I want to make an arraylist with int[] as objects
ArrayList<Integer[]> b= new ArrayList<Integer[]>(); Integer[] a= new Integer[2]; for(Integer i=0;i<20;i++){ a[0]=i; a[1]=i; b.add(a); } for(int i=0;i<20;i++){ System.out.println("line"+i+"= "+b.get(i)[0]+" "+b.get(i)[1]); } and the result I get is this
instead of the values( 0 0 1 1 etc), is seems that has saved only the last. I have also tried with type int instead of Integer but the same result
Consider initializing a within the loop as otherwise you are just writing over the same array the whole time. Instead of
ArrayList<Integer[]> b= new ArrayList<Integer[]>(); Integer[] a= new Integer[2]; for(Integer i=0;i<20;i++){ a[0]=i; a[1]=i; b.add(a); } for(int i=0;i<20;i++){ System.out.println("line"+i+"= "+b.get(i)[0]+" "+b.get(i)[1]); } consider this:
ArrayList<Integer[]> b= new ArrayList<Integer[]>(); for(Integer i=0;i<20;i++){ Integer[] a= new Integer[2]; a[0]=i; a[1]=i; b.add(a); } for(int i=0;i<20;i++){ System.out.println("line"+i+"= "+b.get(i)[0]+" "+b.get(i)[1]); } As the point is for each loop iteration to make a new array and store new values rather than overwrite the existing ones as if you step through your code you may notice you how don't ever allocate a new array within the loop.
You have to declare a new array for each element you're going to add. Otherwise they all reference the same memory. Declare the array inside the for loop instead of before the loop.
