I have a method that gets returns a type SENSOR
In the bold is where I am getting a runtime NullPointerException, cannot understand why.
public Sensor getSensorAt(int x,int y,GridMap grid)
{
/*go through sensor storage array
* for eachsensor index call the get x get y method for that
* compare it to the x,y of the robot
*
*/
for(int i=0;i<s1.length;i++){
if(s1[i].getX() == x){ <======= NullpointerException
if(s1[i].getY()== y){
return s1[i];
}
}
}
return null;
}
You did not show us where s1 is created, but it looks like s1 does not have anything in it for some index i.
I tend to write my for loops like so to make code like this a bit cleaner
Object result = null;
for(int i=0;i<s1.length;i++){
Object current = s1[i]; // Replace Object with whatever your array actually contains
if(current.getX() == x && current.getY() == y) {
result = current;
break; // if you only need the first match
}
}
return result;
Things like formatting are important and will help you prevent bugs in the first place, and make them easier to find when they do happen....
Some of the elements in the array s1 is null, and when you are trying to invoke a method on that null object you are getting NPE.
Hope it helps you.
Related
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.
In this class, I defined a constructor that initializes an array and fill it with Point2D.Double. I want to define a toString method that outputs the Point2D.Double in the array. So inside the toString method, I make a for loop that returns every Point2D.Double in the array. The problem is, I don't know why Eclipse tells me that the update in the for statement is dead code.
import java.awt.geom.Point2D;
public class SimplePolygon {
public int n; // number of vertices of the polygon
public Point2D.Double[] vertices; // vertices[0..n-1] around the polygon
// boundary
public SimplePolygon(int size) {
n = size;
vertices = new Point2D.Double[n]; // creates array with n size. Elements are doubles.
for(int i = 0; i < n; i++)
{
Point2D.Double point = new Point2D.Double(Math.random() * 6, Math.random() * 6);
vertices[i] = point;
}
}
public String toString() {
for(int i = 0 ; i < n ; i++)
{
return "" + vertices[i];
}
return "";
}
I too was puzzled by this. (And the other answers!) So I cut-and-pasted it into Eclipse to see what it actually says.
And what Eclipse is actually says is that i++ is unreachable in this line.
for(int i = 0 ; i < n ; i++)
And in fact, that is correct! If you ever enter the loop body, the body will unconditionally return. Hence the i++ can never be executed.
Note also that this is a warning not an error. This code is not invalid according to the JLS rules about unreachability.
You are right to be puzzled by the other explanations. The final return statement is reachable. Consider the case where the class is instantiated with a negative value for n (or size). In that case, the for loop body will never be executed, and control will go to the final return.
However, their suggestions as to how to fix the problem are correct. You should not have a return in the loop body.
The problem is because of the return statement in the for loop. Remember, whenever you use return, you immediately end the method and stop running any code. That means that your toString method will loop exactly only once, returning only vertices[0]. The second return below the loop never has a chance to execute, so is considered dead code.
This is actually incorrect! See Stephan's answer for a better/accurate explanation of what's going on.
Regardless, you still need to fix your code. Instead of returning something inside the loop, you probably want to combine the values and return them all at once at the very end. An easy way to do this might be:
public String toString() {
String output = "";
for(int i = 0 ; i < n ; i++)
{
output += vertices[i] + " ";
}
return output;
}
Now, instead of returning immediately, we're accumulating values in the loop and returning at the very end.
(Note that the code here isn't very efficient -- you'd probably want to use something like String.join or StringBuilder instead, but if you're a beginner, this works for now)
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;
I am trying to send two variables from one sketch to another, using the oscP5 library for processing.
The message I am sending is created like this:
OscMessage myMessage = new OscMessage("/test");
myMessage.add(title);
myMessage.add("Zeit");
oscP5.send(myMessage, remoteLocation);
In the second sketch, I receive the data like that:
void oscEvent(OscMessage theOscMessage) {
if(theOscMessage.checkAddrPattern("/test")) {
String title = theOscMessage.get(0).stringValue();
String layoutType = theOscMessage.get(1).stringValue();
addToQueue(title, layoutType);
}
}
And here my simplified addToQueue function:
void addToQueue(String title, String layoutType) {
if(!existsInQueues(title)) {
upcomingHeadlines.add(new Headline(title, printAxis, scrollSpeed, layoutType));
}
}
Every time I start the sketches, I get the error:
ERROR # OscP5 ERROR. an error occured while forwarding an OscMessage to a method in your program. please check your code for any possible errors that might occur in the method where incoming OscMessages are parsed e.g. check for casting errors, possible nullpointers, array overflows ... .
method in charge : oscEvent java.lang.reflect.InvocationTargetException
I have been able to track the problem down to the layoutType-Variable. If I change
String layoutType = theOscMessage.get(1).stringValue();
to
String layoutType = "Zeit";
no error occurs.
That is quite confusing, because both versions should have the same result.
The error message does not help me in any way.
Edit
I have compared the two possible variables like that:
String layoutType = theOscMessage.get(1).stringValue();
String layoutTypeB = "Zeit";
if(layoutType.equals(layoutTypeB)) println("Same String!");
Since gets printed to the console, both have to be the same … I really do not know where to search for an error anymore.
