Question

I have the following class, which performs some calculations to fill its static arrays.

public class Amount implements Serializable{
      private static final long serialVersionUID = 8141477444408242243L;
      public static Amount values1[][] = new Amount[10][30];
      public static Amount values2[][] = new Amount[10][30];
      public static Amount values3[][] = new Amount[10][30];

      double highestValue;
      double highestAmount;
      double lowestAmount;
      double lowestValue;

      ...
}

As the calculations take 20 minutes or so, I am looking to store the arrays on file and load the values when the program starts. I am attempting to use the java serialization method and have the following functions

public static void loadFile(Amount[][] arr, String filename){
    try {
        FileInputStream fis = new FileInputStream(filename);
        ObjectInputStream in = new ObjectInputStream(fis);
        arr = (Amount[][])in.readObject();
        in.close();
      }
      catch (Exception e) {
          System.out.println(e);
      }
}

public static void saveFile(Amount[][] arr, String filename){
     try {
         FileOutputStream fos = new FileOutputStream(filename);
         ObjectOutputStream out = new ObjectOutputStream(fos);
         out.writeObject(arr);
         out.flush();
         out.close();
      }
      catch (IOException e) {
          System.out.println(e); 
      }
}

which I call like this saveFile(values1, "valueOneSaveFile"); and loadFile(values1, "valueOneSaveFile");

I have run the program once, saving all the arrays to various files. The files have been created and look to be around the correct size. When I change my program to call the loadFile functions, the arrays do not appear to initialize correctly. I am getting null pointer exceptions when trying to read a value from the array (which appears to be empty after the load)

Answer 1

The problem is in your LoadFile method.
Java passes parameters by value. In the case of objects a copy of the “pointer” is passed.
When you update the array:

arr = (Amount[][])in.readObject();

You are not updating Amount.values1 array, instead the local arr variable points to a new array.

You should change the method signature to:

public static Amount[][] loadFile(String filename)

And use it accordingly.