error: char cannot be dereferenced
Error code

	if(row.charAt(m).equals('0')){
		matrix[i][j]=0;
	}

Where the row is a string.
2. Error reason
Ross: Yeah, I mean, derefrence. The value that points to

The type char is a primitive — not an object — so it cannot be dereferenced
derefer0 derefer0 is The process of authentication The value green to by a reference. Since a char is already a value (not a) reference), it can not be dereferenced.
use Character class:
if(Character.isLetter())

char
char
char is a primitive type.
Character is its wrapper class, reference type, and equals() method. == instead of directly comparing values.