Return value in a class from another class and method

Question

Note that I am new with Java. I have a class and and a method in this class hat computes some values and I want to return them in the first class. I have a code but it returns 0.

That's in class that computes the values

public class ImageProcessing{
int i,j;
int R[][]=new int[640][320];
int G[][]=new int[640][320];
int B[][]=new int[640][320];


public double meanR[]=new double[320];
public double meanG[]=new double[320];
public double meanB[]=new double[320];
public double varianceR[]=new double[320];
public double varianceG[]=new double[320];
public double varianceB[]=new double[320];
public double skewnessR[]=new double[320];
public double skewnessG[]=new double[320];
public double skewnessB[]=new double[320];

public double round(double value){
    //  int decimalPlace = 2;

    BigDecimal bd = new BigDecimal(value);
    bd = bd.setScale(2,BigDecimal.ROUND_UP);
    return (bd.doubleValue());
}

public void Mean(Bitmap image){

    int width = image.getWidth();
    int height = image.getHeight();
    int pixel = 0;


    for (i=0; i<width;i++){
        for (j=0; j<height; j++){
            pixel = image.getPixel(i,j);
            R[i][j] = Color.red(pixel);
            G[i][j]= Color.green(pixel);
            B[i][j] = Color.blue(pixel);


            meanR[j]=meanR[j]+R[i][j];
            meanG[j]=meanG[j]+G[i][j];
            meanB[j]=meanB[j]+B[i][j];
        }
    }

In the main class I have:

    method.Mean(rescaledBitmap1);

    meanR1=method.meanR;
    meanG1=method.meanG;
    meanB1=method.meanB;
    System.out.println(meanR1);

Answer 1

You must specify what you want to return in the definition of the method and actually return a value using the keyword return :

public int sum(int a, int b){
 int result = a + b;
 return result;
}

You've declared Mean() as void public void Mean(Bitmap image) and thus it returns no value.

Also, you can only return 1 variable so you should either put those 3 values in some sort of array or create a new class and encapsulate the variables inside an object. Here is an example:

public class MeanResult(){
private double meanR[]=new double[320];
private double meanG[]=new double[320];
private double meanB[]=new double[320];
//Maybe declare more stuff here

 public MeanResult(Bitmap image){
  //... code n stuff here to calculate width, height and pixel
 }
 public double getMeanR(){ return this.meanR[]; }
 public double getMeanG(){ return this.meanG[]; }
 public double getMeanB(){ return this.meanB[]; }
}

And you can use it like this:

MeanResult mean = new MeanResult(image);
meanR1=mean.getMeanR();
meanG1=mean.getMeanG();
meanB1=mean.getMeanB();

Answer 2

The method mean(Bitmap image) is void . For it to return something, you must change void to what it will return, and at the end of the method return the variable.

Example:

public int add(int x, int y)
{
    return x + y;
}

Then if you called add(1,2) it would have a value of 3.