error from bottom up:
at MainClass.main(MainClass.java:7)
This means that your error started when Java was at this line doing all the function calls. Line 7 of MainClass.java, which jumps to
at SeaBattle.play(SeaBattle.java:36)
That's line 36 in your SeaBattle java. Which goes to
at SeaBattle.onePlayer(SeaBattle.java:76)
to
at Field.(Field.java:39)
to
at Field.setLocations(Field.java:175)
and finally.
at java.lang.String.charAt(Unknown Source)
The last line tells you when you did String.charAt it tried to read from an unknown source, which based on the Exception it gave you, meant that it happened at index 10, possibly meaning your string was only up to 9 characters. (As a guess you probably did a for-loop and tried reading the wrong index)
A good approach is to look at that line and see if you can figure out what's wrong. (Double clicking on the exceptions in IDEs usually brings focus to that point).
If that didn't work..
Use the debugger
If you use Eclipse or Netbeans to write java, they all have a debugger that assists you with well, debugging your code. Set a breakpoint at line 7 of MainClass, or any of those lines in that stack trace and run the debugger to step through your code. Here's a great tutorial on how to do that. Manually stepping through your code will let you see exactly what went wrong, that way you can find your problem. Good luck with your homework assignment, this is a good one to learn about for-loops and reading arrays, go ask your TA if you can't figure out the bug, we're more helpful in person.

Changing an array attempt

I posted this question up earlier and it was pretty much a lazy post as I didn't provide the code I had and as a result got negged pretty badly. Thought I'd create a new one of these ....
I have an array dogArray which takes ( name, secondname, dogname )
I want to be able to change the dogname:
here's my attempt :
public void changeName(Entry [] dogArray) {
Scanner rp = new Scanner(System.in);
String namgeChange = rp.next(); {
for (int i = 0; i < dogArray.length; i++){
for (int j = 0; j < dogArray[i].length; j++){
if (dogArray[i][j] == name){
dogArray[i][j] = nameChange;
}
}
}
}
For a start it doesn't like the fact I've used ' name ' although it is defined in the dogArray. I was hoping this would read input so that users are able to change 'name' to whatever they input. This will change the value of name in the array.
Do you guys think I'm getting anywhere with my method or is it a pretty stupid way of doing it?

Only one loop is necessary, also move your call to next.
public void changeName(Entry [] dogArray) {
Scanner rp = new Scanner(System.in);
for (int i = 0; i < dogArray.length; i++){
String nameChange = rp.next(); {
if(!dogArray[i].name.equals(nameChange)){
dogArray[i].name = nameChange;
}
}
}
You might want to make this function static as well and use accessors and mutators to change and get the name. Might want to tell the user what you want them to type as well. Also there is no point in testing if the name changed, if it changed then set it, if it didnt change then set it (it doesnt hurt). Just get rid of if and keep the code inside.

2D advice for nullpointer exception

I apologize in advance, I am a java noob.
I have this in a statement
if(a==0 && b<4)
{
value = ((elev[a][b]-elev[a+1][b])*0.00001* double "variable" ) ;
}
So my main question is would the following....
(elev[a][b]-elev[a+1][b])
return an int value (assuming that the array was initialized and populated with int values, and that for a==0 and b<4 none of the references are null.
Sorry in advance if this is silly. Please don't feel inclined to comment, but help would be appreciated. I haven't done a lot of this java stuff.
When i populated the array, I printed it's contents to make sure I was populating correctly, and everything is where it should be...
Alas, I get a null pointer error wherever that (elev[a][b] - elev[a+1][b]) is first referenced....yet i know that the values are being put there.
Next question. When i populate an array, if i want to reference the values,
while(input.hasNextInt())
{
elev[i][j] = input.nextInt(); <-- this is how i was doing it
}
of elev[][]... do i need to say
elev[i][j] = new input.nextInt();
or is how i was doing it sufficient. When i populated an ArrayList from a file I had to use the "new" prefix So i was trying to figure out why i would get a null there.
Like i said I did print the array after reading it from the file and it printed out everything was in its place.
Thanks everyone.
EDIT
ok sorry for simplicity sake i didn't put in the actual code of "value"
it is actually
double randKg = getRandKg(avgKgNitrogen[z]);
double gradient = 0.00001
double under = ((randKg *(elev[a][b] - elev[a+1][b]) * gradient));
2nd Edit
This is the code for how i populated.
try{
File file = new File(filename);
Scanner input = new Scanner(file);
int rows = 30;
int columns = 10;
int elev[][] = new int[30][10];
for(int i = 0; i < rows; ++i){
for(int j = 0; j < columns; ++j)
{
while(input.hasNextInt())
{
elev[i][j] = input.nextInt();
}
}
}
input.close();
}
catch (java.io.FileNotFoundException e) {
System.out.println("Error opening "+filename+", ending program");
System.exit(1);
}
3rd edit
So i am getting the null pointer here.....
(elev[a][b] - elev[a+1][b]) > 0 )
Which is why i originally asked. I have printed the array before when i populated and everything is where it should be.

