Loops in Java
Repeat code with for, enhanced for, while and do-while.
Loops let you run the same code many times. Java's loops look much like C's, plus an extra "for-each" form for collections.
The for loop
A for loop sets up a counter once, checks a condition before every round, and updates the counter after every round:
public class Main {
public static void main(String[] args) {
for (int i = 1; i <= 3; i++) {
System.out.println("7 x " + i + " = " + (7 * i));
}
}
}Output
7 x 1 = 7 7 x 2 = 14 7 x 3 = 21
The enhanced for loop
When you just want every item of an array or list, the enhanced for loop is shorter and avoids index mistakes.
public class Main {
public static void main(String[] args) {
String[] languages = {"Python", "C", "Java", "JavaScript"};
for (String lang : languages) {
System.out.println(lang + " has " + lang.length() + " letters");
}
}
}Output
Python has 6 letters C has 1 letters Java has 4 letters JavaScript has 10 letters
while and do-while
A while loop checks first; a do-while loop runs once before checking.
public class Main {
public static void main(String[] args) {
int n = 27;
int steps = 0;
while (n != 1) {
n = (n % 2 == 0) ? n / 2 : 3 * n + 1;
steps++;
}
System.out.println("Reached 1 after " + steps + " steps");
int tries = 0;
do {
tries++;
} while (tries < 3);
System.out.println("tries = " + tries);
}
}Output
Reached 1 after 111 steps tries = 3
break and continue
public class Main {
public static void main(String[] args) {
int[] readings = {12, -1, 15, 999, 8};
int sum = 0;
for (int r : readings) {
if (r < 0) continue; // ignore bad readings
if (r == 999) break; // 999 marks the end
sum += r;
}
System.out.println("Sum = " + sum);
}
}Output
Sum = 27