List of 2D Arrays return last array

Question

I have a problem.

List<Color[][]> list = new ArrayList<Color[][]>();
for (int i = 0; i < 5; i++) {           
    if (i == 0) tab[1][1] = Color.white;
    if (i == 1) tab[1][1] = Color.blue;
    if (i == 2) tab[1][1] = Color.black;
    if (i == 3) tab[1][1] = Color.red;
    if (i == 4) tab[1][1] = Color.cyan;
    System.out.println(i + " add " + tab[1][1]);
    list.add(tab);  
}
System.out.println("_");
for (int i = 0; i < 5; i++)
    System.out.println(i + ". read " + list.get(i)[1][1]);

And output is:

0 add java.awt.Color[r=255,g=255,b=255]
1 add java.awt.Color[r=0,g=0,b=255]
2 add java.awt.Color[r=0,g=0,b=0]
3 add java.awt.Color[r=255,g=0,b=0]
4 add java.awt.Color[r=0,g=255,b=255]

0. read java.awt.Color[r=0,g=255,b=255]
1. read java.awt.Color[r=0,g=255,b=255]
2. read java.awt.Color[r=0,g=255,b=255]
3. read java.awt.Color[r=0,g=255,b=255]
4. read java.awt.Color[r=0,g=255,b=255]

Why list return only last array?

Answer 1

You only have one instance of tab and store 5 references to it. Then you change tab , and the change is reflected everywhere it is referenced.

In other words, System.out.println(i+" add "+tab[1][1]); prints the state of tab at each addition, which is different after each loop iteration, System.out.println(i+". read "+list.get(i)[1][1]); prints the state of after the last addition, which does not change between iterations of the respective loop.

Answer 2

Here you are changing value for the same reference, hence last value is getting overided. so create new object for Color[][] inside for loop.

Answer 3

Because you are always adding elements at 1,1 and adding same tab 5 times. Instead i think you need:

for(int i=0;i<5;i++){           
    if(i==0) tab[i][i] = Color.white;
    if(i==1) tab[i][i] = Color.blue;
    if(i==2) tab[i][i] = Color.black;
    if(i==3) tab[i][i] = Color.red;
    if(i==4) tab[i][i] = Color.cyan;
    System.out.println(i+" add "+tab[i][i]);
}
list.add(tab);  //add just once

And print like:

for(int i=0;i<5;i++)
    System.out.println(i+". read "+list.get(0)[i][i]);//using dynamic index

Answer 4

You should read up the basic of java. First off you are using the same index of the array in the loop instead of iterating. This is how i think you mean to write it.

List<Color[][]> list = new ArrayList<Color[][]>();
for(int i=0;i<5;i++){           
    if(i==0) tab[i][i] = Color.white;
    if(i==1) tab[i][i] = Color.blue;
    if(i==2) tab[i][i] = Color.black;
    if(i==3) tab[i][i] = Color.red;
    if(i==4) tab[i][i] = Color.cyan;
    System.out.println(i+" add "+tab[i][i]);
    list.add(tab);  
}
System.out.println("_");
for(int i=0;i<5;i++)
    System.out.println(i+". read "+list.get(i)[i][i]);

Also there is no need in this code to use multi dimensional array nor a list of it.