Java Crash Course: Loops смотреть онлайн

Loops

Follow along with the instructions at: http://fullstackclouddeveloper.com/course/java-for-beginners/index.html#loops

for loops

for loops provide a way to loop on a incremental or decremental basis.

Here is an incrementing example

for (int i = 0; i ﹤ 5; i++) {
System.out.println(i);
}

output

0
1
2
3
4

Increment by 2 using an addition assignment operator

for (int i = 2; i ﹤= 8; i += 2) {
System.out.println(i);
}

output

2
4
6
8

Here is a decrementing example

for (int = 5; i ﹥ 0; i--) {
System.out.println(i);
}

output

5
4
3
2
1

for loops can also iterate through a collection of objects

List﹤String﹥ names = Arrays.asList("Ricky", "Lucy", "Fred", "Ethel");

for (String name : names) {
System.out.println(name);
}

output

Ricky
Lucy
Fred
Ethel

Or, you can increment through the list like this

List﹤String﹥ names = Arrays.asList("Ricky", "Lucy", "Fred", "Ethel");

for (int i = 0; i ﹤ names.size(); i++) {
System.out.println(i + " = " + names.get(i));
}

output

0 = Ricky
1 = Lucy
2 = Fred
3 = Ethel

while loops

A while loop will loop the execution of its code while the condition it evaluates is true.

Example

To count by 2's

int i = 2;

while (i ﹤= 8) {
System.out.println(i);
i += 2;
}

output

2
4
6
8

iterators

An iterator is an object that manages iteration through a collection of objects.

Example

List﹤String﹥ names = Arrays.asList("Ricky", "Lucy", "Fred", "Ethel");

Iterator﹤String﹥ iterator = names.iterator();

while (iterator.hasNext()) {
System.out.println(iterator.next());
}

output

Ricky
Lucy
Fred
Ethel

Exercise 5: Print a calendar using loops

In this example, we will use what we have learned so far to create a calendar for the current month. We will use conditional statements, for loops and a while loop.

import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.Month;

public class HelloWorld {
public static void main(String[] args) {
// Get today.
LocalDate today = LocalDate.now();
System.out.println("Today is: " + today);

// Get the current month.
Month currentMonth = today.getMonth();

// Get the first day of the month.
LocalDate day = today.withDayOfMonth(1);

// Print the days of the week at the top of
// the calendar.
String[] daysOfWeek = { "Su", "Mo", "Tu", "We", "Th", "Fr", "Sa" };
for (int i = 0; i ﹤ daysOfWeek.length; i++) {
if (i ﹥ 0 && i ﹤ daysOfWeek.length) {
System.out.print(' ');
}
System.out.print(daysOfWeek[i]);
}
System.out.println();

// Print spaces until we are at the current day of
// the week.
int dayOfWeek = day.getDayOfWeek().getValue();
for (int i = 0; i ﹤ dayOfWeek; i++) {
System.out.print(" ");
}

// While we are still in this month...
while (currentMonth.equals(day.getMonth())) {
// Get the day of the month for this day.
int dayOfMonth = day.getDayOfMonth();

// If the day number is less than 10, print
// a space to line up the days.
if (dayOfMonth ﹤ 10) {
System.out.print(' ');
}

// Print the day of the month.
System.out.print(dayOfMonth);

if (!day.getDayOfWeek().equals(DayOfWeek.SATURDAY)) {
// Print a space between days if not Saturday.
// The ! means not in a conditional.
System.out.print(' ');
}

// If we are on Saturday, the last day of the week,
// go to the next line.
if (day.getDayOfWeek().equals(DayOfWeek.SATURDAY)) {
System.out.println();
}

// Move to the next day.
day = day.plusDays(1);
}
}
}

output

*prints today's calendar*