AP CSA 3.1 Exercise 1: One Field, Three Views

Unit 3: Class Creation · Lesson 3.1 · Exercise 1

One Field, Three Views

Abstraction and Program Design. Write real Java, run it, and submit it to be graded against hidden test cases.

Back to the Lesson 3.1 page All Unit 3 lessons

Why this one is worth doing

Abstraction is deciding what the outside world gets to know. This class stores a single number and offers three different views of it, and nobody using it can tell which one is the real one.

What to write

Write a class named Duration that stores a length of time in ONE private instance variable holding a whole number of seconds.
The constructor takes the total number of seconds.
getMinutes returns the number of whole minutes, and getSeconds returns the seconds left over after those minutes.
getTotalSeconds returns the stored total, and add(seconds) increases it.
format returns the minutes, then a colon, then the leftover seconds, with no spaces. For 125 seconds that is 2:5.

Your program reads

The grader supplies the input. Your class does not read anything.

Your program prints

Your class prints nothing. The grader prints what your methods return.

The class the grader will call

Your answer must declare exactly these, spelled exactly like this. The grader creates the object and calls the methods itself, so a method under a different name is a method it cannot find.

class Duration
Duration(int totalSeconds)
int getMinutes()
int getSeconds()
int getTotalSeconds()
void add(int seconds)
String format()

Do not write a class named Main and do not write a main method. The grader supplies its own.

Worked examples

These are the cases you can see. There are more you cannot, and they use different values, so an answer that prints these numbers as constants will fail.

Example 1 input

125 60

Example 1 output

Example 2 input

59 1

Example 2 output

Your answer

Main.java

class Duration {
  // One private instance variable. Not two.

  public Duration(int totalSeconds) {

  }

  public int getMinutes() {
    return 0;
  }

  public int getSeconds() {
    return 0;
  }

  public int getTotalSeconds() {
    return 0;
  }

  public void add(int seconds) {

  }

  public String format() {
    return "";
  }
}

Input for the Run button

Run uses the sample main shown below, which is not the one that grades you. Submitting runs your class against the real cases, including hidden ones.

The sample main the Run button uses

public class Main {
  public static void main(String[] args) {
    Duration d = new Duration(125);
    System.out.println(d.format());
    d.add(60);
    System.out.println(d.format());
  }
}

Stuck?

Hint 1

Storing minutes and seconds separately makes add() hard, because 45 seconds plus 30 has to carry. One total is the design that keeps every method short.

Hint 2

getMinutes and getSeconds compute from the stored total every time they are called. They do not store anything of their own.

Hint 3

format can call your own getMinutes and getSeconds rather than repeating the arithmetic.

Where to go next

Rework the Lesson 3.1 page Pick another Unit 3 lesson

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