You are getting a null pointer exception because double "variable" does not indicate to any integer or double value. Compiler is just trying to convert String 'variable' into double which is not possible. So, try eliminating the Double Quotes from "variable". Moreover you have not declared the data type of value variable.

Ignoring other problems in your code (covered by other answers), here's about your actual question:
If the code
if(a==0 && b<4) {
value = (elev[a][b] - elev[a+1][b]);
}
crashes with NullPointerException, it means elev is null. Assuming a and b are of type int, then there is no other way this can generate that exception (array out of bounds exception would be different). There are two options for the cause:
You execute above code before you do int elev[][] = new int[30][10];, so that elev still has the initial null value.
elev in the crashing line is a different variable than elev in initialization shown in the question.
And in you code, it seems to be 2. You create local elev in the initialization. It goes out of scope and is forgotten. You probably should have this initialization line in your method:
elev = new int[30][10];
And then you should have a class member variable instead of local variable in a method:
private int[][] elev;

Ambiguous java error

public static int intersectionSizeMergeAndSort(studentList L1, studentList L2) {
int intersectionSize = 0;
int[] C = new int[L1.studentID.length+L2.studentID.length];
for(int i = 0; i<L1.studentID.length; i++){
C[i] = L1.studentID[i];
}
for(int i = 0; i<L2.studentID.length; i++){
C[i+L1.studentID.length] = L2.studentID[i];
}
Arrays.sort(C);
int pointer = 0;
while(pointer<((L1.studentID.length)+(L2.studentId.length))){
if(C[pointer] = C[pointer+1]){
intersectionSize = intersectionSize + 1;
pointer = pointer + 2;
}
else {
pointer == pointer + 1;
}
return intersectionSize;
}
}
I have this algorithm I am writing for an assignment. Every time I compile my code, I get an error that I cannot understand in order to debug.
Error is as follows:
Error: /Users/nah/Desktop/studentList.java:137: operator < cannot be applied to int,<nulltype>
the error is pointing to the while loop statement

Not sure if this is directly related to the same error but your line
while(pointer<((L1.studentID.length)+(L2.studentId.length))){
has mis-spelt the second "studentId" and should be "studentID"
while(pointer<((L1.studentID.length)+(L2.studentID.length))){
that is of course unless you happen to have two arrays defined for your studentID class, each with a different case.
I also agree with the other answers related to the "==" comparison operator, but you're telling us the error is related to the while statement

The only error I can see is:
if(C[pointer] = C[pointer+1]){
this should be:
if(C[pointer] == C[pointer+1]){

The following will not work in Java:
if (C[pointer] = C[pointer+1])
This is assignment, its type is int, it cannot be used in the if statement

While loop ? Ok. Debugging is done by simplifying expressions.
The only thing where we see < is the loop condition. Instead of
int pointer = 0;
while(pointer<((L1.studentID.length)+(L2.studentId.length))){
do
int pointer = 0;
int len = ((L1.studentID.length)+(L2.studentId.length));
while(pointer< len){
If error again, simplify the error-raising expression.
I think that this question is mistitled. It is ** ambigous only because you do not attempt to localize it. Post a question only if "<" does not work for two integers. The current question title must be: "how do you debug Java programs?"