数组列表 <Integer[]> 在Java中不起作用

Question

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

Answer 1

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.

Answer 2

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.