error: incompatible types: int[][] cannot be converted to int

Question

As you can see in below code, I am trying to add two (2-D integer ARRAYS),I know that variable (c) in the first method is a reference to the array , why can not I return that array or it's reference , Can I return 2-D array in general ,how?

import java.util.*;

public class Matrix_Addition {

    public static int sum (int[][]a,int[][]b){
        int[][]c=new int [a.length][b[1].length];
        for(int i=0;i<a.length;i++){
            for(int j=0;j<a[i].length;j++)
                c[i][j]=a[i][j]+b[i][j];
        }
        return c;
    }
    public static void display(int[][]a){
        for(int i=0;i<a.length;i++){
            for(int j=0;j<a[i].length;j++)
                System.out.println(a[i][j]);
        }
    }

    public static void main(String[] args) {
        int[][]a={{1,2,3},
                  {4,5,6},
                  {7,8,9}};

        int[][]b={{9,8,7}
                 ,{6,5,4},
                  {3,2,1}};

        int[][]c=sum(a,b);
        display(a);
        display(c);
    }

}

Answer 1

c is an array/matrix but you try to return an int, try

public static int[][] sum (int[][]a,int[][]b) {

In general you should specificy in which line java tells you the error appears when asking a question.

Hope it helps!

Answer 2

Since you have declared c as a 2-D array (int[][]c=new int [a.length][b[1].length])

Your sum method returns int and you are trying to return int 2-D array so it is giving you an error that incompatible types: int[][] cannot be converted to int

You should change return type to your sum method from int to int[][].

public static int[][] sum (int[][]a,int[][]b){

Answer 3

您可以使用与输入相同的类型：

public static int[][] sum(int[][] a, int[][] b)

Answer 4

Change you main function this line of code..

int[][]c=sum(a,b);

with

int c=sum(a,b);

and all the downline code like this...

display(a);
display(b);
System.out.println(c);

this will works. because if you see your add() function definition

public static int sum (int[][]a,int[][]b){.....} // return type is int not int[][].

Reason : Return type of sum(int[][], int[][]) is int not int[][] .