澄清ArrayList语法

Question

What is being done in this for loop, counter and arr are two different ArrayLists.

// 3. store count of each number as we iterate through arr
for(int i = 0; i< arr.size(); i++){
    counter[arr.get(i)]++;
}

Answer 1

arr contains indexes that the corresponding elements for them on counter should be incremented.

For example:

arr = [1,3,4]

Then the elements in 1,3 and 4 will be incremented in the array counter .

I highly recommend you to debug your code to better understand the flow of the program. You should also be careful with ArrayIndexOutOfBoundsException :

for(int i = 0; i< arr.size(); i++) {
    if(arr.get(i) < 0 || arr.get(i) >= counter.length) {
        continue;
    }
    counter[arr.get(i)]++;
}

Or doing something else, depends on the logic of your program.

Answer 2

If arr contains integer numbers it is index of counter array. Then you are incrementing the value of counter array on arr.get(i) index. I hope you understand what I am saying.