问java.lang.IndexOutOfBoundsException:索引0超出长度0的界限

EN

import java.util.*;
 class stockbuysell{
    //Function to find the days of buying and selling stock for max profit.
    public static void main(String []args){
        int A[] = {100,180,260,310,40,535,695};
         ArrayList<ArrayList<Integer>> al = stockBuySell(A,7);
    }
    static ArrayList<ArrayList<Integer> > stockBuySell(int A[], int n) {
        // code here
       // buy = 0;
        //sell = 1
        ArrayList<ArrayList<Integer>> al = new ArrayList<ArrayList<Integer>>();
        int flag = 0;
        int idx = 0;
        for(int i=1;i<n;i++){
            if(i==n-1 && flag==1){
                al.get(idx).add(i);
                flag = 0;
            }
            if(flag == 0 && A[i]>A[i-1]){//
                al.add(new ArrayList<Integer>());
                al.get(idx).add(i-1);
                flag = 1;
            }
            if(flag == 1 && A[i]<A[i-1]){//
                al.get(idx).add(i-1);
                flag = 0;
                idx++;
            }
        }
        for(int i=0;i<al.size();i++){
            System.out.println(al.get(i).get(0)+" "+al.get(i).get(1));
        }
        return al;
    }
}

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length 0
        at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)
        at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)
        at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:266)
        at java.base/java.util.Objects.checkIndex(Objects.java:359)
        at java.base/java.util.ArrayList.get(ArrayList.java:427)
        at stockbuysell.stockBuySell(stockBuySell.java:32)
        at stockbuysell.main(stockBuySell.java:6)