Edit 2
I have wrapped my second sketch in try {...} catch(Exception ex) {ex.printStackTrace();} like that:
void oscEvent(OscMessage theOscMessage) {
try {
if(theOscMessage.checkAddrPattern("/test")) {
if(debug && debugFeed) println("Received message from other sketch.");
String title = theOscMessage.get(0).stringValue();
String layoutTypeO = (String)theOscMessage.get(1).stringValue();
String layoutType = "Zeit";
if(debug && debugTemp) {
if(layoutType.equals(layoutTypeO)) println("IS DOCH GLEICH!");
}
if(debug && debugFeed) println("Parsed Information.");
if(debug && debugFeed) println("-----");
addToQueue(title, layoutTypeO);
}
} catch(Exception ex) {ex.printStackTrace();}
}
That gives me this error as result:
java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(ArrayList.java:635)
at java.util.ArrayList.get(ArrayList.java:411)
at printer$Headline.useLayout(printer.java:260)
at printer$Headline.<init>(printer.java:188)
at printer.addToQueue(printer.java:407)
at printer.oscEvent(printer.java:395)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at oscP5.OscP5.invoke(Unknown Source)
at oscP5.OscP5.callMethod(Unknown Source)
at oscP5.OscP5.process(Unknown Source)
at oscP5.OscNetManager.process(Unknown Source)
at netP5.AbstractUdpServer.run(Unknown Source)
at java.lang.Thread.run(Thread.java:744)
Edit 4
Constructor for my Headline-Class:
class Headline {
//Define Variables
Layout layout;
String title, lastHeadline;
float yPos, speed;
float transparency = 255;
boolean fullyPrinted = false;
int boundingBoxHeight;
// Initialize Class Function
Headline(String t, float y, float s, String lay) {
title = t;
yPos = y;
speed = s;
layout = useLayout(lay);
boundingBoxHeight = calculateTextHeight(title);
}
You might want to know about useLayout() too, so here it is:
Layout useLayout(String name) {
ArrayList layoutVariants = new ArrayList<Layout>();
int existingLayouts = layouts.size();
Layout chosenLayout;
for(int i = 0; i < existingLayouts; i++) {
Layout currentLayout = (Layout)layouts.get(i);
if(currentLayout.layoutType == name) {
layoutVariants.add(currentLayout);
}
}
if(layoutVariants != null) {
int rand = (int)(Math.random() * layoutVariants.size());
chosenLayout = (Layout)layoutVariants.get(rand);
} else {
chosenLayout = (Layout)layouts.get((int)(Math.random() * existingLayouts));
}
return chosenLayout;
}
There are two problems with your code, and both of them are in your useLayout method.
The first problem is that you are not comparing Stringss correctly on this line:
if(currentLayout.layoutType == name) {
name is a String, and I assume currentLayout.layoutType is too. Two Strings that are equal but not the same will not compare equal under ==. As a result of this, your layoutVariants list will quite probably be empty at the end of the for loop.
This line should read:
if(currentLayout.layoutType.equals(name)) {
See also this question.
The second problem is that you don't correctly handle the case that the layoutVariants list is empty. The problem is on this line:
if(layoutVariants != null) {
layoutVariants will never be null, so the else branch of this if statement will never execute. Because layoutVariants.size() will be zero, rand will always be zero. Trying to get the element at index 0 in an empty ArrayList will give you precisely the IndexOutOfBoundsException you are seeing.
I imagine you want the else block to execute if the layout name given isn't recognised, in other words, if the layoutVariants list is empty, rather than null. In that case, change this line to
if(!layoutVariants.isEmpty()) {
Note the ! (not-operator) before layoutVariants. You want the code under the if statement to run if the layoutVariants element is not empty.
EDIT in response to your comments: a null ArrayList is very much not the same as an empty one. null is a special value meaning that the variable doesn't have an object of a given type.
Let's try a real-world analogy: a shopping bag. If you have an empty bag, or no bag at all, then you have no shopping either way. However, you can put things into an empty bag, and count how many items it contains, for example. If you don't have a bag, then it doesn't make sense to put an item in it, as there's no bag to put the item into. null represents the case where you don't have a bag.
Similarly, a String is a collection of characters, and the collection of characters can exist even if it doesn't contain any characters.
isEmpty() can be used for any collection, and, if you're using Java 6 or later, Strings as well. Off the top of my head I can't name any other classes that have an isEmpty method. You'll just have to consult the documentation for these classes to find out.
I've not worked with Processing much, but I am aware that Processing is built on Java, so I would expect any standard Java method to work. Also, I wouldn't worry about 'clearing' a variable: the JVM is generally very good at clearing up after you. There's certainly nothing I can see wrong with your code in this respect.
EDIT 2 in response to your further comment: ArrayList arr; declares a variable of type ArrayList. However, the variable arr is uninitialized: it does not have a value (not even null) and it is an error to try to read the value of this variable before you have assigned a value to it:
ArrayList arr;
System.out.println(arr); // compiler error: arr might not have been initialised.
