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LESSON 269 OF 671 · RECORDS, SORTING & BUILT-INS

Min, Max and Sum in Java: Collections and Streams

Lesson Read the full explanation

TL ; DR:

Collections.min(list) gives the smallest value of a list.

Collections.max(list) gives the largest value.

A stream adds all the values up.

List<Integer> prices = new ArrayList<>(List.of(4, 9, 2, 7));

System.out.println(Collections.min(prices));    // 2

System.out.println(Collections.max(prices));    // 9

int total = prices.stream().mapToInt(x -> x).sum();

System.out.println(total);                      // 22

Full lesson:

In Maximum Number In Array, a loop found the largest value. In Sum of Numbers, a loop added numbers up. These jobs come up all the time. Java has ready-made methods for them.

Collections.min(list) gives the smallest value of the list.
Collections.max(list) gives the largest value.
list.stream().mapToInt(x -> x).sum() adds all the values up.

The sum uses a stream, as in Map and Filter. The call mapToInt(x -> x) turns each Integer into a plain int. Then sum() adds them.

Math.max(a, b) from Maximum Number In Array compares two values. Collections.max looks at a whole list.

Collections.min takes a list, like an ArrayList. It doesn't take an int[] array.

Watch out for an empty list. Collections.min and Collections.max throw a NoSuchElementException on it. An empty list has no smallest value. Check list.isEmpty() first. The stream's sum() of an empty list is 0.

Your task

minutes holds how many minutes you practiced coding on each day of a week.

Store the shortest day in shortest and the longest day in longest. Store the week's total in total.

Use the built-ins from this lesson. The code at the bottom prints all three.

Expected output:

shortest = 15
longest = 60
total = 235

STEP 1 OF 3

Guided

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JAVA · RECORDS, SORTING & BUILT-INS

Min Max Sum in Java, explained

Ready-made methods for common jobs

Finding the smallest value, the largest value and the total of a list are jobs that come up all the time, and you can write each as a loop. Java also has ready-made ways to do them:

List<Integer> prices = new ArrayList<>(List.of(4, 9, 2, 7));
System.out.println(Collections.min(prices));
System.out.println(Collections.max(prices));
int total = prices.stream().mapToInt(x -> x).sum();
System.out.println(total);

The three lines print 2, 9 and 22.

Collections.min(list) returns the smallest value of the list.
Collections.max(list) returns the largest value.
list.stream().mapToInt(x -> x).sum() adds all the values up.

Why the sum needs mapToInt

A List<Integer> holds Integer objects, and a plain stream of them has no sum(). The call mapToInt(x -> x) turns each Integer into a plain int and gives a stream of numbers that can be added.

Math.max(a, b) compares two values, such as Math.max(3, 8), which gives 8. Collections.max looks at a whole list.

Gotchas

Lists, not arrays. Collections.min takes a collection such as an ArrayList. Passing an int[] is a compile error: no suitable method found for min(int[]).
An empty list. An empty list has no smallest value, so Collections.min and Collections.max throw a NoSuchElementException. Check list.isEmpty() first when the list may be empty.
The sum of an empty list is 0. The stream's sum() doesn't throw: adding up nothing gives 0.

The lesson's task

minutes holds the minutes of coding practice on each day of a week. The task stores the shortest day in shortest, the longest in longest and the total in total, using the built-ins above.

int shortest = Collections.min(minutes); looks at every value in one call.
int longest = Collections.max(minutes); does the same for the largest value.
int total = minutes.stream().mapToInt(x -> x).sum(); makes a stream, turns each value into an int and adds them.

The assignment to an int variable works because Java converts the Integer that Collections.min returns into an int automatically.

The solution in Java

List<Integer> minutes = new ArrayList<>(List.of(25, 40, 15, 60, 30, 45, 20));

int shortest = Collections.min(minutes);
int longest = Collections.max(minutes);
int total = minutes.stream().mapToInt(x -> x).sum();

System.out.println("shortest = " + shortest);
System.out.println("longest = " + longest);
System.out.println("total = " + total);

The output is:

shortest = 15
longest = 60
total = 235

Key takeaways

Collections.min and Collections.max take a list and return its extremes.
For the sum, use mapToInt(x -> x).sum() on the list's stream.
Guard against an empty list before calling min or max.

Loop or built-in?

A loop that tracks the smallest value is fine for learning, but the built-ins say what they do in a single line and are harder to get wrong. They cannot start from a wrong value, for example, and they cannot forget a number. Use a loop when the rule is not a plain min, max or sum, for example when you need the position of the smallest value too.

Each of the three scans the list once, so they take time in proportion to the list length. Running all three, as the task does, is three passes, which is fine for a list of seven days or seven thousand.

Both Collections.min and Collections.max compare the items by their natural order. For numbers that is smaller to larger, and for strings it is alphabetical.

The names in the task say what the numbers mean. Calling the results shortest, longest and total makes the printed lines read like sentences. A name like min would work too, but it would say less about the data.

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