Sums, averages and the largest value

A loop over the indexes can combine all the values into one result. For a sum, start from 0 and add each value:

ArrayList<Integer> minutes = new ArrayList<>();
minutes.add(25);
minutes.add(40);
minutes.add(15);
minutes.add(30);

int total = 0;
for (int i = 0; i < minutes.size(); i++) {
    total += minutes.get(i);
}
System.out.println("Total: " + total);
System.out.println("Average: " + (double) total / minutes.size());

Total: 110
Average: 27.5

The average divides the sum by size(), and (double) keeps the decimals. (On an empty list, that divides by 0, and the average comes out as NaN, "not a number". Check the size first when the list may be empty.)

The largest value

To find the largest value, keep the largest one seen so far in a variable, and compare every value with it. Start with the first value in the list, not with 0:

ArrayList<Integer> temperatures = new ArrayList<>();
temperatures.add(-3);
temperatures.add(-8);
temperatures.add(-1);
temperatures.add(-5);

int highest = temperatures.get(0);
for (int i = 1; i < temperatures.size(); i++) {
    if (temperatures.get(i) > highest) {
        highest = temperatures.get(i);
    }
}
System.out.println("Highest: " + highest);

Highest: -1

Starting with 0 would give the wrong answer here: no temperature is above 0, so the program would print Highest: 0, a value that isn't even in the list. The first value is always a real candidate. The loop can then start at index 1, since index 0 has been counted already. (get(0) needs a list with at least one value in it.)

For the smallest value, turn the comparison around, to <. And highest = Math.max(highest, temperatures.get(i)); does the same as the whole if.

Where the largest value is

Sometimes the place matters more than the value: which day was the warmest? Then keep the index of the best value so far, and read the value with get when you need it:

ArrayList<Integer> temperatures = new ArrayList<>();
temperatures.add(-3);
temperatures.add(-8);
temperatures.add(-1);
temperatures.add(-5);

int warmest = 0;
for (int i = 1; i < temperatures.size(); i++) {
    if (temperatures.get(i) > temperatures.get(warmest)) {
        warmest = i;
    }
}
System.out.println("Warmest: day " + (warmest + 1) + ", " + temperatures.get(warmest) + " degrees");

Warmest: day 3, -1 degrees

With >, a tie keeps the earlier index; with >=, the later one would win.