In Java a 2D array is essentially an array of arrays (possibly with different lengths). It is important to remember this. For example, this is OK:
int[][] ar = new int[2][];
ar[0] = new int[8]; // ar[0][0..7]
ar[1] = new int[4]; // ar[1][0..3]
The syntax new int[8][10] can be used as a convenience to create 8 separate arrays of 10 elements each.
If you are familiar with C: an int[][] in Java is more similar to an int** in C.
Note: Map is a terrible name for a variable in Java; variable names generally start with lowercase letters and there is also a very common base container interface of the same name.
1) Why do we put the Map.length in the first set of square brackets but not Map[0].length in the second set of square brackets for int [][] copy = new int[Map.length][]; ?
Because we are starting with an array of Map.length int[]'s, and then cloning those int[]s one at a time.
Don't we have to initialize the length of the columns as well?
No, because when we go through each int[] in Map, we just use clone() to copy it: copy[i] = Map[i].clone().
By cloning the columns one column at a time and putting it into our 2D array it sets the length of the columns for us?
A "column" is just a concept you made up that is only relevant to tabular data (column-major tabular data in your specific context). Anyways, "setting the length" isn't exactly accurate because it implied that something whose length is being set existed in the first place; but when you do int x[][] = new x[5][], x[0] is null until you assign it to something. By cloning the int[]s one at a time, we're just... cloning them one at a time. So, yes, each clone will be the same size as the original.
3) Could we reverse this code by doing this
public int[][] CopyMap(int[][] Map)
{
int [][] copy = new int[][Map[0].length];
for(int i = 0; i < Map[0].length; i++)
copy[i] = Map[i].clone();
return copy;
}
No; and hopefully the reason why is clear now that you know that an int[][] is an array of arrays. The expression new int[][size] doesn't make much sense; it says that we want each int[] in the array to have a given size, but it doesn't say how many int[]s are in the array. It's wrong for the same reason that int[] x = new int[] is wrong.
4) Also copy[i] ? This is a 2D array, so shouldn't it be copy[i][] ? Or something like that.
No, it should be copy[i]. I'll leave figuring out the reasons as an exercise.
