Declaring 2d arrays different ways 

int[][] intArray;
double[][] doubleArray;
String[][] stringArray;
boolean[][] booleanArray;

Initializing 2d arrays 

//Method 1
int[][] intArray = new int[3][2]; //will have to manually write out values though if random:
intArray[0][0] = 20; //... till [2][1]
System.out.println(intArray[0][0]);

//Method 2
int[][] intArray2 = {{1,2},{3,4},{5,6}};
System.out.println(intArray2[1][1]);

//String Method 1
String[][] stringArray =  new String[2][2];
stringArray[0][0] = ("hello world"); //... till [1][1]
System.out.println(stringArray[0][0]);

//String Method 2
String[][] stringArray2 = {{"one","two"},{"three","four"}};
System.out.println(stringArray2[1][1]);

20
4
hello world
four

Printing 2d arrays with for loops vertically vs horizontall 

System.out.println("Vertically:");
int[][] matrix = {{1,3,7,8}, {4,5,9,1}, {1,9,6,2}};
for (int i = 0; i<3; i++) {
    for (int j = 0; j<4; j++){
    System.out.println(matrix[i][j]);
    }
}

System.out.println("Horizontally:");
int[][] matrix = {{1,3,7,8}, {4,5,9,1}, {1,9,6,2}};
for (int i = 0; i<3; i++) {
    for (int j = 0; j<4; j++){
    System.out.print(matrix[i][j]+ " ");
    }
}
System.out.println();

System.out.println("Horizontally decreasing:") ;
for (int count = 3; count >=1; count--){
for (int col = 0; col<matrix[0].length; col++) {
    for (int row = 0; row<count; row++){
    System.out.print(matrix[row][col] + " ");
    }
    System.out.println();
}
}

Vertically:
1
3
7
8
4
5
9
1
1
9
6
2
Horizontally:
1 3 7 8 4 5 9 1 1 9 6 2 
Horizontally decreasing:
1 4 1 
3 5 9 
7 9 6 
8 1 2 
1 4 
3 5 
7 9 
8 1 
1 
3 
7 
8

Printing 2d arrays backwards 

System.out.println("Forwards:");
int[][] matrix = {{1,3,7,8}, {4,5,9,1}, {1,9,6,2}};
for (int i = 0; i<3; i++) {
    for (int j = 0; j<4; j++){
    System.out.print(matrix[i][j]+ " ");
    }
}
System.out.println();

System.out.println("Backwards:");
int[][] matrix = {{1,3,7,8}, {4,5,9,1}, {1,9,6,2}};
for (int row = matrix.length-1; row>=0; row--) {
    for (int col = matrix[0].length-1; col>=0; col--){
    System.out.print(matrix[row][col]+ " ");
    }
}

Forwards:
1 3 7 8 4 5 9 1 1 9 6 2 
Backwards:
2 6 9 1 1 9 5 4 8 7 3 1