A square matrix is said to be diagonal matrix if the elements of matrix except main diagonal are zero.

A square null matrix is also a diagonal matrix whose main diagonal elements are zero.

import java.util.*;
class DiagonalMatrix
{
	public static void main(String args[])throws Exception
	{
		Scanner sc=new Scanner(System.in);
		System.out.print("Enter the size of the matrix : ");
		int m=sc.nextInt();
		int A[][]=new int[m][m];
		
		/* Inputting the matrix */
		for(int i=0;i<m;i++)
		{
			for(int j=0;j<m;j++)
			{
				System.out.print("Enter an element : ");
				A[i][j]=sc.nextInt();
			}
		}

		
		System.out.println("The Matrix is : ");
		for(int i=0;i<m;i++)
		{
			for(int j=0;j<m;j++)
			{
				System.out.print(A[i][j]+"\t");
			}
			System.out.println();
		}
		
		int p=0, q=0;
		
		for(int i=0;i<m;i++)
		{
			for(int j=0;j<m;j++)
			{
				if(i!=j && A[i][j]!=0) // Checking non-diagonal elements
				{
					p=1;
					break;
				}
				if(i==j && A[i][j]==0) // Checking diagonal elements
				{
					q++;
				}
		    }
		}
		
		if(p==0 && q<m)
			System.out.println("The matrix is Diagonal");
		else
			System.out.println("The matrix is not Diagonal");
	}
}

Output:

Enter the size of the matrix : 3
Enter an element : (0,0)1
Enter an element : (0,1)0
Enter an element : (0,2)0
Enter an element : (1,0)0
Enter an element : (1,1)3
Enter an element : (1,2)0
Enter an element : (2,0)0
Enter an element : (2,1)0
Enter an element : (2,2)45
The Matrix is :
1 0 0
0 3 0
0 0 45
The matrix is Diagonal