Create a 2d array in a square shape

Pattern

The pattern here is

• in the first and in the last row you have to print the dash "-".
• in the first and the last column you have to print a "|".
• For the other fields here we are just printing blank whitespaces " ";

Solution

• So you declare a 2D Array

String[][] grid = new String[5][5]; // chose your dimension

• Iniitialize your grid loop through both dimensions and do the check for the 3 cases mentioned in the section Pattern.
  Note i corresponds to your rows and j corresponds to your columns This means i=0 is your first row and grid.length-1 is your last row Same applies to the columns.

if (i == 0 || i == grid.length - 1) {
    grid[i][j] = "-";
} else if (j == 0 || j == grid[i].length - 1) {
    grid[i][j] = "|";
} else {
    grid[i][j] = " ";
}

The Code

public static void main(String[] args) {
    String[][] grid = new String[5][5];
    for (int i = 0; i < grid.length; i++) {
        for (int j = 0; j < grid[i].length; j++) {
            if (i == 0 || i == grid.length - 1) {
                grid[i][j] = "-";
            } else if (j == 0 || j == grid[i].length - 1) {
                grid[i][j] = "|";
            } else {
                grid[i][j] = " ";
            }
        }
    }
}

Output

-----
|   |
|   |
|   |
-----

If I understood the problem correctly, I would propose this solution:

• conceptually, you need two functions:
• one to initialise floor an ceiling ---------------

static void initialiseCeilingAndFloor(char plane[][], int n) {
    for (int i = 0; i < plane.length; i++) {
        plane[0][i] = '-'; // ceiling is initialised
        plane[plane.length - 1][i] = '-'; //floor is initialised
    }
}

The other one to draw one line of each wall |. . . . . . . . .|. Here I actually broke this method in two, for clarity: InitialiseInternalRow does what it says, and InitialiseBody uses it to initialise the whole body of the plane

static void initialiseBody(char plane[][]) {
    // we already initialised floor and ceiling, so here we'll
    for (int i = 1; i < plane.length - 1; i++) {
        // reduce the cycle at the internal lines
        initialiseInternalRow(plane, i);
    }
}

static void initialiseInternalRow(char plane[][], int i) {
    for (int j = 0; j < plane.length; j++) {
        // if we are at the extremes, we draw a wall: '|'
        if (j == 0 || j == plane.length - 1)
            plane[i][j] = '|';
        else // otherwise we leave space
            plane[i][j] = ' ';
    }
}

So that you can easily initialise your array in your main:

char plane[][] = new char[n][n];
initialiseCeilingAndFloor(plane);
initialiseBody(plane);

You could even think to wrap the two methods together to initialise the thing in one hit, like this:

static void initialisePlane(char plane[][]) {
    initialiseCeilingAndFloor(plane);
    initialiseBody(plane);
}

So that you could easily call from your main:

char plane[][] = new char[n][n];
initialisePlane(plane);

No worries if your terminal displays the output more as a rectangle. this depends on the formatting used by the particular application you are using to display the text. just replace the space with dots, and count them. Serious testings here.

You can use streams to create such an array:

int m = 5;
String[][] arr = IntStream.range(0, m)
        .mapToObj(i -> IntStream.range(0, m)
                .mapToObj(j -> {
                    String val = " ";
                    if (i == 0 || i == m - 1) {
                        val += "-";
                    } else if (j == 0 || j == m - 1) {
                        val += "|";
                    } else {
                        val += " ";
                    }
                    return val;
                }).toArray(String[]::new))
        .toArray(String[][]::new);

// output
Arrays.stream(arr).map(row -> String.join("", row)).forEach(System.out::println);

Output:

 - - - - -
 |       |
 |       |
 |       |
 - - - - -

^{See also:
• How do I return such an multi-dimensional array?
• Drawing a rectangle with asterisks using methods and loops}