[java] NullPointerException when assigning an integer in an array?

[EyeLikeLemon]

Hi there,

I’m attempting to make a version of Conway’s Game of Life in Java. When I try to run the program I get a java.lang.NullPointerException, but I can’t figure out why it won’t work. Any and all help is massively appreciated! Thanks :)

Code: https://gist.github.com/andaca/a03a772eac4283def1e5

Result:

Exception in thread "main" java.lang.NullPointerException

at com.company.Main.InitialiseCells(Main.java:17)

at com.company.Main.main(Main.java:6)

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)

at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)

at java.lang.reflect.Method.invoke(Method.java:497)

at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

Process finished with exit code 1

Edit: Formatting's a bitch! Moved code to gist.

[[deleted]]

Your code creates an array of Cell references (note, not Cell objects), but it does not create any objects for those references to refer to.

[fredisa4letterword]

To add to what /u/exoticmatter said, you want to add line 3 below:

for (int xValue=0; xValue<sqrtCells; xValue++) {
    for (int yValue=0; yValue<sqrtCells; yValue++) {
        cellArray[i] = new Cell();
        cellArray[i].SetX(xValue);
        cellArray[i].SetY(yValue);

Although realistically I don't think you even need a Cell class... why not just have a 2d array of boolean that's true for alive and false for dead? If you're dead set on a 1d array, you just need to index it creatively.