Java ArrayList for循环lcv被认为是一个对象，而不是一个int

Question

I'm trying to make an array with a sequence of variable length, so I decided to use an ArrayList.

Since I want to find the sum of the sequence of numbers, I tried to use the ArrayList.get(int index) method to get the value of an element in the ArrayList, but NetBeans is complaining at me about incompatible types: Object cannot be converted to int for the following code:

int seqLen = sequence.size(); // number of elements in ArrayList sequence
int seqSum = 0; // sum of all elements in the sequence
for(int i = 0; i <= seqLen; i++) {
   int seqPart = sequence.get(i);
   seqSum+= seqPart;
}

Specifically, the line int seqPart = sequence.get(i);

I'm not sure why it thinks that i is an object and not an int , and this block of code is crucial for the program to work.

Basically, what am I doing wrong here, I've gone over the JavaDoc for ArrayList a couple times now, but I still haven't figured out what's wrong.

Answer 1

If you look at the JavaDoc , you'll notice that get returns an E . If you don't define what that E is, it'll default to Object. An Object can't be assigned to an int. Define your arraylist like this:

List<Integer> sequence = new ArrayList<>();

This says, "I'm going to fill this ArrayList with Integers". That way when you call get() it'll return an Integer instead of an Object .

This part is a little more advanced and not super important so don't worry if it goes over your head: The reason you can assign that Integer to an int is because of a concept called Auto unboxing .