Question

I’m working with two-dimensional array-values that should be inserted into a ArrayList. But this is done in a for-loop and the value of the two-dimensional array-value gets changed as the loop runs since it is just used as an temp-variable (which makes all of the variables stored in the ArrayList gets changed as this variable changes).
So if I try to print out the content of the ArrayList when the loop is done all the values are the same.

    for(int i = 0; i <= Counter; i++)
    {

        if(Xhavetomove >= i)
            arrayvalue[0][0] = this.Xspeed;
        else
            arrayvalue[0][0] = 0;

        if(Yhavetomove >= i)
            arrayvalue[0][1] = this.Xspeed;
        else
            arrayvalue[0][1] = 1;

        System.out.println(arrayvalue[0][1]);
        Object.movement.add(arrayvalue); 
    }

Are there anyway I can make it store the value itself?

For example: The first time the loop runs the value is “5,5” but if I print out the ArrayList when the loop is done all the values has turned into “5,1”.

Answer 1

The problem is the way Array is added to the Object here. You are not adding the Array to the Object. What is happening is you are adding the address to the location in memory where the Array resides. So every time you add the Array to the Object, you are adding the same address every time. So every Array in the Object is actually the same Array over and over since they all point to a single location in memory. So when you change the Array, it will appear to change all of them inside the Object.

The best thing to do is either create a new Array every time through the loop, essentially creating a new location in memory for the Array to reside, or clone() the Array which will create a new reference.

Example:

String[] houseOfStark = {"Eddard", "Catelyn", 
                       "Robb", "Sansa", "Arya", "Bran", "Rickon"}; // Sorry Jon
String[] copyOfStark  = houseOfStark;
String[] cloneOfStark = houseOfStark.clone();

houseOfStark[1] = "Lady Catelyn";

System.out.println(houseOfStark[1]);
System.out.println(copyOfStark[1]);
System.out.println(cloneOfStark[1]);

Will produce:

Lady Catelyn
Lady Catelyn
Catelyn

Good blog post explaining the difference