Null Pointer Exception after making copy of instance variable?

Question

I have a GridMonitorClass that reads in a file to get a basegrid and then from there we have to get a surrounding sum grid, average grid etc, I set up an algorithm to get the surrounding sum grid then in the get function I made (I believe) a copy of that grid and then set the method to return it. However, when the test scenario tries to run the function it spits back a NullPointerException and I'm not sure why... below is my GridMonitor constructor and the getSurroundingSumGrid method as well

GridMonitor Contructor

// Instance Variables
private double[][] baseGrid;
private double[][] surroundingSumGrid;
private double[][] surroundingAvgGrid;
private double[][] deltaGrid;
private boolean[][] dangerGrid;

//Constructor
public GridMonitor(String fileName) {

        try {
            this.baseGrid = readFile(fileName);
            // Get Grid Dimensions for the Remaining Grids
            int baseGridXDimension = this.baseGrid.length;
            int baseGridYDimension = this.baseGrid[0].length;

            // Set Up Surrounding Sum Grid
            this.surroundingSumGrid = new double[baseGridXDimension][baseGridYDimension];
            for (int i = 0; i < baseGridXDimension; i++) {
                for (int j = 0; j < baseGridYDimension; j++) {
                    if (i == 0 && j == 0) {
                        this.surroundingSumGrid[0][0] = (2 * this.baseGrid[0][0]) + this.baseGrid[0][1]
                                + this.baseGrid[1][0];
                    } else if (i == 0 && j != 0 && j < baseGridYDimension - 1) {
                        this.surroundingSumGrid[0][j] = this.baseGrid[0][j] + this.baseGrid[1][j]
                                + this.baseGrid[0][j - 1] + this.baseGrid[0][j + 1];
                    } else if (i != 0 && j != 0 && i < baseGridXDimension - 1 && j < baseGridYDimension - 1) {
                        this.surroundingSumGrid[i][j] = this.baseGrid[i - 1][j] + this.baseGrid[i + 1][j]
                                + this.baseGrid[i][j + 1] + this.baseGrid[i][j - 1];
                    } else if (i == 0 && j == baseGridYDimension - 1) {
                        this.surroundingSumGrid[0][j] = (2 * this.baseGrid[0][j]) + this.baseGrid[1][j]
                                + this.baseGrid[0][j - 1];
                    } else if (i == baseGridXDimension - 1 && j == 0) {
                        this.surroundingSumGrid[i][0] = (2 * this.baseGrid[i][0]) + this.baseGrid[i][1]
                                + this.baseGrid[i - 1][0];
                    } else if (i == baseGridXDimension - 1 && j == baseGridYDimension - 1) {
                        this.surroundingSumGrid[i][j] = (2 * this.baseGrid[i][j]) + this.baseGrid[i - 1][j]
                                + this.baseGrid[i][j - 1];
                    }
                }
            }

        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
        }
    }

Get SurroundingSumGrid Method

public double[][] getSurroundingSumGrid() {
        // TODO Auto-generated method stub
        double[][] surroundingSumGrid = this.surroundingSumGrid;
        return surroundingSumGrid;
    }

Edit

I tried just returning this.surroundingSumGrid and am still faced with a null-pointer exception so I am effectively lost in why this error is happening

Answer 1

In Java, variables refer to arrays by reference (whereas they refer to primitive data types by value). That means that when you make this assignment:

double[][] surroundingSumGrid = this.surroundingSumGrid;

You are just saying surroundingSumGrid should point to the same place in memory as this.surroundingSumGrid . To make a copy of the entire array, you'll need to instantiate a new array:

double[][] surroundingSumGrid = new double[baseGridXDimension][baseGridYDimension];

And then copy the array contents from the old array into the new one . Fortunately, since double is a primitive data type, you don't have to worry about cloning individual objects within the array.

Be a little careful, though, because if you're creating a whole new array every time the getter is called, you may end up allocating a lot of memory that you never use. Consider renaming the method to make it clearer that calling this method will cause more work to happen than simply getting a field's value.