Array values change unexpectedly-CodePudding

I have an array with Coordinate objects. In the beginning, a for-loop assigns the values so that v(1] becomes v[0] (also what I want). Then I change v[0] independently but v(1] is now changing too. This is not what is suppose to happen. How can you solve this? Used input: direction = "R", width = 26, heigth = 26.(code under screenshot).

code:

  package Snake;

import java.util.Random;
import java.util.Locale;
import java.util.Scanner;
import ui.Event;
import ui.SnakeUserInterface;
import ui.UserInterfaceFactory;

public class Snake {

    int lengthOfSnake;
    Coordinate[] snakePosition = new Coordinate[20];

    Snake() {
        lengthOfSnake = 2;
    }

    void initializeSnake() {
        for(int i = 0; i< 20; i  ) {
            snakePosition[i] = new Coordinate();
        }
        snakePosition[0].x = 1;
        snakePosition[0].y = 0;
    }

    void move(String direction, int width, int height) {
        for(int i = lengthOfSnake - 1; i >0; i--) {
            snakePosition[i].x = snakePosition[i-1].x;
            snakePosition[i].y = snakePosition[i-1].y;
            System.out.printf("index %d: %d.%d\n", i, snakePosition[i].x, snakePosition[i].y);
        }
        System.out.println();
        if(direction.equals("R")) {
            snakePosition[0].x  = 1;
            snakePosition[0].x = snakePosition[0].x % width;
        }
        else if(direction.equals("L")) {
            snakePosition[0].x -= 1;
            if(snakePosition[0].x ==- 1) {
                snakePosition[0].x = width - 1;
            }
        }
        else if(direction.equals("U")) {
            snakePosition[0].y -= 1;
            if(snakePosition[0].y ==- 1) {
                snakePosition[0].y = height - 1;
            }
        }
        else {
            snakePosition[0].y  = 1;
            snakePosition[0].y =snakePosition[0].y % height;
        }
        for(int i = lengthOfSnake - 1; i >0; i--) {
            System.out.printf("index %d: %d.%d\n", i, snakePosition[i].x, snakePosition[i].y);
        }
        System.out.print("end move method\n");
    }

    void growTail() {
        lengthOfSnake  ;
        snakePosition[lengthOfSnake] = snakePosition[lengthOfSnake - 1];
    }

    boolean didSnakeEatItself() {
        for(int i = 1; i < lengthOfSnake; i  ) {
            if((snakePosition[0].x == snakePosition[i].x) && (snakePosition[0].y == snakePosition[i].y)) {
                return false;
            }
        }
        return true;
    }
}

CodePudding user response：

I assume that your snakePosition array consists of objects like that:

public class SnakePosition {
 public int x;
 public int y;
 public SnakePosition(int x, int y) {
  this.x=x;
  this.y=y;
 }
 public String toString() {
  return "x=" x ",y=" y;
 }
}

When you have to SnakePosition objects like that:

var pos1 = new SnakePosition(1,1);
var pos2 = new SnakePosition(2,2);

and do the following assignments and object changes

pos1 = pos2;
pos1.x = 0;

your two objects pos1 and pos2 will be both x=0,y=2.

You can do the following and making a deep copy method:

public class SnakePosition {
 ..

 public SnakePosition deepCopy() {
   return new SnakePosition(this.x, this.y);
 }
}

and use it like that:

for(int i = lengthOfSnake - 1; i >0; i--) {
    snakePosition[i] = snakePosition[i-1].deepCopy();
}

Another positibility would be to directly copy the x and y values:

for(int i = lengthOfSnake - 1; i >0; i--) {
    snakePosition[i].x = snakePosition[i-1].x;
    snakePosition[i].y = snakePosition[i-1].y;
}