Assign null and the code then compiles:
ArrayList arr = null;
System.out.println(arr); // prints 'null'.
It's not often you need to declare a variable and not give it a name, but one common case is where you want to assign different values to the same variable on both sides of an if statement. The following code doesn't compile:
int y = getMeSomeInteger(); // assume this function exists
if (y == 4) {
int x = 2;
} else {
int x = 5;
}
System.out.println(x); // compiler error: cannot find symbol x
The reason it doesn't compile is that each variable x is only available within the braces { and } that contain it. At the bottom, neither variable x is available and so you get a compiler error.
We need to declare x further up. We could instead write the following;
int y = getMeSomeInteger(); // assume this function exists
int x = 0;
if (y == 4) {
x = 2;
} else {
x = 5;
}
System.out.println(x);
This code compiles and runs, but the value 0 initially assigned to x is never used. There isn't a lot of point in doing this, and we can get rid of this unused value by declaring the variable but not immediately giving it a value.
int y = getMeSomeInteger(); // assume this function exists
int x;
if (y == 4) {
x = 2;
} else {
x = 5;
}
System.out.println(x);
I am building a Sudoku solver that use the Try and Fail technique to solve any problem. My algorithm is:
1)Update (method that remove any possible value that already given as a final value to element in the same Row, column or squar)
2)Get the minimum element that has minimum number of possible values
3)start solve assuming the first possible value is the final value
4)save the current sate into a stack
5)Try to solve
5-a)If solved, return
5-b)if not solved and with invalid Sudoku, then Pop previous state
6)Repeat step 3) for all possible vaues (9)
7)Repeat step 2) until the puzzel is solved
This is my code
Stack<Element[][]> myStack= new Stack<>();
private Element[][] mySudoku;
public void solve(){
update();//remove all final values from all possible values for each element
if(isSudokuSolved(mySudoku)){
return;
}
//find a cell that is not confirmed and has the minimal candidates
int celli=-1,cellj=-1, p=10;
for(int i=0;i<9;i++){
for(int j=0;j<9;j++){
if(mySudoku[i][j].getValue()==0){
if(mySudoku[i][j].getPossibleValues().size()<p){
celli=i;
cellj=j;
p=mySudoku[i][j].getPossibleValues().size();
}
}
}
}
try {
for (int c = 0; c < mySudoku[celli][cellj].getPossibleValues().size() - 1; c++) {
//save state
Element[][] copy=deepCopy(mySudoku);//copy the current state
myStack.push(copy);
//apply candidate to cell
mySudoku[celli][cellj].setValue(mySudoku[celli][cellj].getPossibleValues().get(c));
update();//check is solved
if(checkValidInputSudoku(mySudoku)){
solve();
}else{
try {
mySudoku = myStack.pop();
} catch (EmptyStackException est) {
//do nothing
}
}
}
} catch (Exception e) {
}
//if we have reached here then we are at the last possible value for the candidates so confirm candidate in cell
if(celli!=-1 && cellj!=-1 && p!=10) {//Some problems happen here "out of Boundry -1 Error"
mySudoku[celli][cellj].setValue(mySudoku[celli][cellj].getPossibleValues().get(mySudoku[celli][cellj].getPossibleValues().size()-1));
}
}//end of solve method
I have spent more than 6 hours trying to find out the problem. I have checked for the Update() method, deepCopy() method and checkValidInputSudoku() method. They all works fine. Thank you in Advance
I can see one problem in your code. You have a loop that is sawing off the branch it sits on:
for(int c = 0; c < mySudoku[celli][cellj].getPossibleValues().size() - 1; c++) {
...
mySudoku[celli][cellj].setValue(mySudoku[celli]cellj].getPossibleValues().get(c));
...
}
Apart from that, you are missing one of the values, it should be for(c=0; c!=size; ++c), i.e. not size - 1. Also, calling getPossibleValues() just once would make this code much more readable. Lastly, catching and ignoring a stack underflow is just stupid, because it hides errors in your algorithm, as far as I can tell. If you don't know how to handle an error, don't just silence it. Since java requires you to catch it, put it in the outermost place possible or at least abort or do something, but don't ignore it!
One more thing: You are recursing and passing the context data via mySodoku and myStack. This is completely missing the point of recursion (or at least the way it's typically used), because the function call stack is the only stack you need. Using these to pass parameters only makes things more complicated than necessary. Instead, the function should return a partial sodoku puzzle and return either the fully solved puzzle or null. Using is easier to distinguish than the exception you're using now, and it's a regular and expected thing, not really exceptional. Then, when trying different choices, you set the cell to the values in turn and recurse, until the call doesn't return null. If none of the choices returns a solution, you clear the cell and return null yourself.
solve(sodoku):
if sodoku is solved:
return true
if sodoku is invalid:
return false
c = some empty cell
for v in 1...9:
// set to a value and recurse
c = v
if solve(sodoku):
// found a solution
return true
// no solution found, clear cell and return failure
c = null
return false
BTW: This strategy is called "backtracking". Using a cell with the least amount of possible values is called "pruning", which allows you to cut off whole branches from the search tree. Actually determining the possible values also helps avoiding a few futile attempts.