find number of columns in a 2D array Java

I'm new to java and am trying to find the transpose of a matrix X with m rows and x columns. I'm not familiar with java syntax but I think I can access the number of rows using double[x.length] but how do I figure out the number of columns? Here is my code:

import stdlib.StdArrayIO;

public class Transpose {
    // Entry point
    public static void main(String[] args) {
        double[][] x = StdArrayIO.readDouble2D();
        StdArrayIO.print(transpose(x));
    }

    // Returns a new matrix that is the transpose of x.
    private static double[][] transpose(double[][] x) {
        // Create a new 2D matrix t (for transpose) with dimensions n x m, where m x n are the
        // dimensions of x.
        double[][] t = new double[x.length][x[1.length]]

        // For each 0 <= i < m and 0 <= j < n, set t[j][i] to x[i][j].
        


        // Return t.
    


    }
}

Also, I was trying to figure it out by printing a 2D list in a scrap file but It wouldn't work for some reason, where is what I had:

import stdlib.StdArrayIO;
import stdlib.StdOut;

public class scrap {
    public static void main(String[] args){
        double[][] x = new double[10][20];
        StdOut.println(x.length);
    }


}

Could you point out what I was doing wrong? It kept waiting on input. I could also use any other tips that would help, I am transitioning from Python.