[JAVA] I got stuck in a clone of a two-dimensional array

If it is a one-dimensional array of ordinary primitive type, you can copy it with clone (). If it is an array of classes, you can make a shallow copy by cloning (). Then, if it is a primitive type two-dimensional array ...

clone () of int type 2D array

I used to make another two-dimensional array and copy the values, but when I saw someone's source, I thought I could write it in one line.

	public static void main(String[] args) {
		int[][] map = new int[3][3];
		for (int y=0; y<3; y++) {
			for (int x=0; x<3; x++) {
				map[y][x] = x+y;
			}
		}
		System.out.println("before");
		for (int y=0; y<3; y++) {
			for (int x=0; x<3; x++) {
				System.out.print(map[y][x] + " ");;
			}
			System.out.println("");
		}
		int[][] nmap = map.clone();
		for (int y=0; y<3; y++) {
			for (int x=0; x<3; x++) {
				nmap[y][x] = 4-(x+y);
			}
		}
		System.out.println("after");
		for (int y=0; y<3; y++) {
			for (int x=0; x<3; x++) {
				System.out.print(map[y][x] + " ");;
			}
			System.out.println("");
		}
	}

When you run

before
0 1 2 
1 2 3 
2 3 4 
after
4 3 2 
3 2 1 
2 1 0 

I intended to make clone () in nmap, but map has been rewritten.

Array hash value

Try to output the hash value of int [].

	public static void main(String[] args) {
		int[][] map = new int[3][3];
		System.out.println("before");
		for (int y=0; y<3; y++) {
			System.out.println(map[y]);
		}
		int[][] nmap = map.clone();
		System.out.println("after");
		for (int y=0; y<3; y++) {
			System.out.println(nmap[y]);
		}
	}

When you run

before
[I@15db9742
[I@6d06d69c
[I@7852e922
after
[I@15db9742
[I@6d06d69c
[I@7852e922

It's the same. If you think about it carefully, it's natural, but since int [] is a reference type, it is a shallow copy.

Loop and clone () a one-dimensional array

Then clone () int [].

	public static void main(String[] args) {
		int[][] map = new int[3][3];
		System.out.println("before");
		for (int y=0; y<3; y++) {
			System.out.println(map[y]);
		}
		int[][] nmap = new int[3][];
		for (int y=0; y<3; y++) {
			nmap[y] = map[y].clone();
		}
		System.out.println("after");
		for (int y=0; y<3; y++) {
			System.out.println(nmap[y]);
		}
	}

When you run

before
[I@15db9742
[I@6d06d69c
[I@7852e922
after
[I@4e25154f
[I@70dea4e
[I@5c647e05

This time it's good.

Try cloning () the first source as a one-dimensional array

Try clone () with int [].

	public static void main(String[] args) {
		int[][] map = new int[3][3];
		for (int y=0; y<3; y++) {
			for (int x=0; x<3; x++) {
				map[y][x] = x+y;
			}
		}
		System.out.println("before");
		for (int y=0; y<3; y++) {
			for (int x=0; x<3; x++) {
				System.out.print(map[y][x] + " ");;
			}
			System.out.println("");
		}
		int[][] nmap = new int[3][];
		for (int y=0; y<3; y++) {
			nmap[y] = map[y].clone();
		}
		for (int y=0; y<3; y++) {
			for (int x=0; x<3; x++) {
				nmap[y][x] = 4-(x+y);
			}
		}
		System.out.println("after");
		for (int y=0; y<3; y++) {
			for (int x=0; x<3; x++) {
				System.out.print(map[y][x] + " ");;
			}
			System.out.println("");
		}
	}

When you run

before
0 1 2 
1 2 3 
2 3 4 
after
0 1 2 
1 2 3 
2 3 4 

nmap has a copy of the map.

Summary

Two-dimensional arrays cannot be clone () in one row. Even if you write it in someone's source, there is no guarantee that it is correct.