Lesson 3

Welcome! In this lesson, we will explore the practical applications of **string operations** and **type conversions** in **Java**. These concepts are crucial and are used in many programming areas. We'll examine a real-world example: **time parsing**. Have you ever wondered how to add a certain number of seconds to a specific time? By the end of today's session, you'll be equipped to calculate this using Java. Let's get started!

Imagine this: You receive a time formatted as a string in `HH:MM:SS`

, where `HH`

, `MM`

, and `SS`

denote the hour, minute, and second, respectively. You are also given an integer representing a certain number of seconds. Your task is to calculate the new time after adding the provided seconds, then output the result in the `HH:MM:SS`

format.

For example, if the input time is `05:10:30`

and the number of seconds to add is `123`

, the output should be `05:12:33`

since `123`

seconds translate to `2`

minutes and `3`

seconds.

Please note these points when resolving this task:

- The input time is always a valid time string in the
`HH:MM:SS`

format, with`HH`

ranging from 00 to 23 and`MM`

and`SS`

ranging from 00 to 59. - The output should be a time in the same format.

Now let's go ahead and break down how to achieve this in our step-by-step solution guide.

Our first step involves parsing the input time string. We aim to extract the hours, minutes, and seconds from this string as integer values for further calculations. In Java, we can use the `split`

method to divide the string at `":"`

and convert each substring into an integer:

Java`1String time = "12:34:56"; 2int secondsToAdd = 10; 3String[] timeParts = time.split(":"); 4int hours = Integer.parseInt(timeParts[0]); 5int minutes = Integer.parseInt(timeParts[1]); 6int seconds = Integer.parseInt(timeParts[2]);`

By executing this operation, we've successfully parsed the time string and converted the hours, minutes, and seconds into integers.

Now that we have the hours, minutes, and seconds in integer format, we can efficiently calculate the total number of seconds that have elapsed since the start of the day:

Java`1int secondsSinceStart = hours * 3600 + minutes * 60 + seconds;`

At this point, your function should compute the cumulative number of seconds from the start of the day.

Now we need to add the integer representing the number of seconds to our computed `secondsSinceStart`

and also consider cases where the added seconds roll over into the next day:

Java`1int totalSeconds = (secondsSinceStart + secondsToAdd) % (24 * 3600);`

The modulus operator `%`

ensures that our `totalSeconds`

value doesn't exceed the total number of seconds in a day ($86400$ seconds or $24 \cdot 3600$ seconds).

In this step, we reverse the previous operation. We're given an integer number of seconds, and we have to convert this into a time string in the `HH:MM:SS`

format.

We will use the division `/`

and modulo `%`

operations directly to convert these values:

- First, we calculate the number of hours in the total seconds:
`int newHours = totalSeconds / 3600;`

. Here,`3600`

is the number of seconds in an hour. - Next, we update the
`totalSeconds`

to the remainder after extracting the hours:`totalSeconds %= 3600;`

. This step ensures that`totalSeconds`

now holds the seconds remaining after accounting for hours.

Now, we repeat this process for minutes:

- We calculate the number of minutes in the updated
`totalSeconds`

:`int newMinutes = totalSeconds / 60;`

. Here,`60`

is the number of seconds in a minute. - Following a similar pattern, update the
`totalSeconds`

to the remainder after extracting minutes:`totalSeconds %= 60;`

. This step ensures that`totalSeconds`

contains only the seconds remaining after accounting for both hours and minutes.

Finally:

- The remaining
`totalSeconds`

directly gives us the seconds:`int newSeconds = totalSeconds % 60;`

.

So the complete code for this step is:

Java`1int newHours = totalSeconds / 3600; 2totalSeconds %= 3600; 3int newMinutes = totalSeconds / 60; 4int newSeconds = totalSeconds % 60;`

The final step is to assemble these values into our `HH:MM:SS`

format string. Java provides the `String.format`

method for easily formatting strings:

Java`1String result = String.format("%02d:%02d:%02d", newHours, newMinutes, newSeconds);`

Here, `%02d`

ensures that each time unit (hours, minutes, and seconds) will be at least 2 characters wide, padding with `0`

if necessary.

Let's collate all the individual steps and formulate the final function:

Java`1public class TimeAdder { 2 3 public static String addSeconds(String time, int secondsToAdd) { 4 String[] timeParts = time.split(":"); 5 int hours = Integer.parseInt(timeParts[0]); 6 int minutes = Integer.parseInt(timeParts[1]); 7 int seconds = Integer.parseInt(timeParts[2]); 8 9 int secondsSinceStart = hours * 3600 + minutes * 60 + seconds; 10 int totalSeconds = (secondsSinceStart + secondsToAdd) % (24 * 3600); 11 12 int newHours = totalSeconds / 3600; 13 totalSeconds %= 3600; 14 int newMinutes = totalSeconds / 60; 15 int newSeconds = totalSeconds % 60; 16 17 return String.format("%02d:%02d:%02d", newHours, newMinutes, newSeconds); 18 } 19 20 public static void main(String[] args) { 21 // Call the function 22 System.out.println(addSeconds("05:10:30", 123)); 23 } 24}`

And voila! You've crafted a function that accurately calculates the new time based on the provided time and the number of seconds that have elapsed since then.

Congratulations! You skillfully learned how to parse a time string and utilize type conversions to calculate the number of seconds elapsed from the beginning of the day. Following this, you learned how to perform the reverse operation: to calculate the time based on the number of seconds that have passed since the beginning of the day. In this lesson, we practiced essential Java skills, including string operations and fundamental integer operations. Continue practicing with similar problems to reinforce your learning, and these tasks will soon become second nature. See you in our next session, and happy coding!