整数数组列表的打印内容在末尾显示负数

Question

I created an integer arraylist and added 0 to 10 into it. when I try to print in a for loop it prints negative 1 as the last element of the arraylist.

New to java

  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.List;

 public class StudentList {

public static void main(String[] args) {

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

   for (int i=0; i<=10; i++){
       avarlist.add(i);

    }
    for (int i= 0; i<= avarlist.size(); i++){

        System.out.println(avarlist.indexOf(i));
    }

}

}

Answer 1

Because you are using <= instead of < . Your avarlist will have size 11 after you have added the elements. Then you iterate 12 times (from 0 to 11 inclusive) over the list.

Remember: the last index of a list is always size() - 1 . When iterating over a list, you almost certainly need to use < size() .

Answer 2

Question is already answered, however you can improve your code:

For iterating through your ArrayList you can use for-each:

for(Integer i:avarlist){
  System.out.println(i);
}

Answer 3

Because you are using the terminating condition of the for loop as i<= avarlist.size() . In a Array or ArrayList in most programming languages indexing starts from 0 . So your last element's index value is equals to avarlist.size() -1 .So refactor your code like one of the ways below.

for (int i= 0; i < avarlist.size(); i++){

        System.out.println(avarlist.indexOf(i));
    }

for (int i= 0; i <= avarlist.size()-1; i++){

        System.out.println(avarlist.indexOf(i));
    }

int index =0;

for(int i :avarlist){//Using an enhanced for loop
    
          System.out.println(index);
          index++;
}